Lecture notes on Plant Physiology. After reading these notes you will learn about: 1. Introduction to Plant Physiology 2. Need for the Study of Plant Physiology 3. Role of Water in Plant Physiology 4. Role of Hydroponics in Plant Physiology 5. Cell Wall of a Plant 6. Osmosis in Plants 7. Role of Transpiration-Recent Synthesis in Plants and Others.
Contents:
Notes on the Introduction to Plant Physiology
Notes on the Need for the Study of Plant Physiology
Notes on the Role of Water in Plant Physiology
Notes on the Role of Hydroponics in Plant Physiology
Notes on the Cell Wall of a Plant
Notes on Osmosis in Plants
Notes on the Role of Transpiration-Recent Synthesis in Plants
Notes on Factors Influencing Nutrient Uptake in Physiology of Plants
Notes on Plants as Indicators of Mineral Nutrients
Notes on the Physiology of Foliar Nutrition in Plants
Notes on Physiology of Photosynthesis
Notes on the Photorespiration and Plants
Notes on the Physiology of Insectivorous Plants
Notes on the Phenomena of Growth in Plants
Notes on Plant Development
Notes on Growth Hormones in Plants
Notes on the Physiology of Flowering Plants
Notes on the Physiology of Seed Development in Plants
Notes on the Physiology of Fruit Development
Notes on Aging and Senescence in Plants
Notes on the Physiology of Plant Biotechnology
Notes on the Relationship of Plant Physiology to Other Sciences
Note # 1. Introduction to Plant Physiology:
In plant physiology, natural phenomena operating in the living plants and plant parts are studied. It is a discipline of botany where the structure of the cell, tissues and organs is associated with processes and functions. The different responses of organisms to environmental alternations and the resultant growth and development which are the outcome of such responses are also studied in plant physiology.
A process comprises a series of sequential events operating under natural conditions. Some of the most important processes operating in plants are stomatal mechanism; water and mineral absorption; photosynthesis, respiration, etc. A plant physiologist tends to understand, describe and explain such processes.
The natural activity of the cells, tissues, organs and organisms is referred to as their function. A plant physiologist must understand, describe and explain these functions as well. These functions are explained at the cellular and molecular levels. In plant physiology, an attempt is also made to study the factors which modify growth and development.
In other words, a plant physiologist must understand and explain how external stimuli and factors modify plant responses. In brief, in plant physiology, comprehensive information on the structure, processes and functions operative at the cells, tissues, organs is desired in order to explain the processes of growth and development in an organism.
These days, a plant physiologist must have a sound knowledge of chemistry, biochemistry, physics, statistics and plant molecular biology. In fact, these sciences have contributed immensely in elucidating the functional and structural aspects of plants.
Indeed the development of these sciences has contributed to the evolution and discovery of several instruments and techniques which helped in the elaboration of plant anatomy. A sound knowledge of plant anatomy is essential for interpreting the functions and processes operating in a plant.
Several of the plant processes could only be revealed by the use of sophisticated instruments and techniques provided by the chemical and physical sciences.
The availability of the techniques of radioisotopes, antimetabolites, chromatography, scanning, transmission electron microscopy, and mass spectrometry has helped in the elaboration of several processes.
In fact, elaboration of cell structure has considerably aided in understanding plant structure in relation to function. The discovery in plants of lysosomes, peroxisomes and other organelles in recent years is the outcome of combined technology of biochemists, biophysicists and anatomists.
Clearly plant physiology is not a static discipline but shall always change and be enrichened by the discovery and usage of new methodology and instruments. This is also a reason why for one process divergent opinions exist. The observations are dependent upon techniques employed while the interpretations are dependent upon understanding of divergent chemical and physical phenomena.
Obviously a plant physiologist is always open to self or other corrections. It is an enlivened science which will change with time and be enrichened with more and increased experiences over the years. Take, for instance, the process of photosynthesis and even the discovery of photorespiration or the processes operating in C4 plants.
Since plants possess different patterns and habitats as compared to animals it is important to study plant physiology. Plants are also static and possess the capacity to manufacture their own food. Therefore, they possess the capacity to grow and add cells throughout their life span.
Clearly in plants organs develop, grow and die. The regulating systems controlling growth differ from those of animals. The control of plant development is the outcome of action of several hormones, environments and nutritional factors.
Most of these factors interact with each other and cause effective growth of the plant. As compared to animals plants possess special features which point towards their unique physiological characteristics.
These have non-motile habit, autotrophy, and dependence on soil for mineral supply; possess several devices in the terrestrial habitat to protect against excessive evaporation, heating and transport of water from the soil to the plant apex. The presence of rigid cell wall, occurrence of circadian rhythms for several metabolic processes, development of devices to overcome drought, frost-injury, also add to the uniqueness of plants.
These characteristics are best studied and explained on the basis of physiological organization. A successful plant must have the ability to compete in its environments to its best advantage. Thus, plants have undergone physiological evolution and adaptation to the environments.
Note # 2. Need for the Study of Plant Physiology:
The knowledge of plant physiology will help in forging several advances in agriculture, horticulture, forestry, plant pathology and other disciplines of botany. In fact, researches in plant physiology have been and are likely to contribute immensely to crop improvement. Increase in crop production is based on exploiting maximal levels or plant metabolic processes.
The production of new varieties and strains shall have to take into account the physiological attributes of basic material or genotypes. The control of soil fertility, overcoming presence of excessive salts in the soil through the knowledge of plant physiology has helped in increased crop production.
A basic knowledge of plant metabolic processes can help in the increase of photosynthetic conversion of solar energy for the production of food materials that are utilized by human beings.
