In this article we will discuss about:- 1. Introduction to Water Relations for Crop Growth 2. Water Potential for Crop Growth 3. Moisture Stress 4. Water Stress Effects 5. Water Use Efficiency.
Contents:
- Introduction to Water Relations for Crop Growth
- Water Potential for Crop Growth
- Moisture Stress for Crop Growth
- Water Stress Effects on Crop Yield
- Water Use Efficiency for Crop Growth
1. Introduction to Water Relations for Crop Growth:
A rapidly growing herbaceous crop is composed primarily of water. Water content varies between 70 and 90 depending on age, species, particular tissue and environment.
It is indispensable for numerous plant functions:
1. Solvent and medium for chemical reactions.
2. Medium for organic and inorganic solute transport.
3. Medium that gives turgor to plant cells. Turgor promotes cell enlargement, plant structure, and foliar display.
4. Hydration and neutralization of charges on colloidal molecules. For enzymes, water of hydration helps to maintain structure and facilitates catalytic functions.
5. Raw material for photosynthesis, hydrolytic processes, and other chemical reactions in the plant.
6. Water evaporation (transpiration) for cooling plant surfaces.
Under field conditions, the roots permeate a relatively moist soil whiles the stems and leaves grow into a relatively dry atmosphere. This causes a continuous flow of water from soil through the plant to the atmosphere along a gradient of decreasing energy potential. On a daily basis, this flow amounts to 1 to 10 times the amount of water held in plant tissues, 10 to 100 times the amount used in expansion of new cells, and 100 to 1,000 times the amount used in photosynthesis. The primary movement of water, therefore, is from the soil to the leaf to replace transpiration loss.
Because of the high demand for and importance of water, a plant requires a consistent water source for growth and development. Anytime water becomes limiting, growth is reduced and usually also crop yield. The amount of yield reduction is affected by genotype, water deficit severity, and the stage of development.
2.
Water Potential for Crop Growth:
The system that describes the behavior of water and water movement in soils and plants is based on a potential energy relationship. Water has the capacity to do work; it will move from an area of high potential energy to an area of low potential energy. The potential energy in an aqueous system is expressed by comparing it with the potential energy of pure water. Since the water in plants and soils is usually not chemically pure due to solutes and is physically constrained by forces such as polar attractions, gravity, and pressure, the potential energy is less than that of pure water.
In the plant and soil, potential energy of water is called the water potential, symbolized by the Greek letter psi (ψw) and expressed as force per unit of area. The unit measurement is usually the bar or pascal (Pa): 1 bar = 105 Pa = 106 dynes. cm-2 = 0.99 atmospheres, or 102 J . kg-1. Pure water has a water potential equal to 0 bar. The water potential in plants and soils is usually less than 0 bar, which means that it is of negative value. The more negative the value, the lower the water potential.
The water potential of plants and soils is the sum of a number of component potentials:
ψw = ψm + ψs + ψp + ψz …(4.1)
Where,
ψm = matrix potential, the force with which water is held to plant and soil constituents by forces of adsorption and capillarity. It can only be removed by force and so has a negative value.
ψs = solute potential (osmotic potential), the potential energy of water as influenced by solute concentration. Solutes lower the potential energy of water and result in a solution with a negative ψw.
ψp = pressure potential (turgor pressure), the force caused by hydrostatic pressure. Since it is a force, it usually has a positive value.
ψz = gravitational potential, which is always present but usually insignificant in short plants, compared with the other three potentials. It can be significant in tall trees.
3. Moisture Stress for Crop Growth:
Water often limits crop growth and development. The plant’s response to water stress is relative to its metabolic activity, morphology, stage of growth, and yield potential. The sequence of the response to a drying cycle follows.
Cellular growth is the plant function most sensitive to water deficits. The daytime ψw of meristematic tissues often causes the ψP to decrease below that needed for cell enlargement. This in turn causes a reduction in protein synthesis, cell wall synthesis, and cell enlargement, which may account for the observation that many species have their greatest growth at night when the ψw is greatest.
The effect of stress during the vegetative stage is the development of smaller leaves, which can reduce the LAI at maturity and result in less light interception by the crop. Chlorophyll synthesis is inhibited at greater water deficits. With moisture stress most enzymes show reduced activity (e.g., nitrate reductase), but some hydrolytic enzymes show increased activity (e.g., amylase). The breakdown of reserved polymer molecules decreases the ψs, resulting in increased ψp and counteracting the effects of a water deficit.
With reduced water potentials, plant hormones also change in concentration. For example, abscisic acid (ABA) increases in leaves and fruits. ABA accumulation induces stomatal closure, which results in reduced CO2 assimilation; older leaves and fruits often abscise if accumulation is high. Not all plants show an ABA increase with moisture stress. Cytokinins and ethylene can counter the ABA effect and often increase when ABA increases. This may account for the more rapid fruit ripening under water stress conditions.
Under moderate to severe stress conditions, the amino acid proline increases in concentration more than any other amino acid. Proline seems to aid in drought tolerance, acting as a storage pool for nitrogen and/or as a solute molecule reducing the ψs of the cytoplasm. At extreme levels of stress (water potential greater than —15 bars), respiration, CO2 assimilation, assimilate translocation, and xylem transport rapidly diminish to lower levels while the activity of hydrolytic enzymes increases.
Stressed plants growing in a soil water level at permanent wilting will usually recover when irrigated if wilting is of short duration. However, old leaves may abscise, new leaves will be reduced in size, and several days may be required for leaf photosynthesis to reach prestress levels.
