Mechanisms of Environmental Stress Resistance in Plants

Amarjit S. Basra, Ranjit K. Basra

• Publisher: Routledge
• Year: 2022
• eBook ISBN: 9781351432405
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Plant growth and productivity are limited in many areas of the world by a wide variety of environmental stresses. This book discusses progress made toward the major goal of uncovering the plant resistance mechanisms to biotic and abiotic stresses; the purpose being to utilise this knowledge in genetic modification of plants for achieving improved stress resistance. This volume achieves a new synthesis in considering the mechanisms of resistance at various levels of organisation -- from individual cells and tissues, through whole plants, to communities. Chapters are written by internationally acknowledged experts, who have a wealth of research and teaching experience. With comprehensive and up-to-date coverage, this book analyses many outstanding problems and poses important questions for future research.

1. DROUGHT RESISTANCE IN PLANTS

DOI: 10.1201/9780203747803-1

HENRY THOMAS Institute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth, Dyfed SY23 3EB, Wales, UK.

INTRODUCTION

Plants have been coping with water stress for some 400 million years, ever since they first left the seas and colonized dry land. When drought occurs, higher plants have always been obliged to endure it or to adjust their life cycle to avoid it: unlike animals, they cannot move to a better place. Thus a major driving force behind the evolution of land plants has been their need to search for water, to absorb it, to transport it and to retain it. Even so, drought is still the major constraint to crop production (e.g. Boyer, 1982; McWilliam, 1986).

The mechanisms underlying drought resistance, having evolved over such a long period of time, are numerous and still imperfectly understood. The aim of this chapter is to describe the current state of our knowledge, and how it might be put to practical use.

THE DROUGHT ENVIRONMENT

The Components of Drought

To the meteorologist, drought is defined as a period (15d, say) without appreciable rain. In agriculture, a drought is a dry spell that results in a loss of yield below that expected under optimal water supply. To the plant physiologist, drought is more than a lack of rainfall: it is a concurrence of at least seven environmental stresses. These are:

1. Low soil moisture availability, limiting the supply of water to the roots.
2. High evaporative load, due to low humidity, high temperature, high insolation and strong winds. The potential loss of water from the leaves exceeds that which can be taken up by the roots, even in well-watered soil. Monteith (1986) has calculated that in equatorial regions, 30-50% of the loss in yield during drought is due to low humidity.
3. High temperature, causing high respiration and damage to metabolic processes and cell structure (Chapter 7, this volume).
4. High solar irradiance, leading to photo-inhibition, photo-oxidation, and eventually death of leaves.
5. Soil hardness increases as soil dries; this can adversely affect root growth and lead to reduced leaf growth and photosynthesis, especially in seedlings (Masle and Passioura, 1987).
6. Unavailability of nutrients, in particular in the upper soil horizons which dry most rapidly but are most mineral rich (Garwood and Williams, 1967).
7. Accumulation of salts in the topsoil and around the roots, leading to osmotic and toxic stress (Chapter, 4 this volume).

In practice, the researcher faces the major problem of disentangling the relative contributions of these phenomena to plant growth and adaptation.

Geographical Variation in Drought Patterns

Drought is caused by an imbalance between evaporation, including transpiration, and rainfall. For the purposes of this discussion it is convenient to recognize six climatic regions on the basis of seasonal rainfall (R) and evaporation (E) patterns (Kendrew, 1961; Edmeades et al., 1989). These descriptions are based on monthly means, and give little idea of the reliability of rainfall, or the probabilities of serious drought.

1. Equatorial climate, having fairly stable E, and two rainy seasons at either side of the solstice (when R > E), and two dry seasons (when R is low but not zero).
2. Tropical climate, with rain in the hottest, sunniest months and a long, dry winter; intermediate between (1) and (3).
3. Monsoonal climate, occurring in the tropics and to the east of large landmasses, with a very dry winter and spring, highest E in spring, and R>E in summer.
4. Continental interior climate, a less extreme version of climate (3). R and E tend to run in phase, being at their maxima in summer and minima in winter. In summer, at higher latitudes or altitudes R and E may be similar, but in more extreme climates E will exceed R.
5. Mediterranean climate, with R concentrated in the winter half of the year, possibly with maxima in autumn and spring, and a reliably dry summer; E low in winter, high in summer.
6. Maritime temperate climate, with R evenly distributed or rather higher in autumn and winter, and E ranging from zero in winter to moderate in summer. Droughts occur only in some years, but may be serious since crops are less well adapted.