Basic knowledge on nitrogen fixation will help in increased utilization of atmospheric nitrogen by different plant species. In the past few years several tissue culture techniques have been developed which have shortened the life cycle of several plant species, helped in raising plants from seeds with shrunken endosperm, and have increased our understanding of cell wall formation and mineral uptake.
The detailed knowledge of plant hormones, their synthesis and mode of action, has considerably facilitated their application in checking water loss, manipulating growth and development of certain crops and improving the quality of food materials. The usage of certain hormonal weedicides has minimized the occurrence of weeds in the crop fields.
Crop yields have increased by the judicial usage of auxins, cycocel, gibberellins, phenols and aliphatic alcohols, etc. The regulation of flowering, seed formation and fruit setting has been controlled through the application of different hormones at the appropriate time of plant height and age. In recent years most of the breeding projects also seek the help of the plant physiologists.
Physiological processes play a significant role in several interactions between plant and animals. Man changes his environments deliberately or unintentionally and this change affects the physiological behaviour of plants.
The physiological reactions of plants due to their introduction and cultivation are an admitted fact. Increased urbanization, development of industry and excessive utilization of land has led to the modifications of the environments.
As a result of these modifications there is a drastic effect on the accompanying fauna. For successful agricultural practices a sound physiological base is a must. Plant cultivation or agriculture was a crude art or a native effort and plant physiological studies have developed into a regular science. Modern understanding of the physiological mechanisms of growth and development is being increasingly exploited for increased quality and quantity of crops.
This insight has also increased the survival and or extending the range of desirable plants. Controlled fertilizer application and proper water management have been exploited to the best advantage of limited resources. Practices as crop rotation, green crop ploughing, usage of selective fertilizers, use of growth hormones, inhibitors, etc., are all based on our understanding of plant physiological concepts.
Recently computer technology has been used in aiding plant growth and manipulating plant responses to varied environments. With the growing population there are greater demands on production of various food crops.
Agriculture and agricultural produce are becoming industrialized and the role of plant physiologist is ever increasing. Environmental engineering in all aspects including usage of barren lands, growing increased number of crops, and exploitation of solar radiations by the existing and newly bred species shall involve plant physiologists.
In the coming years, teaching and research in plant physiology will occupy a pivotal place in our institutions. The search for deeper understanding and insight of how plants absorb water and minerals, utilize and conserve them will continue with added scientific techniques.
Note # 3. Role of Water in Plant Physiology:
Of all the vital substances essential for plant growth and development, water is needed in enormous amounts. It is present throughout the plant body. Water constitutes more than 70% of the fresh weight of a plant and in some of the growing cells its level may be up to 90%. Water supply affects the growth rate of plants considerably.
(i) It is a major component of the plant body.
(ii) What is an essential solvent in which mineral nutrients are dissolved and translocated from the roots to the apex of the plant body. Minerals are also absorbed through water.
(iii) Large number of metabolic reactions take place in the water medium.
(iv) It maintains the structure of nucleic acids, proteins by supplying hydrogen bonding.
(v) Several processes like photosynthesis use water as a reactant of raw material. Thus formation of complex carbohydrates from the simple ones also involves the removal of water while the reverse reaction requires water as a reactant.
(vi) This essential component is required to maintain the turgidity of the cell. Thus, it helps the cells to retain their tensile strength and provides proper shape to the cells.
Turgidity is essential for the opening of the stomata, and also activity of several organelles. In addition opening of flowers, folding of leaves occurs due to the changes in the turgidity of the cells.
(vii) Water also acts as a temperature buffer since it has an exceptionally high heat capacity for specific heat.
(viii) Water molecules have the unique property of adhesion and cohesion and thus these processes keep the water molecules together. This property helps in upward movement of water in the plant body.
(ix) The elongation phase of cell growth is mostly dependent on water absorption.
(x) Water is also a metabolic end product of respiration.
(xi) Plans absorb enormous quantities of water and simultaneously lose greater amounts of water through transpiration.
Note # 4. Role of Hydroponics in Plant Physiology:
‘Hydroponics’ involves soil-less growth and refers to the raising of crop plants, vegetables and flowering plants in well-balanced nutrient solutions in the absence of soil. The plants are grown in large shallow pots which are full of nutrient solutions. The tubs or tanks are covered with wire-netting to provide support to the seedlings. The solution is aerated at regular intervals by means of an inlet tube (Fig. 9-2).
Sand culture technique is also employed. Except for Hydroponics have certain advantages like: possibility to provide desirable raising crops in green houses, these techniques are not economically feasible at present, nutrient environment; regulation of pH of the nutrient solution conducive to specific crops; it is possible to replace the nutrient solution periodically; it is possible to control pathogens which are otherwise present in the soil; it is feasible to regulate aeration at regular intervals: in situations where equipment is automatic much cost on labour can be saved; there is no need to undertake tillering weeding; uniform plant growth can be easily achieved and nutrition budget can be easily regulated to meet the changing pattern of plant growth and maturity.
However, hydroponics have some disadvantages also and these are: limited plant yield and production; the system is only suitable to more specific and highly valued crop species; considerable technical experience and skill is needed to handle hydroponics; each crop demands specific modifications, and lastly several of the crop plants like potatoes, sweet potatoes, carrots may even change their shape and size.
Despite its several disadvantages and practical difficulties, large scale commercial hydroponics growth is carried out in many floricultural and horticultural species. In several of the European countries it has been possible to obtain high yields in radish, tomato, lettuce, cucumbers, etc.
Note # 5. Cell Wall of a Plant:
Plant cells have the outer hard covering which is known as cell wall. It consists of three layers-middle lamella, primary and secondary walls.
The middle lamella is a common layer of the two adjacent cells and is amorphous and colloidal in nature.
It consists of pectic compounds, chiefly calcium and magnesium pectate. Sometimes lignin and hemicellulose also occur in the middle lamella.
