An Introduction to Plant Physiology and Its Importance in Agriculture

Posted on Aug 1, 2024 in Biology

Introduction to Plant Physiology and Its Importance in Agriculture

Dr. J. Pradhan

Plant Physiology

Plant physiology is the study of vital phenomena in plants. It is the science concerned with processes and functions, the responses of plants to their environment, and the growth and development resulting from these responses. Plant physiology helps us understand various biological processes in plants, such as:

• Photosynthesis
• Respiration
• Transpiration
• Translocation
• Nutrient uptake
• Plant growth regulation through hormones

These processes have a profound impact on crop yield.

Crop Definition

A crop is a group of plants grown as a community in a specific locality for a specific purpose.

Crop Physiology

Crop physiology is the study of how plant physiological processes are integrated to cause whole-plant responses in communities.

Importance of Crop/Plant Physiology in Agriculture

Understanding the physiological aspects of:

• Seed germination
• Seedling growth
• Crop establishment
• Vegetative development
• Flowering
• Fruit and seed setting
• Crop maturity
• Plant hormone interaction
• Nutrient physiology
• Stress (biotic/abiotic) physiology

…provides a reasonable scientific basis for effectively monitoring and beneficially manipulating these phenomena.

Plant physiology provides a platform for achieving better crop yields because, in agriculture, we are interested in economic yield, the output of these phenomena, and the well-being of plants. Studying these phenomena to develop better crop management practices forms the subject matter of crop physiology.

Examples of the Importance of Physiology in Agriculture and Horticulture

1. Seed Physiology

• Seed is the most important input in agriculture.
• Seed germination and proper seedling establishment depend on various internal and external factors.
• Knowledge of seed physiology helps in understanding the different physiological and morphological changes during germination.
• Any deviation in these processes causes seed dormancy. Seed dormancy prevents the immediate use of harvested seed for the next crop, which is important in intensive agriculture.
• By understanding the causes and effects of seed dormancy, crop physiologists have developed different methods of breaking it. Example: When paddy (rice) is used as seed material in the very next season, treating the seed with HNO₃ or GA is recommended.

2. Optimum Seedling Growth and Plant Population

• Knowing the process of radicle and plumule emergence and their function helps achieve the best plant health.
• Knowing the different input requirements of plants (water, nutrients, sunlight) allows for managing plant populations effectively to obtain the highest yield.
• The interaction of inputs within a plant is a matter of plant physiology.

3. Growth Measurement of Crops

• The first prerequisite for higher yields in crops is high total dry matter production per unit area.
• High dry matter production is a function of optimum leaf area (Optimum Leaf Area Index) and Net Assimilation Rate. (CGR = LAI X NAR).
• Example: Pruning in horticultural crops like mango is done based on this principle of proper canopy management for better photosynthesis.

4. Harvest Index

• The difference between the total amount of dry matter produced and the photosynthates used in respiration is the net product of photosynthesis.
• Economic yield depends on how the dry matter is distributed among different plant organs.
• The partitioning of total dry matter among major plant organs is of interest to farmers as they are more interested in its allocation towards economic yield.
• Example: An excessively long vegetative growth period in groundnut (peanut) produces fewer pods as the reproductive period is constricted. Thus, groundnut varieties with a relatively extended period of reproductive growth are desirable.

5. Mode of Action of Different Weedicides

• The use of herbicides to kill unwanted plants is widespread in modern agriculture.
• The majority of herbicides—about half of the commercially important compounds—act by interrupting photosynthetic electron flow (e.g., Paraquat, Diuron) or the electron flow of respiration.
• In photosynthesis, when electron transport is blocked, it virtually stops the light reaction. When the light reaction is stopped, the dark reaction does not happen, and thus CO₂ is not fixed as carbohydrate. Therefore, the weed is killed by starvation.

6. Nutrient Physiology

• Nutrient physiology is another important area for understanding crop physiology.
• Around 17 essential elements are required for the healthy growth of a crop.
• Knowledge of nutrient physiology has helped in identifying essential nutrients, ion uptake mechanisms, their deficiency symptoms, and corrective measures.
• It also helps to check the toxicity symptoms of various nutrients.
• The use of fertilizers and their intake by plants can be fully understood by studying plant physiology.