PRINCIPLES OF CROP WATER RELATIONS

The Role of Water in the Plant

Water is essential for life on earth. In plants it performs a number of functions, which show different sensitivities to water stress. Briefly, these roles are:

1. Acting as a “skeletal” material which has very little metabolic cost. Plants have large vacuoles, consisting mainly of water, which permit a small amount of dry matter to be spread over a large area or volume. This increases the effectiveness with which roots explore the soil and with which foliage exploits the atmosphere and solar energy.
2. As a medium for metabolism, since many gases, salts and organic compounds are soluble in water. Minerals are taken up in solution from the soil and transported to the leaves in the xylem flow; gas exchange occurs through the film of water in and on mesophyll cells; and metabolites are moved through the plant in solution in the phloem. The tensile strength of water allows it to be drawn, without breaking in the xylem vessels, to the leaves of the tallest trees.
3. To reduce fluctuations in temperature. Only about 1% of the solar radiation reaching a plant canopy is used in photosynthesis, and a large proportion (about half) of the remaining radiative load is consumed in evaporating water from the leaves (transpiration), thus cooling them and preventing overheating.

Further details of the physical and chemical properties of water in plants are to be found in Milburn (1979).

Water Relations and Growth of Unstressed Plants

Plants have an enormous demand for water. Consider a closely mown grass sward (the meteorologists’ standard crop) which accumulates dry matter at the rate of 0.1 t ha^-1 d^-1, under a moderate evaporation load of 3 mm d^-1. It will absorb c. 0.5 t ha^-1 d^-1 of water in its newly produced tissues but, in doing so, it will transpire 30 t ha^-1 d^-1 of water through its leaves into the atmosphere. The water moves from the soil, into the roots and up to the leaves in response to gradients in water potential (ψ), which is a measure of free energy per unit volume of water (Slatyer, 1967). When most of the water passing through the plant is used in transpiration (particularly in sunny weather), the flow rate is linearly proportional to the gradient in ψ between soil and leaf, △ψ (Boyer, 1985).

The water in a plant occupies two distinct compartments: (a) the apoplast, which includes the cell walls, the intercellular spaces and the xylem lumen, all outside the plasmalemmas; and (b) the symplast, lying inside the plasmalemma. The symplast in turn includes two sub-compartments, namely (a) the vacuole, separated by the tonoplast from (b) the cytoplasm and organelles. The symplasts must draw water from the apoplastic transport stream in the xylem in order to keep mature cells hydrated and to expand young growing cells. Potential gradients across the semipermeable plasmalemma are generated by solutes in the cytoplasm and vacuole, which lower the osmotic (solute) potential ψ_s and suck water in from the apoplast. The typical value of ψ_s in mesophytic plants i...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Contents
• Dedication
• Preface
• List of Contributors
• 1 Drought resistance in plants
• 2 Mechanisms of desiccation tolerance in resurrection vascular plants
• 3 Mechanisms of plant resistance to waterlogging
• 4 Mechanisms of salt tolerance in higher plants
• 5 Mechanisms of chilling resistance in plants
• 6 Mechanisms of freezing resistance of wood tissues: Recent advancements
• 7 Reduction of high temperature stress in plants
• 8 Resistance of plants to the effects of ultraviolet radiation
• 9 Plant strategies for coping with variable light regimes
• 10 Mechanisms of plant resistance to nutrient deficiency stress
• 11 Mechanisms of plant resistance to toxicity of aluminium and heavy metals
• 12 Mechanisms of mechanical stress resistance in plants
• 13 Mechanisms of virus resistance in plants
• 14 Resistance against fungal pathogens: Its nature and regulation
• 15 Insect resistance in plants: Natural mechanisms and improvement through biotechnology
• Index