In this essay we will discuss about the effects of water deficits on plant growth.
Essay Contents:
- Essay on the Effect of Water Deficits on Crop Growth and Yield
- Essay on the Effect of Water Deficits on Plant Physiological Processes
- Essay on the Plant Mechanisms for Surviving Water Stress
- Essay on the Soil-Water Conditions that Affect Plants
Essay # 1. Effect of Water Deficits on Crop Growth and Yield:
Water stress affects every aspect of plant growth: modifying anatomy, morphology, physiology and biochemistry.
Some of the adverse effects of water stress on plants include:
1. Loss of turgidity leading to reduced cell enlargement and stunted growth.
2. Decrease in photosynthesis due to decreased diffusion of CO2 with the closure of the stomata to conserve water
3. Increase in respiration resulting in decreased assimilation of photosynthesis
4. Breakdown of RNA and DNA
5. Inhibition of synthesis and translocation of growth regulators
6. Hydrolysis of carbohydrates and proteins leading to increase in soluble sugars and nitrogen compounds
7. Rolling and wilting of leaves to reduce leaf area
8. Reduced tillering/branching
9. Forced maturity
10. Reduction in productivity.
Effects of water deficits (drought) on crop growth range from morphological to molecular levels and are evident at all phenological stages of crop growth at whatever stage the water deficit takes place. The first and foremost effect of water deficits on crops is impaired germination and poor stand establishment. Drought stress has been reported to severely reduce germination and seedling stand.
Growth is accomplished through cell division, cell enlargement and differentiation and involves genetic, physiological, ecological and morphological events and their complex interactions. Quality and quantity of plant growth depend on these events, which are affected by water deficits (Fig 6.1).
Cell growth is one of the most drought sensitive physiological processes due to the reduction in turgor pressure. Under severe water deficiency, cell elongation of higher plants can be inhibited by interruption of water flow from the xylem to the surrounding elongating cells.
Injury due to water stress, largely, depends on the stage of crop development at which it occurs.
Generally, life cycle of field crops is divided into three phases:
1. Seed germination and crop establishment
2. Vegetative phase
3. Reproductive phase.
1. Seed Germination and Crop Establishment:
Under field conditions, seed germination and seedling growth are inhibited due to water deficits leading to poor crop stand establishment.
2. Vegetative Phase:
Vegetative growth in general and leaf expansion in particular, is affected by water deficits. Visible injury of water stress is wilting, rolling/curling of leaves, colour change, wax coating on leaves etc. Leaf abscission is often noticed due to the accumulation of ABA under drought. Reduced growth is due to reduction in cell enlargement at low water potential. Reduction in tillering/branching is common in most crops.
Abscissic acid (ABA) is often known as a stress hormone which ascribes drought tolerance to plants. It is very important to recognise the pros and cons of high ABA concentration in various plant organs. This should allow weighing the different effects and their sum totals under given stress scenarios and given agricultural ecosystem.
ABA has evolved as a life conserving mechanism when the plant enters a stress situation. Where drought stress is concerned, the first consequences of ABA activity are to reduce water use and plant hydration via reduced shoot growth, reduced stomatal conductance and promoted root growth and its hydraulic conductance.
As stress increases ABA serve to reduce the sink load on the stressed plants by reducing the number of developing fruit and/or seed. However, few remaining seeds are still retained and filled well. When total plant assimilate production is limited by stress it would be a reasonable strategy to limit the number of sinks in order to produce at least a few viable seeds. Filling of the remaining seed in the cereals is supported by ABA induced stem reserve mobilisation. Dormancy is then affected in order to conserve the seed until the next season.
This survival strategy is extremely important to the plant in terms of its ontogenicity and evolution. However, when this plant is used for the farmer’s livelihood, other considerations can be more important and they may not fit the above built-in strategy of ABA regulation (Table 6.5).