7. Photoperiodism

• Photoperiodism is the response of a plant to the relative length of day and night.
• This concept was used to choose photo-insensitive varieties.
• The semi-dwarf rice varieties that have revolutionized Indian agriculture are lodging resistant, fertilizer responsive, high-yielding, and photo-insensitive. Photo-insensitivity has allowed rice cultivation in non-traditional areas like Punjab. Even in traditional areas, rice-wheat rotation has become possible only due to these varieties.

8. Plant Growth Regulators

• Plants can regulate their growth through internal mechanisms involving the action of extremely low concentrations of chemical substances called plant growth substances, phytohormones, or plant growth regulators.
• 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.

9. Drought Resistance

• Indian agriculture is predominantly rainfed; therefore, developing drought-resistant varieties is very important.
• Root zone depth, root density, plant water potential, relative water content, water use efficiency, and xerophytic characters of leaves are some characteristics that have helped breed drought-tolerant varieties and develop efficient irrigation management practices (sprinkler and drip irrigation).

10. Post-Harvest Physiology

• Post-harvest losses of agriculture and horticulture products cause great distress to the farming community.
• Moisture and temperature are the two important factors causing physiological changes that reduce the post-harvest quality of grains.
• Controlling moisture content and maintaining low temperatures have proven effective in storing grains.
• Being perishable, the magnitude of post-harvest loss is comparatively higher in horticultural crops.
• Example: In recent years, ‘modified atmospheric storage’ has been developed to prolong the post-harvest life of fruits and vegetables. The shelf life of cut flowers can be increased by applying kinetin (cytokinin), which reduces the burst of ethylene and thus reduces the rate of senescence.

Photosynthesis

Introduction

• All life on Earth is solar-powered.
• Plants trap solar energy.

Basic Overview

• Photosynthesis is a physiological process in which carbohydrates are synthesized by the aerial green parts of plants by trapping solar energy, taking CO₂ from the atmosphere, and water from the soil.
• Green plants trap solar energy using their photosynthetic pigments.
• Solar energy is converted into chemical energy and stored in high-energy bonds.
• Photosynthesis is an oxidation-reduction process where water is oxidized to oxygen, and carbon dioxide is reduced to glucose (carbohydrate).

CO₂

• About 90% of all photosynthesis is carried out by marine algae.
• The ratio of the volume of oxygen liberated in photosynthesis to the oxygen used in respiration during the day is 10:1.

Historical Background

The first ideas about light being used in photosynthesis were proposed by Jan Ingenhousz in 1779, who recognized that sunlight falling on plants was required for photosynthesis, although Joseph Priestly had noted the production of oxygen without the association with light in 1772.

Priestly’s Experiment

1. Priestly kept a burning candle and a rat together in a single bell jar.
2. After some time, the candle extinguished, and the rat died.
3. For the second time, he kept a burning candle, a rat, and a green plant together in the bell jar.
4. He observed that neither the candle extinguished nor did the rat die.

Conclusion

• The candle releases something X (CO₂) taken up by the plant, and the plant releases something Y (O₂) that helps the candle burn.
• Joseph Priestly discovered that whatever animals or a burning candle release into the air, plants take it up and release something beneficial into the air.
• He found the role of air in photosynthesis.
• However, at that time, he could not discover that X is CO₂ and Y is O₂.
• Later, in 1774, he discovered O₂.

Priestly’s Hypothesis

Plants restore or give to the air whatever is removed or taken by animals and burning candles.

Jan Ingenhousz’s Experiment

1. First, he repeated Priestly’s experiment and stated that it is only possible in the presence of light. This means light is essential for photosynthesis.
2. Ingenhousz placed submerged plants in sunlight and then in the shade. He noticed that small bubbles were produced by the plants when they were in the sunlight. When transferred to the shade, these plants no longer produced bubbles. Ingenhousz later concluded that plants use light to produce oxygen.

Experiment to Test if Light is Essential for Photosynthesis

1. Only the portions of the leaf exposed to sunlight could photosynthesize and hence turned blue-black when tested with iodine.
2. The region covered by black paper on both sides could not photosynthesize, and hence the color did not appear.

Conclusion

This proves the importance of sunlight in photosynthesis, as it does not occur in the parts of leaves not exposed to sunlight.