The primary wall is made up of cellulose and pectic compounds along with hemicellulose and other polysaccharides (Plate 1). Rarely these walls are also lignified but usually they are elastic and highly hydrated.
The cell wall formation is initiated in the late telophase of mitosis. The tubular fragments of the endoplasmic reticulum migrate to the equatorial region during the telophase and participate in the formation of the cell plate or middle lamella which cements the two adjoining cells due to their calcium pectate composition with greater binding property.
Pectic compounds and hemicellulose play an important role in the initial stage of cell growth.
As the cell matures secondary wall starts developing on the inner surface of the primary wall. The cell wall thickens as layers of cellulose are laid down by the cytoplasm.
The wall becomes plastic and the cell growth ceases. Secondary wall gives the plant cell its structural independence and mostly contains cellulose and other polysaccharides including hemicellulose.
It is less hydrated and hence more dense. The secondary wall is mostly mechanical in function and is usually distinguishable in three layers (S1, S2, and S3).
Of these, the S3 layer is also referred to as the tertiary wall. In some plant species the secondary wall is made up of several layers which are designated as S1, S2 … Sn etc. It is a permanent layer i.e. it cannot be extended by growth.
Cytoplasmic ground substance (cytosol) is not a constant material and its consistency changes from time to time.
Its main component is water and its percentage varies from 90 (parenchyma cells) to 4-5 (in seeds; spores and pollen).
The cytosol contains several living (mitochondria, plastids, ribosomes, lysosomes, etc.) and non-living particles (fat globules, aleurone grains, organic acids, pigments, fat globules, etc.).
Cytosol varies enormously in the same cell at different times and variation also exists from plant to plant.
Note # 6. Osmosis in Plants:
Take a container and separate it into two parts by a differentially permeable membrane. Add pure water in one of the parts of the beaker and a sugar solution in the other. On the side having pure water the water potential (Ψ) is higher than that on the other side. The sugar molecules cannot diffuse across their differentially permeable membrane, though water can.
Water will diffuse from the side where it is pure i.e. having higher Ψ to the side where sugar solution is present i.e. the side with lower Ψ. This diffusion of water across a differentially permeable membrane from a region of higher potential to a region of lower potential is called osmosis.
In the plant system different biological membranes are present which are differentially permeable. These are plasmalemma, tonoplast and the membrane which surrounds organelles. Because of their physical or chemical nature water molecules pass easily through these membrances.
On the other hand, molecules of different solute dissolved in the water either cannot penetrate or do so slower than water molecules. There are also membrances which are permeable or semipermeable depending upon whether both solute and solvent molecules together or only solute molecules can pass through them respectively.
The osmosis in biological systems was regarded as a process of diffusion but recently it has been shown to occur through mass flow. More recent studies have also shown that some of the living membranes in plants are traversed by continuous aqueous channels whereas others are not.
Therefore, osmosis may occur by mass flow only in some of the membranes. It is uncertain whether the porosity of the membrane is species specific or is dependent upon other factors. Possibly the age of an individual cell may also affect its porosity. Osmosis is important in the absorption of water in higher plants (Fig. 6-1).
It has a special role to play in the transport of water into plant cells. The phenomena of plasmolysis depend upon osmosis. We have observed that water moves under the influence of imbibition and thus water potential Ψ) must be affected by these forces. Matric potential (ΨM) is the term commonly employed to account for all the forces causing imbibition or holding the water in a matrix.
The potential of water in a matrix e.g. in a soil, colloid etc., can be explained as follows:
Ψ = Ψπ + Ψp + ΨM
Note # 7. Role of Transpiration-Recent Synthesis in Plants:
Transpiration is completely essential to the life of a land plant since it helps in absorption of CO2 from the atmosphere. It seems that stomata were evolved to accomplish CO2 absorption and transpiration was a consequence of it. The fact that stomata remain closed at night, their role in oxygen absorption does seem feasible.
This is also supported by the concentrations of oxygen and CO2 in the general atmosphere. However, during the day time when the green leaves are undergoing photosynthesis, oxygen released is diffused out of stomata. However, in reality stomata are primarily evolved to enact the role of CO2 absorption. The general argument against the necessity of transpiration could be advanced since several of the alpine plants could be successfully grown when completely submerged.
Admittedly transpiration as such may not be all that useful to a plant but it is a necessity which has turned into an advantage. Thus, several of the plant species could grow without transpiration but transpiration confers several advantages. For a long time it was assumed that transpiration contributed towards the transport of minerals.
Recent studies have shown that there is a kind of circulation of minerals in the plant. Using tracer elements it has been demonstrated that water moves from assimilating organs to the using organs via phloem tissue; even in the absence of transpiration, and this water returns via xylem tissue. Clearly, transpiration does not appear to be essential for the movement of minerals within the plant.
The general inference is that in the presence of transpiration, there is mineral absorption from the soil and then transport it in the various parts of the plant. Experiments in greenhouse grown tomato plants revealed that calcium transport required an active transpiration stream.
Note # 8. Factors Influencing Nutrient Uptake in Physiology of Plants:
1. Light:
Plants growing in bright light show rapid solute uptake than those grown under weak light. This may be due to high photosynthesis rate and greater supply of sugars to enhance respiration in roots. In green algae and some water plants light directly affects mineral uptake. Here light energy is converted into chemical form e.g., ATP which helps in ion uptake.
2. Temperature:
High temperature up to 40°C increases solute uptake. The slow growth of plants growing in cold climate is due to slow uptake of solutes. High temperature also stimulates respiration of roots which may be aiding ion uptake. Temperature above 40°C affects respiration and thus uptake of solutes is slowed down.