3. Reproductive Phase:
Reproductive phase of the crop is highly sensitive to water stress as the final yield of most crops depends on availability of optimum soil moisture in the soil at this phase. Crops such as rice, wheat, maize, sorghum, fingermillet, pearlmillet etc. are adversely affected by water stress during panicle initiation, anthesis and grain formation stages. Since, number of grains per ear head and test weight of the seed are determined at these stages, water deficits at these stages have maximum effect on grain yield.
All yield components taken together constitute the sink, while all assimilate contributing parts of the plant are considered the source. Drought stress can reduce yield by affecting the sink or the source. Source capacity is reduced under drought stress as a result of stress effects on leaf area, gas exchange and carbon storage available for grain filling as well as from an increase in leaf senescence and the increase in rate of certain developmental processes.
Reduction in sink capacity under drought stress is caused by arrested organ differentiation as well as by the dysfunction of the differentiated reproductive organs. Thus, for example, drought stress reduces the number of tillers either by stopping their sequence of differentiation or by death of growing or grown tillers.
Number of flowers (florets) in the inflorescence will be reduced by arrested differentiation or by abortion and degeneration of developed flowers under stress. Reduction in the number of grains developed from a given number of flowers in the inflorescence can be affected by induced sterility of female or male organs as well as by stress induced abortion of embryos.
There is very large volume of evidence that the most drought stress sensitive plant growth stage is flowering. This can be seen in the classical presentation by O’Toole (1982), where yield of rice is reduced most when stress occurs during plant reproduction. Peak stress sensitivity is at anthesis and fertilisation (Fig 6.2). This presentation for rice represents well most if not all other cases of grain and fruit bearing crops.
In the case of fruit crops like citrus and apple, water stress often causes shedding of immature fruits. However, coffee crop must be subjected to water stress before flowering. This results in profuse flowering leading to high yield of berries. Extended period of drought causes premature flowering (earliness in flowering) leading to reduction in yield due to reduced size of pods, seeds, fruits etc.
Essay # 2. Effect of Water Deficits on Plant Physiological Processes:
All the plant physiological processes are influenced by the water deficits (Fig 6.3). Except abscisic acid and solute accumulation all other physiological processes are sensitive to decreasing water potential.
1. Photosynthesis:
A major effect of drought is reduction in photosynthesis, which arises by a decrease in leaf expansion, impaired photosynthetic machinery, premature leaf senescence and associated reduction in food production.
When stomatal and non-stomatal limitations to photosynthesis are compared, the former can be quite small. This implies that other processes besides CO2 uptake are being damaged. The role of drought induced stomatal closure, which limits CO2 uptake by leaves, is very important. In such events, restricted CO2 availability could possibly lead to increased susceptibility to photo-damage.
When the amount of available soil-water is moderately or severely limiting, the first option for plants is to close stomata. This decreases the inflow of CO2 into the leaves and spares more electrons for the formation of active oxygen species (Fig 6.4). As the rate of transpiration decreases, the amount of heat that can be dissipated increases.
Drought stress disturbs the balance between the production of reactive oxygen species and the antioxidant defense, causing accumulation of reactive oxygen species, which induces oxidative stress. Upon reduction in the amount of available water, plants close their stomata, which decrease the CO2 influx.
Reduction in CO2 not only reduces the carboxylation directly but also directs more electrons to form reactive oxygen species. Severe drought conditions limit photosynthesis due to a decrease in the activities of ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPCase), NADP-malic enzyme (NADP-ME), fructose-1, 6-bisphosphatase (FBPase) and pyruvate orthophosphate dikinase (PPDK).
Reduced tissue water contents also increase the activity of Rubisco binding inhibitors. Moreover, non-cyclic electron transport is down-regulated to match the reduced requirements of NADPH production and thus reduces the ATP synthesis.
Impaired mitosis, cell elongation and expansion result in reduced plant height, leaf area and crop growth under water deficits. Many yield-determining physiological processes in plants respond to water stress. Yield integrates many of these physiological processes in a complex way. Thus, it is difficult to interpret how plants accumulate, combine and display the ever-changing and indefinite physiological processes over the entire life cycle of crops.