T.W. Engelmann Experiment

Engelmann grew oxygenic bacteria on photosynthesizing algae. He then passed light through a prism, splitting it into the VIBGYOR spectrum before supplying it to the algae for photosynthesis. He saw that the oxygenic bacteria concentrated in the regions where red and blue light fell. This indicated that more oxygen is produced in these regions due to photosynthesis. Engelmann thus concluded that the red and blue regions of the visible spectrum are more effective for photosynthesis.

Then, in 1939, Robert Hill showed that isolated chloroplasts would produce oxygen but not fix CO₂, showing that the light and dark reactions occurred in different places. This led later to the discovery of photosystems I and II.

Structure of Chloroplast

• The chloroplast is a green-colored plastid.
• It is oval or biconvex, found within the mesophyll of the plant cell.
• The size of the chloroplast usually varies between 4-6 µm in diameter and 1-3 µm in thickness.
• It is a double-membrane organelle with outer, inner, and intermembrane spaces.
• Two distinct regions are present inside a chloroplast: the grana and the stroma.
  • Grana comprise stacks of disc-shaped structures known as thylakoids or lamellae. The grana contain chlorophyll pigments and are the functional units of the chloroplast.
  • Stroma is the homogenous matrix that contains grana and is similar to the cytoplasm in cells, in which all the organelles are embedded. Stroma also contains various enzymes, DNA, ribosomes, and other substances. Stroma lamellae function by connecting the stacks of thylakoid sacs or grana.

The chloroplast structure consists of the following parts:

• Membrane Envelope: Comprises inner and outer lipid bilayer membranes. The inner membrane separates the stroma from the intermembrane space.
• Intermembrane Space: The space between the inner and outer membranes.
• Thylakoid System: Suspended in the stroma. It is a collection of membranous sacs called thylakoids or lamellae. The green-colored pigments called chlorophyll are found in the thylakoid membranes. It is the site for the light-dependent reactions of photosynthesis. The thylakoids are arranged in stacks known as grana, and each granum contains around 10-20 thylakoids.
• Stroma: A colorless, alkaline, aqueous, protein-rich fluid present within the inner membrane of the chloroplast, surrounding the grana.
• Grana: Stacks of lamellae in plastids. These are the sites of the conversion of light energy into chemical energy.

Photosynthetic Pigments

Photosynthetic pigments are colored complex organic molecules in a biological system that absorb radiant energy in the visible range of the electromagnetic spectrum and convert it into chemical energy.

• Pigments absorb light due to conjugated double bonds, which impart color.

Chlorophyll a

• Widely distributed in O₂-evolving organisms.
• Empirical formula: C₅₅H₇₂MgN₄O₅
• Absorbs red and blue light.

Chlorophyll b

• Found in green algae, bryophytes, and angiosperms.
• Empirical formula: C₅₅H₇₀MgN₄O₆
• The CH₃ group of chlorophyll a is replaced by an aldehyde group in chlorophyll b.
• Also absorbs blue and red light.

Chlorophyll a and chlorophyll b reflect green light; hence, they appear green.

• Depending on the nature of absorption, chlorophyll a is of various kinds: Chl a 670-673, Chl a 680-683, P680, P700.
• Chlorophyll b is of two types: Chl b 640 and Chl b 650.
• Chlorophyll a and Chlorophyll b exhibit different absorption spectra.

Chlorophyll c

• Found in diatoms and brown algae.
• Absorbs blue and orange light.

Chlorophyll d

• Found in red algae.
• Absorbs blue light.

Structure of Chlorophyll

• Mg-Porphyrin structure
• Phytol chain
• Porphyrin head – hydrophilic
• Phytol tail – lipophilic

Structure of Xanthophylls

• In photosynthetic bacteria, bacteriochlorophyll a and b and chlorobium chlorophylls are found.
• Carotenoids are of two kinds: carotene and xanthophyll (carotenol).
• Carotene is orange and is represented by the empirical formula C₄₀H₅₆. The most abundant carotenes are α-carotene and β-carotene.
• Xanthophylls are oxygen-containing derivatives of carotene. The common xanthophylls are lutein, zeaxanthin, and neoxanthin. Xanthophylls are yellow.
• Carotenoids shield chlorophyll molecules against photo-oxidation and trap solar energy of short wavelengths of light, transferring it to chlorophyll molecules by inductive resonance transfer during photosynthesis.