3. pH:
pH 4-9 affects maximal plant growth. When the pH of soil is from 5-7 the growth is maximal. Thus some plants prefer to grow in alkaline soils where as other species grow best in acidic soils. There are several ways in which pH affects growth and uptake of solutes. For instance pH influences the ionic charge and affects the phosphate uptake.
The low availability of iron and other elements may be due to slow growth of plants at a high pH. When pH is low hydrogen ions usually decrease the cations absorption whereas anion absorption is enhanced.
4. Aeration:
Roots must have sufficient quantity of oxygen available for absorbing solutes. Plants growing in waterlogged soils have slow growth since aeration is poor. There is also a possibility of high CO2 level about the roots. Roots aerated with oxygen show increased absorption of potassium.
5. Nutrients Level of Plant:
If plant roots have enough amounts of specific solutes then absorption of that solute is comparatively slow as compared with the roots which are deficient. The level of elements affects the uptake of solutes mechanisms. There are also reports available where abundance of one ion influences the uptake of another.
Thus when the level of nitrogen was high plant was able to absorb more of sulphate and phosphate. When phosphate was high then the absorption of nitrogen was also stimulated. Apparently some interaction between nitrogen and phosphate and nitrogen and sulphate occurs inside the plant.
6. Growth:
Rapidly dividing and growing cells accumulate more of ions. On the other hand mature cells accumulate fewer amounts of ions. In the young cells the proteins provide a binding site for the several cations whereas other compounds are rapidly changed in order to become a part of protoplasm. A specific relationship definitely exists between the age of cell and solute uptake.
Note # 9. Plants as Indicators of Mineral Nutrients:
Plants are used in prospecting minerals in three ways mapping the distribution of indicator species by studying physiological and morphological changes in plants growing on high mineral soils; and by chemical analysis of the metal content of the plant. Members of Caryophyllaceae and Labiateae and some mosses grow on copper rich soils.
In Wales (UK) copper depositions were found where plants like Armeriamaritima, Polycarpeaspirostylis, Bulbostylisbarbata, etc., were growing. The ‘copper mosses’ led to the discovery of copper deposits in Sweden. High zinc bearing soils have been found where plants of Violoalutea, Armeriahalleri, etc., were growing.
Crotalaria cobalticola and Silenecobalticola are indicators of cobalt. Polygala baumii and Mechoviagrandiflora inhabit soils which are rich in manganese. In Russia Juncus, Salsolanitraria were used as indicators of soils rich in boron. Similarly Yucca species are reliable indicators of granite and silica.
Artemrsia indicates the occurrence of sand deposits. Same is indicated by the distribution of some members of Liliaceae and Polygonaceae which can tolerate high sulphur contents in the soils. Some plants are indicators of gypsum. While exploring oil in the Caspian sea halophytes like Salsolanitraria were used to locate salt domes.
The absence of vegetation is also used as an indicator in the exploration of minerals. For instance, in Congo soils rich in copper (8-14%) show no tree growth. The quality of particular mineral is also related with changes in the morphology of the species.
Thus when zinc is present in high percentage the petals of Papaver macrostomumbecome incised. The dwarfness of the species and a change in its height or form is also related to a mineral gradient concentration.
In summary, plant analyses have also helped in the discovery of zinc, tin, tungsten, arsenic, copper and vanadium. Further amount of metal content in the plant has helped in providing indications on its distribution in the soil e.g., manganese, chromium, cobalt and molybdenum.
Note # 10. Physiology of Foliar Nutrition in Plants:
In recent year’s variety of chemicals in solution form are directly sprayed on the leaves of the crop plants since they are readily absorbed and utilized more efficiently. Based on this observation mineral nutrients are also supplied to the plants as foliar sprays.
This practise has several advantages; it reaches the desired tissues quickly, the growth and development are accelerated, the process is highly advantageous in crop plants which are tolerant to aerial spraying.
This technique is of immense use where plants have thick cuticle or waxy cuticle layer. Several instances may be mentioned where foliar sprays have been employed with advantage to overcome deficiency of some micronutrients. It is also possible to supply macronutrients during critical growth periods when it is not possible to supply fertilizers in the soil.
In plants where it takes long time for the applied nutrients to reach the place of its need this method is also useful. This is particularly true of annual crops which are growing fast in specific periods. It is also an economical means of fertilizing some crops. There could be lot of saving both on the material and finances especially when u is desired to use micronutrients.
Several companies are manufacturing products which employ a particular formulation of growth substances, enzymes, amino acids chelated micronutrients (Zn, Fe, Mn, Bo, Mo) complex combined with ethoxylatesiloxane derivatives that increase nutrients and water retention. These products are available in several combinations and are produced by different companies.
They are soluble in water and are sprayed on the foliage to provide an effective method of applying nutritional support to plants. The complexes of this kind are especially useful when responses are demanded within short growing seasons.
The foliar penetration of the nutrients begins with the absorption of bio-chemicals through the leaf trichomes during early growth periods when the waxy cuticle concentration is low and non-limiting. Radioisotopic studies have revealed that micronutrients supplied as foliar sprays to trees, potatoes, corn, barley, rice and wheat were well absorbed and translocated to different organs of the plant.
Also when the soil application of these micronutrients was compared with the foliar sprays, a less time lag with foliar feeding system was observed. The penetration of the micronutrients may also occur through stomata or even diffuse through cuticle.
Sometimes there are wounds, breaks, sutures in the leaves and these micronutrients may enter through them. The role of ectodesmata in the translocation of micronutrients applied as foliar sprays also seems feasible. Foliar uptake may also be accompanied by leaching of some nutrients also. Once leached they may be reabsorbed by the roots and used by the plant.
The effect of foliar spray treatments of low doses of phosphorus has been shown in several crop plants. The studies were conducted on cereals, oil-seed plants and vegetables, etc.