For water stress, severity, duration and timing of stress, as well as responses of plants after stress removal, and interaction between stress and other factors are extremely important. For instance, water stress applied at pre-anthesis reduced time to anthesis, while at post- anthesis it shortened the grain filling period in triticale genotypes.
In barley (Hordeum vulgare), drought stress reduced grain yield by reducing the number of tillers, spikes and grains per plant and individual grain weight. Post-anthesis drought stress was detrimental to grain yield regardless of the stress severity.
Drought induced yield reduction has been reported in many crop species, which depends upon the severity and duration of the stress period. In maize, water stress reduced yield by delaying silking, thus increasing the anthesis-to-silking interval. This trait was highly correlated with grain yield, specifically cob and kernel number per plant.
2. Respiration:
Drought tolerance is a cost-intensive phenomenon, as considerable quantity of energy is spent to cope with it. Fraction of carbohydrate that is lost through respiration determines the overall metabolic efficiency of the plant. Root is a major consumer of carbon fixed in photosynthesis and uses it for growth and maintenance as well as dry matter production. Plant growth and developmental processes as well as environmental conditions affect the size of this fraction (i.e. in respiration).
However, the rate of photosynthesis often limits plant growth when soil-water availability is reduced.
A negative carbon balance can occur as a result of diminished photosynthetic capacity during drought, unless simultaneous and proportionate reductions in growth and carbon consumption take place.
In wheat, depending on the growth stage, cultivar and nutritional status, more than 50 per cent of the daily accumulated photosynthates were transported to the root, and around 60 per cent of this fraction was respired. Severe drought reduced the shoot and root biomass, photosynthesis and root respiration rate. Limited root respiration and root biomass under severe soil drying can improve growth and physiological activity of drought tolerant wheat, which is advantageous over a drought sensitive cultivar in arid regions.
3. Water Relations:
Relative water content, leaf water potential, stomatal resistance, rate of transpiration, leaf temperature and canopy temperature are important characteristics that influence plant -water relations. Relative water content of wheat leaves was higher initially during leaf development and decreased as the dry matter accumulated and leaf matured.
Obviously, water stressed wheat and rice plants had lower relative water content than non stressed ones. Exposure of these plants to drought stress substantially decreased the leaf water potential, relative water content and transpiration rate, with a concomitant difference caused by the associated higher leaf temperature. Transpiration rates were similar in both treatments and the lower total water use of the unirrigated stand resulted entirely from a smaller leaf area index.
4. Nutrient Relations:
Decreasing water availability under drought, generally, results in limited total nutrient uptake and their diminished tissue concentrations in crop plants. An important effect of water deficit is on the acquisition of nutrients by the root and their transport to shoots. Lowered absorption of the inorganic nutrients can result from interference in nutrient uptake and the unloading mechanism.
However, plant species and genotypes of a species may vary in their response to mineral uptake under water stress. In general, moisture stress induces an increase in N, a definitive decline in P and no definitive effects on K.
As nutrient and water requirements are closely related, fertiliser application is likely to increase the efficiency of crops in utilising available water. This indicates a significant interaction between soil moisture deficits and nutrient acquisition. Studies show a positive response of crops to improved soil fertility under arid and semi-arid conditions.
Currently, it is evident that crop yields can be substantially improved by enhancing the plant nutrient efficiency under limited moisture supply. It was shown that N and P contents in the plant tissues diminished under drought, possibly because of lowered mobility as result of low moisture availability.
Essay # 3. Plant Mechanisms for Surviving Water Stress:
Crops can be divided into three general groups- determinate, indeterminate and forage crops (Table 6.6). Determinate plants often have a main stem, a preset number of leaves and produce flowers and seeds once they have reached a specific point in their growth. This cycle can occur in a relatively short period of time to avoid drought conditions.
The advantage that determinate crops have over indeterminate crops is a steady use of soil-water, leaving some for the grain stage, while the disadvantage is the possibility of permanent damage to flowers or grain if significant water stress occurs at critical times.
Indeterminate plants have the ability to grow, flower and mature on secondary stalks and can take advantage of erratic rainfall throughout the growing season; yet large, new leaves and energy spent on new growth increases transpiration, bringing on water stress with higher temperatures.