Photo-Oxidation

Photo-oxidation or solarization is oxidizing cellular components, including chlorophyll molecules, in high-intensity light. During photosynthesis, chlorophyll molecules get excited and combine with molecular oxygen to form a chlorophyll-oxygen complex, inactivating and subsequently destroying the chlorophyll molecules.

• Photo-oxidation of chlorophyll molecules is averted by shielding pigments (carotenoids).
• Carotenoids (non-epoxy) bind to the chlorophyll-oxygen complex, forming an epoxide derivative and releasing chlorophyll molecules. The non-epoxy carotenoid itself gets oxidized.

Distribution of Photosynthetic Pigments

Photosynthetic pigments (chlorophylls and carotenoids) are distributed in the chloroplasts of eukaryotes and the chromatophores of prokaryotes. These pigments are not distributed throughout the chloroplasts but are restricted to thylakoids.

Structure of Thylakoids

• Thylakoids are disc-shaped membranous sacs enveloped by a unit membrane structure called the thylakoid membrane. The molecular organization of the thylakoid membrane is based on the fluid mosaic model of the membrane.
• The space enclosed by a thylakoid is called the lumen. The area between two thylakoids is called the partition. The end portion of each thylakoid facing the stroma is called the margin.
• Thylakoids are arranged to form stacks that look like piles of coins. Each stack is called a granum (singular: granum; plural: grana).
• Each granum consists of 10-100 thylakoids. In higher plants, thylakoids in a granum are closely packed, while in red algae, they are widely separated.
• There is no stacking of thylakoids in the chloroplasts of algae; hence, grana are not found there. Thylakoids are uniformly distributed throughout the chloroplast.
• Grana stacks are interconnected by membranous lamellae called stroma lamellae, which help transport various photosynthetic substrates.

Organization of Photosystems

Thylakoids have quantasomes on their inner membranes. Quantasomes are photosynthetic units. A photosynthetic unit comprises all the photosynthetic pigment molecules required to affect a photochemical act, the release of one molecule of oxygen. A quantasome has both pigment systems, PSI and PSII. Each photosystem comprises about 300 primary chlorophyll complexes called antenna chlorophyll and a reaction center. Antenna chlorophyll, along with carotenoids, is called antenna molecules. Antenna molecules gather light and transfer it to the reaction center by inductive resonance transfer.

Reaction Center

Reaction centers comprise protein and special chlorophyll molecules. In the reaction center, light energy is converted into chemical energy. Reaction centers are of the following types:

• P680: Chlorophyll a, which absorbs up to 680nm wavelength of light. It is the reaction center of photosystem II.
• P700: Chlorophyll a molecules that absorb 700nm wavelength of light. It is the reaction center of PSI in higher plants.
• P890: The reaction center of PSI of photosynthetic bacteria absorbs the 890nm wavelength of light.

The two photosystems were discovered by Emerson and Lewis in 1943 while working on the action spectrum for different pigments in Chlorella.

The two photosystems are connected by an electron transport system.

Light

• Light is a source of energy for the entire universe.
• Green plants trap this energy to carry out photosynthesis.
• Other photobiological processes that depend on light include chlorophyll synthesis, photoperiodism, phototropism, and vision in animals.
• Photosynthetic pigments absorb only the visible part of the broad electromagnetic spectrum. The visible part consists of seven colors: VIBGYOR (400nm – 700nm).
• Absorption of light is maximum in the violet-blue and red regions.

Absorption Spectrum

The absorption of light at different wavelengths by different pigments is called the absorption spectrum.

Action Spectrum

• The action spectrum indicates the overall rate of photosynthesis at each wavelength of light.
• Maximum photosynthesis occurs at the wavelengths absorbed by chlorophyll a. Hence, chlorophyll a is the most important and chief pigment, and the rest are accessory pigments.
• Chlorophyll a captures the maximum light and is the most abundant pigment.

Red Drop Effect or Emerson First Effect

• Discovered by Emerson.
• Emerson and coworkers performed an experiment and found the rate of photosynthesis in plants at different wavelengths.
• When monochromatic light of wavelengths greater than 680nm, say 700nm (far-red region light), is used, there is a sudden drop in the quantum yield of photosynthesis.
• Quantum Yield: The amount of O₂ released per quantum of light absorbed.
• It was proposed that only one photosystem operates at this wavelength.