From these investigations it becomes apparent that considerable quantities of phosphaticfertiliser could be saved by spraying the nutrient at the flowering or juvenile stage. Some authors have added nitrogen to the phosphatic spray and when sprayed on the leaves of wheat, protein content of the grain increases.
Note # 11. Physiology of Photosynthesis:
Synthesis of compounds with the aid of radiant energy especially in plants are known as photosynthesis.
1779―Jan Ingen-Housz concluded that O2 production occurred within few hours of photosynthesis and secondly it was involved during the day time. Plants which were green only used CO2 of the atmosphere.
He also proposed the following equation:
1782― Jean Senebier demonstrated the essentiality of CO2 to produce O2 by the green plants.
1804―Nicholas Theodore de Saussure showed importance of water in photosynthesis. Light was required for the evolution of O2.
1837―Dutrochet demonstrated the importance of chlorophyll in photosynthesis.
1842―Mayer described sun as the source of energy.
1845― Liebig indicated the involvement of CO2 in the formation of organic compounds.
1864― Sachs reported formation of carbohydrates in photosynthesis. He used half- leaf experiment for this.
1905― Blackman proposed Law of Limiting factors.
1923― Warburg used Chlorella as a basic system for studying photosynthesis.
1924― CB van Niel showed that some bacteria use H3S instead of H2O for photosynthesis.
1939― Robert Hill performed first experiment to demonstrate that separate light reaction was localized in chloroplasts.
1940-50― Melvin Calvin and Andrew Benson unravelled the outlines of CO2 fixation into carbohydrates in photosynthesis. Calvin used 14CO2 in his experiments.
1941― S. Ruben and M. Kamen used 18O2 to confirm that O2 produced during photosynthesis was derived from H2O.
1950― Ochoa and Vishniac showed that NADP+ could substitute as the hydrogen acceptor in the Hill reaction.
1954―Kortschak described the formation of C4dicarboxylic acids.
1954―Daniel Arnon made discovery that light and of dark reactions photosynthesis were separable.
1957― Emerson and his associates discovered Emerson effect.
1960― Woodward confirmed structure of the chlorophyll-α and synthesized it in the laboratory.
1960― Hill and Bendall showed two-step electron in transport system-photosynthesis.
1962― Fujita and Hattori investigated the effect of the various factors onformation of phycobiliproteins in algae.
1962― Jagendorf introduced the concept of two-state phosphorylation.
1966― Hatch and Slack extended the work of Kortschak.
1967― Arnon proposed two-photosystem scheme of photosynthesis.
1968-69― Laetsch described ultrastructure of bundlesheath and mesophyllchloroplasts.
1973― Rouhani and his associates proposed pathways in CAM plant Sedum.
1980― GJ Lorimer showed that rubisco uses CO2 as a substrate and activator.
Note # 12. Photorespiration and Plants:
Under normal atmospheric conditions, when the O2 is 21%, the rate of photorespiration in Helianthus. Leaves is nearly 17% of gross photosynthesis. Every photorespired CO2 needs an input of two O2 molecules. This implies that the true rate of oxygenation nearly 34% and the ratio of carboxylation to oxygenation is about 3 to 1. The ratio of oxygenation and carboxylation depends on the relative levels of O2 and CO2 since both gases compete to bind to the active site on Rubisco. When the O2 level declines, the relative level of carboxylation increases till at zer O2, photorespiration is also zero. Contrarily, increase in the relative levels of O2 or decreases in CO2 shifts the balance towards oxygenation, Enhancement in oxemperature also favours oxygenation due to decrease in solubility of gases though oxygen solubility is less affected than carbon dioxide. Therefore oxygen will inhibit photosynthesis.
Measured by net CO2 Reduction in Plants with Photorespiration:
With photorespiration energy costs are also associated and same is true of glycolate pathway. Thus, the energy expended (ATP, NADPH) in glycolate pathway following oxygenation is nearly equal to that expended for the reduction of one CO2 during the PCR cycle. There is net loss of carbon. Photorespiration apparently is expensive and inefficient process with regards to energy and carbon. It is reasonable to pose a question regarding existence of this wasteful process in plants.
The answer to this question is highly complicated and several ideas have been advanced. One view is that oxygenase function of Rubisco is inescapable since it evolved during period of earth when CO2 level in the atmosphere was high and oxygen was low. Both oxygen and CO2 react with the enzyme at the same active site and oxygenation needs activation by CO2 and same is true of reverse.
It is suggested that photosynthesis led to accumulation of oxygen in the atmosphere, but high built up of the oxygen was not accompanied by the loss of bifunctionality of the Rubisco. So to say C3 plants created problems for themselves-generating oxygen which was a competent inhibitor of carbon reduction. Thus, oxygenase part of Rubisco is an hangover of the past but has no role.
On the other hand, inefficiencies due to photorespiration if any, are not severe. Some workers believe that plants have turned photorespiration to its metabolic advantage. Thus glycolate pathway acts as a scavenger. For two turns of the cycle, two molecules of phosphoglycolate are formed by oxygenation. Of the four carbon atoms, one is lost as CO2 and three are returned to the chloroplast.
Glycolate pathway thus recovers 75% of the carbon that could be lost through glycolate pathway, otherwise. Amino acids like serine, glycine are also produced and are used in other biosynthetic pathways. The third groups of workers suggest that photorespiration acts as a safety valve in circumstances that need dissipation of excess excitation energy. In some situations, in the absence of O2 and CO2, normal light, the rate of photosynthesis declines. Photorespiration could save the leaf in situations when excess of oxygen is present.
The CO2 generated through photorespiration protects the plants from photo-oxidation damage by operating electron transport chain continuously. In situations when the plant has high light and low carbon dioxide, this could be of immense ecological significance. In conditions when the plant experiences water stress and the moisture in the surroundings is low, stomata close and this could be a protective measure.
