Introduction to crop physiology

AGR-302 Introduction to Physiology of Crop Plants

AGRON Lectures

• Introduction
• Brief history
• Importance of Crop Physiology in Agriculture
  • Drought studies
  • Physiology in crop breeding programs
  • Seed Physiology
  • Seedling growth
  • Growth measurements
  • Harvest index
  • Weed control
  • Nutrition Physiology
  • Photoperiodism
  • Plant growth regulators
  • Post-harvest Physiology
• Limitations on the contribution of crop physiology
• How to overcome limitations in crop physiology

Introduction

Genetic potential of a plant and its interaction with environmental factors decides its growth and development by influencing or modifying certain internal processes. Plant physiology studies about these internal processes and their functional aspects.

Physiology

is the science of life. It is the study of all the processes happening in living organisms, from the basis of cell function at the ionic and molecular level to the integrated behavior of the whole body and the influence of the external environment. The processes involved in living organisms include such as respiration, excretion and in the case of plants, photosynthesis, transpiration etc.

Plant

is a living organism of the kind exemplified by trees, shrubs, herbs, grasses, ferns, and mosses, typically growing in a permanent site, absorbing water and inorganic substances through its roots, and synthesizing nutrients in its leaves by photosynthesis using the green pigment chlorophyll.

Plant physiology

encompasses the study of plant structure and function. Plants had to evolve methods to deal with the separation of CO₂ and water as they adapted to life on soil. Plant physiology examines the functions of various plant components, either separately or collectively. It is the science concerned with processes and functions, the responses of plants to environment and the growth and development that results from the responses. It helps to understand various biological processes of the plants like Photosynthesis, respiration, transpiration, translocation, nutrient uptake, plant growth regulation through hormones and such other processes which have profound impact on crop yield.

Crop

is a group of plants grown as a community in a specific locality and, for a specific purpose.

Crop physiology

is the study of the ways in which plant physiological processes are integrated to cause whole plant responses in communities. The subject matter of crop physiology includes the ways in which the knowledge of plant physiology is applied for better management of crops. Crop physiologists work on a whole plant which studies all its growth and problems, nutrient/ water uptake, air exchange, photosynthesis/ respiration and production and partitioning of different resources affecting growth. They deal with a plant in terms of knowledge from the different field such as soil science, plant physiology, botany etc. Their main aim is to increase the yield of crops economically.

Processes

are a collection of natural events or a sequence of events that occurred in order to accomplish a specific goal. Transpiration, respiration, photosynthesis, translocation etc are some examples of processes that occur in living plants.

Functions

are components in an organism that contributes to its fitness.

Both processes and functions are influenced by environmental factors such as light and temperature. The major task of plant physiology is to identify and demonstrate how processes and functions react to changes in the environment, as these two aspects are influenced by external factors. Plant physiology’s ultimate aim is to establish a thorough and systematic understanding of all natural phenomena that occur in living plants and, as a result, to comprehend the essence of plant growth, development, and productivity. More comprehensive plant physiology research may benefit many aspects of practical agriculture.

Brief history

W.L. Balls' work paved the way for crop physiology, which aimed to understand the dynamics of crop growth and yield. He studied the effects of plant spacing and sowing date on the growth and yield of Egyptian Cotton plants in crop stands, not isolated plants. The word crop physiology was coined as a result of his work. Since then, numerous scientists have begun to use advancements in physiological knowledge to improve crop management.

A rapid development of growth and yield analysis methods by numerous investigators began in England in \(1924\). They began explaining ‘the physiology of crop yield’ with the introduction of various methods of growth analysis. D.J. Watson created the definition of LAI (Leaf area index) in \(1947\). This index has sparked renewed interest in crop physiology by providing a more meaningful way of evaluating crop growth.

In the \(1950s\), the Infrared Gas Analysis (IRGA) method was introduced to study the photosynthetic rate of the leaf and the loss of photosynthates due to respiration. This approach has made estimating short-term photosynthesis and respiration rates by crops in the field much easier. With their definition of light interception coefficient, Monsi and Saeki demonstrated how light is intercepted by the crop canopy in \(1953\).

Later, several studies were conducted to better understand processes such as food material translocation, partitioning toward economic yield, storage mechanisms, flowering physiology, impact of stressful environmental factors on crop growth and production, and the role of plant growth regulators in growing crop productivity, among others. All of these fields have added to our understanding of physiological processes and their effect on crop yield.

Importance of Crop Physiology in Agriculture

Genetic potential of a plant and its interaction with environmental factors decides its growth and development by influencing or modifying certain internal processes. Plant physiology studies about these internal processes and their functional aspects. Many areas of agriculture could benefit from more rigorous plant physiology study, which could lead to practical solutions in agriculture. Crop yield is determined by the relationship of crop plants’ genetic ability and the ecosystem in which they emerge. Variations in the genotype and the environment, such as temperature and cultural patterns, regulate development through physiological processes. Plant physiological mechanisms are therefore the machinery that enables both genetic potentialities and the environment to work together to create the quantity and quality of growth or phenotype that we call yield.