Enhancement Effect or Emerson Second Effect

• Photosynthesis at 700nm – 10
• Photosynthesis at 653nm – 53
• Photosynthesis at 700nm and 653nm together – 72

When plants were exposed to 700nm and 653nm wavelengths simultaneously, the rate of photosynthesis was enhanced compared to individual exposure.

Conclusion

This enhancement is because two photosystems are present in plants. One operates at 700nm, and the other operates at (653-680nm). PSI – 700nm and PSII – 680nm.

Mechanisms of Light Reaction

• The light reaction is very fast.
• It is completed in 0.00001 seconds.
• The light reaction is not affected by temperature.
• The rate of the light reaction is regulated by the intensity and quality of light and pigments.

The light reaction can be divided into two phases:

1. Photo-oxidation phase
2. Photo-chemical phase

The photo-oxidation phase comprises the following events:

1. Absorption of light energy.
2. Transfer of light energy from accessory pigments to reaction centers.
3. Activation of the chlorophyll a molecule.

The photo-chemical phase comprises the following events:

1. Photolysis of water and oxygen evolution.
2. Electron transport and synthesis of assimilatory power.

Chemiosmotic Hypothesis

• Proposed by Peter Mitchell to understand the mechanism of ATP synthesis during photosynthesis and respiration.
• Phosphorylation: Synthesis of ATP; it is the addition of a phosphate group. ADP + iP — ATP
• Photophosphorylation: Formation of ATP during photosynthesis.
• Basis: Development of a proton gradient across the thylakoid membrane.

Light falls on PSII, electrons are photoexcited and ejected, accepted by the primary acceptor (Phaeophytin), and passed to PQ (a hydrogen carrier). PQ takes up an H⁺ ion from the stroma and is reduced to PQH₂, which transfers the electron to the cyt. b6f complex (the next electron carrier), and H⁺ is released into the lumen of the thylakoids. PQ is then oxidized back to PQ. From the cyt. b6f complex, the electron is transferred to PC, a blue-colored copper-containing pigment. Then the electron is transferred to PSI. Photoexcitation and ejection of electrons occur from PSI. The electron is accepted by the primary electron acceptor of PSI (FRS – FD – FNR), which accepts two electrons and two H⁺ (from the stroma) and transfers them to NADP⁺, which is reduced to form NADPH + H⁺.

Oxygen Evolving Complex

Associated with PSII, it catalyzes the oxidation or photolysis of water in the thylakoid lumen.

Photolysis of H₂O

:  Lumen of thylakoid . 2H2O → 4H+ + O2 + 4e–  Photolysis of water is only associated with PA II never with PS I. 3 reason for H+ gradient across the thylakoid membrane are: (higher concentration in lumen and lower concentration in stroma)  Breakdown of water in lumen .  Transport of H+ ion through electron transport chain.  Use of H+ in stroma to form NADPH from NADP. This pmf ( proton motive force ) or proton gradient acts as driving force for synthesis of ATP. F0- particle embedded in the thylakoid membrane act as a transmembrane channel which helps in the facilitated diffusion of protons across the membrane. F1– particle present towards the outer surface of membrane facing stroma ,ATP Synthase is present in F1 particle helps in ATP formation. Summary of light reaction : 1.It involves PS I and PSII 2. The electron expelled from P680 of PSII is transferred to PS I and hence it is a non cyclic electron transport. 3. In non cyclic electron transport, photolysis of water (Hill’s reaction and evolution of O2 ) takes place. 4. Phosphorylation (synthesis of ATP molecules) takes place at only one place. 5. The electron released during photolysis of water is transferred to PS II. 6. The hydrogen ions (H+) released from water are accepted by NADP and it becomes NADPH2 7. At the end of non cyclic electron transport, energy rich ATP, assimilatory power NADPH2 and oxygen from photolysis of water are observed 8. The ATP and NADPH2 are essential for the dark reaction wherein, reduction of CO2 to carbohydrate takes place.  It involves only PS I.  Occurs cyclic flow of electrons within photosystem.  ATP synthesis occurs.  Possible site : stroma lamellae  Does not involve splitting of water.  No oxygen evolution.  No formation of NADPH2 Thank you