Continued attempts are being made to suppress/inhibit photorespiration through the use of chemicals or genetically, to increase yield. Efforts are also on to identify species with Rubisco having low affinity for oxygen. To date such efforts have not yielded substantial success in this direction.
Obviously success in increasing photosynthesis and improving crop productivity has to be sought in other ways. Thus, a mechanism of concentrating CO2 in the photosynthetic cells could be a way to suppress photorespiration losses and improve the overall efficiency of carbon assimilation.
Note # 13. Physiology of Insectivorous Plants:
Several of the plant species inhabit marshy and swampy soils which are deficient in nitrogen. They are otherwise autotrophic but are dependent upon the nitrogen requirement through several insects. They break animal proteins and use the nitrogen rich compounds thus produced.
In tissue culture studies conducted recently in Utricularia by Mohan Ram and his students it has been proved that these insectivorous plants do not necessarily require animal proteins as source of nitrogen.
Several of the insectivorous plants are known and these include Drosera, Dionaea, Nepenthes, Utricularia sp. In all these plants the leaves or their parts are diversely modified to prey upon the different types of insects and trap them. Through the enzyme(s) action these insects are decomposed and used as nitrogen source. The products of digestion are absorbed by the plant.
These plants possess several specialized trichomes which increase the ability of the plant to trap insects. These trichomes possess proteolytic enzymes and possibly were evolved from hydathodes. In the leaves of Pinguicula two types of glands are present (Fig. 16-1) and these are either sessile or stalked. The stalked glands have a large basal cell, stalk and columellar cell with a head of 10-13 cells.
On the head drop of mucilage oozes out which helps in the capture of the insect. On the other hand the sessile glands have basal and columellar cells bearing a distinct head of 2-8 cells. The lateral walls of columellar cell is cutinized and thus resembles endodermis. The mature secretory cells of all the insectivorous plants have wall ingrowths resembling transfer cells.
These are well developed on the radial walls of both the glands. The heads of the trichomes have no well-developed cuticle and therefore secretion can easily pass through it. The glandular trichomes show high activity of several hydrolases including esterase, acid phosphatase and ribonuclease.
It has been shown that the chief site of activity of these enzymes was in the anticlinal or radial walls of the head cells. It is suggested that enzymes are moved in the radial walls and then through a poorly developed cuticle through the pores outside. If the sessile glands of Pinguicula are stimulated secretion begins within one hour and within two to three hours it oozes out.
This secretion has a detergent characteristic and possibly wets the exoskeleton of the insect. The glands of insectivorous plants are interesting since in them the secretion products move out of the cuticle and the digested products move in the glands simultaneously.
In Drosophyllum also both types of glands are present. The mucilage is, however, secreted by the outer stalked or tentacle glands. These glands are rich in dictyosomes. In fact during the period of secretion these glands abound in dictyosomes.
The mucilage formation is respiration dependent and temperature also affects the rate of secretion. The closure of the leaf is shown to be brought about by electrical signals which originate in the multicellular sensory hairs. In Drosera the large tentacles on the leaf may be regarded as large stalked glands which are not wholly epidermal in origin.
These glands have multicellular stalks and have a bundle of tracheids and a head with 2-3 cells. The cuticle of these cells is perforated. Plasmodesmata are present between different cells in the head. Transport of calcium has been traced in these glands. Through series of experiments it has been demonstrated that digestive material can pass to the inner side against the direction of flow of mucilage and digestive enzymes.
The situation in pitcher plant (Nepenthes sp.) is very interesting. The pitchers in this plant possess different kinds of glands. These glands are similar in structure but differ in location and function. There are ‘alluring glands’ which are multicellular and are present on the under surface of the lid of the pitcher. These glands secrete nectar. On the inner wall of the pitcher several digestive glands are distributed.
These glands have digestive and absorptive functions. They are more in distribution towards the base of the pitcher. These glands also bring about a short distance transport of the material from the pitcher cavity to the tissue. These glands also secrete several hydrolases like acid phosphatase, esterase and ribonuclease, etc. The pitchers may have as much as one litre of digestive fluid.
In Utricularia the leaves are modified into bladder-like traps and trap the insects in several ways. On the outer side of the bladders there are sessile and stalked glands which secrete mucilage and nectar in order to attract insects.
The inner wall of the bladder is lined with sessile glands which extract water from the contents and cause tension in the bladder. Consequently water stream alone with the insects is moved in. The door of the bladder is guarded by hair which open towards the inner side. The trapped insects are not allowed to escape.
Cuscuta (Cuscutaceae) species are among the best known angiosperm parasites and about 180 species are reported in the genus. The parasite joins the host plant through a wedge-shaped physiological bridge called haustorium.
In recent years development, ultrastructure and physiology of haustorium have attracted much attention. Recent studies have shown that from the distal end of the haustorium hyphae like structures develop which penetrate the individual cells the host cortex or pith.
These are search or contact hyphae. Of the two types, search hyphae grow inter and intracellularly. Tripodi (1967) has studied details of the haustorium and also changes in the host cytoplasm through electron microscope. This author reported that Golgi vesicles increased in the host cytoplasm.
Dorr (1969) has reported several plasmodesmatal links between the protoplasts of host and parasites. She has also reported hand like furrow at the tip of the hypha where it attaches to host’s sieve tube membrane. Cuscuta is unique in developing a highly specialized contact with the host phloem. Recent studies by Malik and Komal (1979) have shown that formation of haustoria is sequential and is regulated by cellular relations within the host.
Haustoria fail to penetrate the secondary tissues, especially those which are lignified. There are four stages in the developmental sequence of the Cuscutahaustorium and these are dependent upon at least two conditions.