Unfavorable environmental conditions like drought or temperature, cultural activities like fertilisation, an insect pest, or a plant pathogen may only impact yield by interfering with the plant’s physiological processes. Plant Physiology offers a forum for improving crop yield since we are interested in economic yield, which is the productivity of these phenomena and plant well-being in agriculture. Crop physiology is the study of these phenomena with the aim of developing better crop management practices.

The following examples illustrate the importance of physiology in agriculture:

Drought studies

Drought, in the sense of agriculture, is a prolonged period of insufficient precipitation that has a detrimental effect on crop growth or yield. Droughts are likely to become more frequent and severe in the coming future as the atmosphere warms. Drought reduces development because a lack of water in the plant allows stomata to close, reducing photosynthesis, turgor depletion, and cell enlargement. At the cellular and molecular level, more extreme stress can result in permanent damage. In order to improve yield stability in a changing environment, it will be necessary to recognize core physiological limitations to productivity under drought and pathways of crop resistance to water deficit stress. Furthermore, in order to adapt crops to episodic drought events in the future, restricted genetic variation among essential crop populations, as well as ecological productivity constraints, must be resolved. Plants’ ability to retain physiological functions at low plant water status and rapidly rebound once the tension is eliminated would be critical for maintaining long-term crop productivity during occasional droughts. In several crop species, the consequences of drought stress have been well-documented; however, studies on physiological responses to gradual drought and recovery after re-watering are scarce.

Physiology in crop breeding programs

Plant breeders and agricultural scientists in the \(21\)^st century are faced with the problem of increasing crop yield capacity in order to feed the world’s developing population. Climate change, its effects, and rising nutrient shortages are all considerations to remember when constructing new plant breeding pipelines. Modern genomics and genetic methods, in combination with developments in precision phenotyping and breeding methodologies, are supposed to help researchers better understand the genes and metabolic pathways that impart drought tolerance in plants. Molecular biology and genomics methods classify candidate genes and quantitative trait loci associated with these traits, while plant physiology enhances our understanding of the dynamic network of drought tolerance-related traits, enhancing selection performance.

Seed Physiology

Seed is the most significant agricultural input. Various internal and external factors affect seed germination and seedling establishment. Seed physiology awareness aids in comprehending the various physiological and morphological changes that occur during germination. Seed dormancy is induced by any deviation in these processes. Crop physiologists have devised various methods for breaking seed dormancy by gaining a better understanding of the causes and effects of the issue. Seed physiology is one of the most significant facets of seed genebank operations, since it necessitates an appreciation of seed efficiency during growth and maturation, seed dormancy and germination, and seed survival in target species storage. When paddy is used as a seed material, it is recommended that the seed be treated with either HNO₃ or gibberellic acid (GA) to break dormancy.

Pre-sowing priming triggers a complex physiological condition in seeds and has emerged as a promising method for enhancing field plant behaviour. Farmers and seed companies are keen to not only discover effective low-cost priming therapies, but also to pinpoint the agronomical properties that have increased as a result of priming in cultivated plants. Under various environmental conditions, seed priming techniques such as hydropriming, nutrient priming, hormonal priming, chemical priming, osmopriming, and redox priming may be efficient.

Seedling growth

We can achieve best plant health, which is the product of best plant physiology, by understanding the process of radical and plumule emergence and their work. We can easily control the plant population to get the highest yield by understanding the different input requirements of plants (water, nutrients, and sunlight). Plant physiology is concerned with the input-output relationship of plants within their bodies.

Growth measurements

High total dry matter production per unit area is the first requirement for higher crop yields. The optimum leaf area (LAI) and the net assimilation rate (NAR) are two factors that influence dry matter production (\(LAI \times NAR = CGR\)). For example, Pruning in horticultural crops such as mango is based on this concept of proper canopy management for improved photosynthesis. Absolute and relative growth rates, leaf weight ratios, compound growth rates, and integral durations are all examples of various growth measurements.

Harvest index

The harvest index (HI) is a measure of reproductive efficiency that is calculated as the ratio of grain to total shoot dry matter. In wheat, historical genetic yield gains have mainly been accomplished by increasing HI. Environmental factors such as seasonal patterns of water flow and intense temperatures during crop reproductive production are important determinants of HI. The net result of photosynthesis is the difference between the total amount of dry matter produced and the photosynthates used in respiration. The distribution of dry matter among the plant’s various organs determines the economic yield. Farmers are interested in the distribution of total dry matter among the major plant organs because they are more concerned with its distribution in terms of economic yield.