Since a variety of plants fall under the forage crops, one single characteristic does not describe their growth habit or drought tolerance. Many of the more drought tolerant forage species have deep taproots. Other forage species tolerate saline soils or alkaline pH conditions that frequently exist in semi-arid land and exacerbate drought conditions.
Mechanisms of Drought Tolerance in Plants:
Drought tolerant plants are defined as plants that can grow satisfactorily under periods of water stress.
Mechanisms for tolerating drought include:
1. Avoiding growth in the dry season (quick growing annuals, perennials that go dormant or plants that adjust their growth when conditions are favorable, such as indeterminate crops)
2. Water conservation measures (greater uptake, control of transpirational loss and greater storage in tissues).
3. Reducing leaf area (temporary leaf rolling)
4. Seasonal changes in the cuticle and leaf configuration and hairiness.
Beneficial Effects of Water Stress in Crops:
Water stress is not always a disadvantage to field crops.
It sometimes improves the quality of the produce as indicated below:
1. Increase in rubber content
2. Improvement in desirable aromatic properties of Turkish tobacco
3. Increase in alkaloid content of belladonna, datura, digitalis etc.
4. Improvement in oil content of mint, olive and soybean
5. Moderate water stress improves the quality of fruits like apples, plum, cherry peaches etc. by increasing the soluble sugar content and by developing fascinating colours of the fruits
6. An increase in protein content of wheat is often noticed
7. Under air pollution situations, water stressed plants are injured lesser than the normal and well-watered plants because of the hindrance in the entry of polluting gases/particles as the stomata are closed due to water stress
8. Moderate degree of water stress is often desirable before uprooting the seedlings and also after transplanting in the main field for better root development and quicker establishment in the main field.
Essay # 4. Soil-Water Conditions that Affect Plants:
1. Soil Saturation:
Saturated soils have all macro and micro-pores filled with water. This creates anoxia (no oxygen) or hypoxia (subnormal oxygen) and triggers anaerobic respiration in the plant. Soil- water potential at this condition is close to zero and water rapidly enters the roots in excess of crop needs.
Effects on non-wetland plants from reduced oxygen levels include reduced stem and root growth, decreased rates of photosynthesis, changes in cellular structure and a build up of toxic products from anaerobic respiration such as pyruviate, ethanol and lactate.
Low soil temperatures and low oxygen levels from flooding have been shown to retard shoot development in several irrigated crops. With a decrease in oxygen, the concentration of CO2 and other gases increase; this can slow plant growth due to ethylene production in plant tissues. Depending on the length of time that the soil is saturated, plants may experience mineral nutrient deficiencies as active uptake is slowed.
2. Field Capacity:
With soil-water content relatively high at field capacity, mass flow of nutrients like nitrate, sulphate, calcium and magnesium is often sufficient for plant needs. In addition, diffusion rates of phosphorus (P) and potassium (K) are often relatively high.
When soils reach field capacity, 50-80 per cent of P fertiliser can diffuse from fertiliser granules into the soil solution within 24 h. Knowledge of nutrient movement at different water potentials can assist irrigation timing for quick delivery of nutrients to crops.
3. Permanent Wilting Point:
Under these conditions, plants cannot lower their internal water potential enough to maintain cell turgor, even if transpiration stops. Increased solute concentration in soil water can cause water stress in plants even at potentials greater than PWP. In an effort to conserve water, transpiration does not occur and plant metabolism decreases; both functions cause lower conductance of water throughout the plant.
Soil-water is carried through xylem tissue which is made up of conduits of dead cells connected by narrow openings or pits. As the xylem pressure decreases from lack of water, air moves into the xylem through these pits, causing cavitation or air bubbles.
In extreme cases of PWP, cavitation can cause cell walls to crack and break whole stems. Consequently, producers who can irrigate generally avoid reaching PWP by irrigating frequently. Farmers without irrigation capabilities rely on drought tolerant crops and conservation strategies.