The first is the initiation of haustorialprimordium within the prehaustorium and the second is the physical and chemical environment existing in the immediate surroundings of developing haustorium. The haustorialprimordium has a programmed set of requirements and these must be met with to affect hasutorium formation.
The initiation of the haustorium does not require the presence of host and that haustorial extensions and basal xylem formation can be induced by specific chemical stimulants or even physcialcondtions. Wolswinkel (1979) has reviewed the transport of assimilates and minerals and the role of phloems unloading in the parasitic relationships.
Though Cuscuta is traditionally regarded as classical example of total parasite which is unable to grow autotrophically, some reports are available in recent years which suggest the possibility of photosynthesis by the Cuscuta vines.
In a recent review paper Malik and Singh (1980) have discussed the biochemical aspects of parasitism by Cuscuta and discussed the available literature on the physiology of host-parasite relationships. In the haustorium diverse types of digestive enzymes are reported. Most of these hydrolases are not excreted enmasse into the host tissues.
Where host-parasite tissues did not contact no hydrolases were localized. Assumingly haustorial cells contained lysosomes and the latter fused with the cell membranes and released their contents into the host cells. Hydrolases act upon the host cells plasmalemma and destroy its semi-permeability attributes.
As a result its turgor pressure is disturbed. When followed by the mechanical force of the haustorium, these host cells get distrupted and are crushed due to the pressure and hence their cytoplasm is released. The space thus made is occupied by the tip of the haustorium which subsequently grows within the host tissues.
Apparently both enzymatic and mechanical processes were involved in facilitating the entry of the haustorium in the host tissues. Major function of the cells in the tip of the haustorium is to cause digestion and disruption of the host cells. Thus weakening of surrounding cells enzymatically appears to be an essential precondition for the entry and pentration of the haustoria.
There are several semi-parasites like Loranthus, Viscum, Arceuthobium, etc. Most of these species grow on the tree branches and possess green leaves. Viscum species have a dichotomously branched shoot bearing green leaves. Each branch produces a wedge shaped haustorium which penetrates the host tissues.
There are also secondary haustoria which establish contact with the xylem of the host. It draws water and minerals from the host tissue. Haustoria are also regarded as sucking roots. Loranthus species are also referred to as Dandropthae sp.
They are semi-parasites growing on the branches of mango, dalbergia, fig and other species. Here also primary haustoria penetrate the host tissue and obtain water and mineral supply whereas leaves of the parasite are green and manufacture food material themselves.
In Punjab and United Province one of the most prominent root parasites is Orobanchecernui and other species. This plant infests the roots of Brassica and Solanaceous members. The tall shoots arise from the roots and bear pink flowers and white scaly leaves. Similarly species of Rafflesia are also specialized total root parasites. Members of family Santalaceae are partial root parasites.
Note # 14. Phenomena of Growth in Plants:
The dictionary meanings of growth are many the advancement towards or attainment of full size or maturity; development; a gradual increase in size, etc. In plant physiology arbitrarily we separate the concepts of growth and development, using the term growth to denote increase in size, thus not considering any qualitative concepts such as full size or maturity which are clearly unrelated to the process of increase.
The first requirement for studying growth is a suitable method of measuring it. Growth can be measured as an increase in length, width or area; often it is measured as an increase in volume, mass or weight (either fresh or dry weight). Each of these parameters describe something different and in a growing organism, there is rarely a simple relationship among them. The simplest definition of growth is an increase in size.
Since size is a synonym for volume, this means that:
Growth mark = Vt – Vo
and growth rate = Vt – Vo/t
where Vt = volume at end of time t and
V0 = original volume at time zero.
The ideal method of measuring growth would, therefore, be to determine the volume of the plant part, but this is usually difficult to do with any degree of accuracy.
In case of growing leaves the area is more readily determined:
Growth = St − So
where St = surface area at end of time t
So = original surface area.
In case of stems and roots, a simple measurement is length:
Growth = Lt— Lo
Where Lt = length at end of time t
Lo = original length,
The easiest and the most accurate measurement, however, is usually the weight of the plant or plant part and hence it is represented as:
Growth = Wt− Wo
where Wt = weight at end of time t
Wo= original weight.
The measurement of the weight may lead to some complicating factors. It must be decided whether fresh or dry matter is a more correct measurement of growth; for example, during seed germination there is an initial uptake of water which is not accompanied by any significant growth as we normally consider it.
There is an increase in fresh weight as well as volume but not in dry matter. Subsequently the seedling increases enormously in length but there is a net decrease in dry matter. Of course, growth by any reasonable definition has occurred.
On the other hand during mid-winter, when the tree has stopped growing, there is a slow increase in the dry matter because of accumulation of reserve food materials. Dry matter can, therefore, be used as a parameter for the measurement of growth only when it does not include any significant alterations in plant reserve food materials.
It is a satisfactory measure of growth of seedling only if the endosperm or cotyledonary leaves are not included. In other words, weight can only be used as a measure of growth if W < V.
There seems to be no acceptable solution to this problem presently because growth, development and simple changes in size frequently overlap and can occur separately or together in any combination.
Some of the physiologists have suggested that the best definition of growth is the self- multiplication of the protoplasm, living material itself. The germinating seeds convert much of their stored food materials into more functional compounds within the protoplasm of the growing and newly formed cells. Although this definition appears to be appealing yet it is not very practical. The problem arises while considering how one could measure the functional protoplasm in a plant.
Thus, perhaps, if we sampled the organism, count the cells, and try to estimate the rate of cellular increase this would provide more meaningful definition. Since the growth of a plant is initiated in the meristems, the cellular changes in these tissues might throw some light on the process. The three main changes in the complete development of a cell are cell division, cell elongation and cell differentiation.