Weed control

Herbicides have made a huge difference in modern crop production. However, understanding how these compounds act in plants and their environments is critical for the production of more efficient and safer agrochemicals and to destroy unwanted plants. The majority of herbicides (roughly half of commercially important compounds) operate by interrupting photosynthetic electron flow (e.g., Paraquat, Diuron) or respiration electron flow. As electron transport is blocked in photosynthesis, the light reaction is effectively halted. When the light reaction is stopped, the dark reaction is disrupted as well, and CO₂ is not fixed as a carbohydrate. As a result, the weed is starved to death.

Nutrition Physiology

Nutrition involves various chemical and physiological activities which transform food elements into body elements. About \(17\) essential elements are needed for a crop’s healthy growth. Nutritional physiology knowledge has aided in the detection of essential nutrients, ion absorption pathways, deficiency symptoms, and treatment options. It also aids in the detection of different nutrients’ toxicity symptoms. Nitrogen and mineral nutrient shortages hinder development since these compounds are necessary for certain physiological processes. Studying nutrition physiology can help you understand how nutrients are used and how they are absorbed by plants.

Photoperiodism

Almost all plants can photosynthesize, and it is important for their survival because it allows them to produce sugar molecules for fuel and building materials. Plants, on the other hand, react to light in a number of ways, often to particular wavelengths of light. These non-photosynthesis related responses help plants adapt to their surroundings and grow more efficiently.This concept was used to choose photo insensitive varieties. Lodging resistant, fertilizer tolerant, high yielding, and photo insensitive, semi dwarf rice varieties have revolutionized agriculture sector.

Plant growth regulators

Plant hormones are essential biochemicals that influence plant growth and yield development under a variety of conditions, including stress. Auxin, abscisic acid, ethylene, gibberellins, cytokinins, salicylic acid, strigolactones, brassinosteroids, and nitrous (nitric) oxide are examples of plant hormones. Plant hormones control how plants work under stress, and they may help the plant deal with environmental stresses. Plant hormones have been shown to have a wide variety of behaviour. The development of resistant plant species would be more likely if it is possible to control the activities of plant hormones under stress.

Plant growth substances, phytohormones, and plant growth regulators serve at low concentrations to enable plants to regulate their growth. The application of different hormones at the appropriate time of plant height and age has also been used to regulate flowering, seed forming, and fruit setting.

Post-harvest Physiology

Agriculture post-harvest losses are causing a lot of grief among farmers. The two most significant factors that cause physiological changes in grains after harvest are moisture and temperature. Grain storage has proven to be effective when moisture content is controlled and low temperatures are maintained.

Limitations on the contribution of crop physiology

Following are some of the reasons why crop physiology hasn’t had as much of an impact on agriculture as it should have to:

• One is because crop physiologists are typically more concerned with collecting data on physiological processes than with addressing crop development issues. They focus on the dynamics of processes like photosynthesis, respiration, and translocation rather than their effect on crop growth and yield.

• The reductionist theory that influenced biology during the \(1950\)s and \(1960\)s likely slowed the introduction of basic crop physiology to agriculture by focusing emphasis on cellular and molecular biology at the expense of whole organism biology. As a result, a cohort of physiologists has grown up with a restricted knowledge of whole-organism physiology.

• Because of the contact distance between laboratory and field scientists, most theoretically valuable physiological knowledge is never used in agriculture. Since knowledge about these two classes is often left to chance, field scientists are rarely informed about valuable laboratory experiments, and laboratory scientists are frequently unaware of fascinating and significant field problems. Departmental structure also separates the two classes of scientists, and field and laboratory staff have an unfortunate inclination to discredit each others work. Laboratory physicists, for example, often underestimate the significance of problem-oriented field study, while field investigators view laboratory research as impractical.

How to overcome limitations in crop physiology

The following approaches can effectively control crop physiology limitations:

• Plant physiology has historically offered reasons for poor crop yields and also proposed solutions. It can be used in the future to determine which physiological mechanisms are likely to reduce plant yields in different soils and climates. Plant breeders will indeed be aided by this knowledge in developing varieties with the best mix of physiological and morphological characteristics to withstand the stresses of specific environments.

• Since the physiological mechanisms that regulate yield are heavily influenced by the environment. So, environmental and stress physiology study is expected to make the most significant contributions to crop development. This necessitates a different approach from that used in conventional crop physiology, where the primary focus is generally on plant processes. The first step is to determine which environmental factors are limiting yield in a given situation. This can be supplemented by regulated environment and laboratory experiments to determine the physiological mechanisms that are inhibiting growth and crop yield as a result of environmental stresses.

• We also need to encourage graduate students, especially those studying plant physiology, to even more field and entire plant problems to expand their mental outlook and flexibility. Typically, their expertise is restricted to laboratory research on a single mechanism, such as photosynthesis, or in a narrow field, such as membrane physiology, electron transfer, a certain enzyme system, or stomatal mechanisms. However, they often overlook the link between their study and broader plant growth issues. Such a limited emphasis on a single area can help researchers develop a reputation, but it will never solve the problems of plant development. The scope and complexity of these issues necessitate multidisciplinary study by teams of versatile, well-trained scientists.