The growth of a plant part may result from either cell division or cell enlargement. However, it must be realized that even growth resulting from cell division is possible only because the daughter cells enlarge to the size of the mother cell before they divide again. Cell enlargement must not include any simple turgor-induced changes in cell volume if it is a true cell growth.
Consequently, the growth of a plant or plant part is an increase in size caused by cell enlargement when measured by cell constant positive cell turgor. In the simplest instances of cell growth there is an uptake of water, producing a turgor pressure to overcome the force of attraction between the particles in the cell wall.
Consequently, wall stretches to become thin and the osmotic pressure drops because of water uptake which dilutes the cell content. The consequent increase in cells water potential soon brings end of water uptake (and, therefore, growth). Under normal conditions, cells are capable to increase in size more than 15-folds.
In these cells, as water enters during enlargement solutes are also absorbed, and cell maintains a sufficient turgor pressure to continue stretching its wall. Besides absorption of water and solute, other changes occur during cell growth.
The new wall material must be laid down due to an insertion of new microfibrils between the old wall components that have become separated, i.e., wall thick, opposition (a deposit of new material on the inner surface of the wall) occurs during cell maturation.
Biochemical studies have revealed that cell wall formation is accompanied by many biochemical changes which include synthesis of proteins, nucleic acids, phospholipids, multiplication of organelles and utilisation of energy as ATP, etc.
The concept of growth in cellular terms can be summarized as follows:
Kinetics of Growth:
Many investigators have measured size of an organism and have plotted the data as a function of time. Growth of a plant or plant part characteristically passes through stages represented by an S- shaped curve. Size versus time plot is the most direct way of handling growth data.
We can also plot the longrithum of the size as a function of time. In a size plot the units are length, volume or weight but in the rate curve (Fig. 19-1) the units are length, volume or weight per unit time (Fig. 19-2).
Growth curves plotted in any of the two ways have two clearly distinguishable phases (Fig. 19-2). The first is rapid or exponential (a) phase during which the rate curve as well as size curve is increasing. In the last phase, the size continues to increase but more slowly so that the rate decreases (c).
The first phase has been called logarithmatic or exponential phase and the last phase is referred to as senescence or decreasing phase. In many examples of growth curve, another phase is also clearly evident. This is a phase of minimum growth rate or the linear phase (called by Sachs, the grand period of growth), and it lies between the logarithmatic phase and the senescence phases.
Blackman (1919) put forward the idea that growth of a plant could be represented by the equation
Wt = Wo.ert
where W1 is the final size (weight, height, etc. after time t), Wo is the initial size at the beginning of the time period, r is the rate at which plant substance is laid down during time t, and e is the base of natural logarithms. It should be noted that r is the relative growth rate as discussed above.
Black-man pointed out that this equation also describes the manner in which money placed at compound interest increases with time, and the term compound interest law is used to describe such phenomena. Banks usually apply compound interest quarterly or annually so that the increase in amount occurs at a jump. With plants or other biological systems, compound interest is applied continuously, and size increases as a smooth curve.
From the same equation, it is also seen that the size of an organism (Wt) depends on the initial size (Wo). It has been shown that seedling growth in seeds of different sizes follows such an expectation, with the large seeds giving a large plant. This is true, however, only during the early stages of growth, and if t is large, then the final size of plants from small and/or large seeds frequently are equal.
From this equation it is seen that plant size also depends on the magnitude of r, the relative growth rate. Blackman suggested that r might be used as a measure of the ability of a plant to produce new plant material and called r the efficiency index. Plants with a high efficiency index could be expected to perform better than those with a low efficiency index.
While r does differ among plant species, it is not constant during the life of aslant. Furthermore, all parts of a plant are not equally involved in synthesizing new plant substances; some of the material goes for storage or is catabolized. The usefulness of the efficiency index as a predictive factor in evaluating performance or yield is limited.
Note # 15. Plant Development:
Plant development comprises a series of events. Some are repetitive whereas others occur only once in the life of the plant. These include flowering or seed germination. Divergent types of controls are operative during the initiation of these events. In the chapter on Physiology of Flowering we shall discuss the role of day length in controlling flowering in plants.
Plant seems to possess the potentiality to measure the length of light and thus flowering occurs at the proper time and season of the year as determined by the night length, specific to that season. Some plants must experience cold treatment before they can flower.
This process checks premature flowering in such species. The possible mechanism through which a plant is able to perceive, measure and react to light and temperature.
Similarly processes like seed germination and leaf abscission are regulated by external factors. For instance seed germination is dependent upon water absorption and provision of proper physiological conditions whereas leaf abscission is mostly influenced by physiological and environmental factors.
Plant behaviour in terms of its development is rhythmic. Thus some flowers open in the morning while others do so at night. Similar aspects are exhibited by the leaves. Photosynthesis and respiration and flowering are rhythmic changes which are dependent upon alternation of day and night.
Plants have a built in clock which can measure time and rhythm. Though several models have been advanced yet the nature and mode of regulation of plant behaviour remains conjunctural.
Note # 16. Growth Hormones in Plants:
In intact plants endogenous hormones must move from the site of synthesis to several other locations where they direct growth, differentiation and morphogenesis. Understanding the direction and pattern of their movement is highly important in elucidating their mechanism of control. Most information is based on exogenous applications and then tracing them in different parts of the plant.
Auxins:
Using isolated sections as a system one can estimate uptake, distribution and movement of auxin. Uptake is measured by comparing the loss and gain of auxin by the gainer and the donor. Because of the complexity of the system it is difficult to infer that uptake from the donor is equivalent of uptake by a polar transport system.
Uptake due to diffusion, utilization, etc. is difficult to ascertain presently. The available data show that transport is polar because growth causes a sink for auxin. Auxin can also pass through cytoplasm and pass a