4. Water Stress Effects on Crop Yield:
The effects of water stress on yield are manifold. During vegetative development even minor stresses can reduce the rate of leaf expansion and LAI at later stages of development. Severe water stress can cause stomatal closure, which reduces CO2 uptake and dry matter production.
A continuation of stress can cause such a severe reduction in photosynthesis rate that it may take several days after irrigation to resume the original rate. A number of soybean growth parameters, for example, were found to be greatly affected by water stress.
Root elongation and dry weight were not affected as much as leaf area, stem elongation, and dry weight of tops. Roots were expanding into areas where available water was not depleted, resulting in less reduction of cell elongation.
Seed yield was not affected as drastically as vegetative yield, possibly reflecting the greater water availability during seed fill and remobilization of assimilate stored in vegetative parts. The most dramatic effect of the early vegetative moisture deficit was reduction in leaf area.
For seed yield the timing of water stress may be as important as the degree of stress. To a determinate species such as maize, a severe 4-day stress at certain stages of the reproductive cycle may be critical. Pollination (silking) and the 2 week following was the period most sensitive to water stress; the number of kernels per ear was the yield component most drastically affected.
That the plants could produce more photosynthate than the kernels would accept is indicated by the increase in stalk weight. Three week after pollination, water stress no longer affected kernel number but did decrease kernel weight, indicating that the period of kernel loss had passed and that moisture stress reduced leaf photosynthesis and/or translocation. A similar pattern also exists for wheat, another determinate species.
A relatively short but severe stress may have no influence on grain yield if imposed during the vegetative stage of development. Longer periods of less severe stress might have a greater influence on yield.
Indeterminate species that have the potential to flower over a longer period of time may not be as sensitive to water stress. Short-term severe water stress during early flowering of soybean caused little reduction in seed yield; even though water stress caused flower abortion, the plant had time to generate more flowers after stress was removed. Flowers produced late in the flowering period, however, were less likely to produce mature pods by harvest.
The yield component most influenced by water stress at flowering was the number of pods per plant. The stages most sensitive to water stress were late pod development and mid-bean filling. At late pod development water stress still caused pod abortion, poorer pod development (fewer seeds per pod), and reduced photosynthesis (reduced weight per seed). In later stages of seed filling, although there was some effect on pods per plant and seeds per pod the greatest influence was on the weight per seed.
The effect of drought during pollination in areas where determinate maize and indeterminate soybeans are grown usually causes more severe seed yield losses in the maize.
5. Water Use Efficiency for Crop Growth:
The water use efficiency (WUE) of field crops for this discussion is defined as:
WUE = dry matter production (DM)/evapotranspiration
And is expressed as g DM . kg-1 water, kg DM . (ha/cm)-1 water, or lb DM . (acre/in.)-1 water. WUE measurements have been made on plants in containers, on individual plants in the field, and on crop communities. They can be used for economic yield as well as total dry matter. A related term, water requirement, is the reciprocal of WUE.
Water requirement = evapotranspiration/dry matter production
Water requirement is usually expressed in weights of equal magnitude, such as g water. (g . DM)-1.
WUE is not the same as drought, resistance. WUE refers to yield in relation to the water used to produce the yield. Most of the research on WUE has been oriented toward attaining high WUE while maintaining high productivity.
In drought resistance research the emphasis is often placed on survival during periods of high atmospheric demand and low water availability. In many cases, the ability to withstand severe moisture stress is negatively correlated with productivity. Many species that can tolerate severe water deficits do not make efficient use of water in the absence of stress.
Some species, well adapted to severe water deficits, have moderate efficiency even in the presence of stress. The succulents are one such group. Their crassulacean acid metabolism (CAM), stomata that are closed during the day and open at night during water deficits, and leaf structure that allows minimum water loss when stomata are closed reduce transpiration more than photosynthesis and result in a higher WUE than most other species.
Crop yields have increased considerably over the past 40 years. These yields have been achieved without much increase in seasonal ET. For this reason the WUE has increased along with increases in yields. Any management factors that reduce the limitations to growth without significantly increasing ET will increase WUE. Such factors as fertilizer application control of weeds and other crop pests, water conservation, improved tillage techniques, timely planting, and improved crop varieties have substantially increased both yield and WUE.
Large differences in WUE occur when species are categorized by CO2 fixation pathways. It is now accepted that the WUE of C4 species is generally higher than that of C3 species. Earlier field data for WUE, when regrouped into C3 and C4 species, illustrate a 2-fold increase for C4 species when calculated for either grasses or dicots (Table 4.2). Differences between C3 and C4 species increase as the temperature rises from 20 to 35°C.
The factors contributing to the higher WUE of C4 species include higher photosynthesis and growth rates under high light and temperature and higher stomatal resistance. The higher WUE of C4 species is thus a result of higher photosynthetic rates under high light and temperature and lower transpiration rates under low light. So WUE can be increased by growing C4 crops in high solar energy regions or seasons and growing C3 crops only in temperate humid regions or seasons.
The WUE values for both C3 and C4 species are low compared with CAM plants. One CAM species, pineapple (Ananas comosus), has shown a WUE of 20 g. DM . kg water-1. Use of crop species with CAM is limited because the CO2 fixation and overall productivity of CAM plants is low.
In most crop species, field ET is influenced more by atmospheric demand, amount of ground cover, and water availability than by the specific crop species. Table 4.3 illustrates that well-watered crops differed in average daily ET from 4.2 to 5.7 mm. d-1. The primary factors that influenced ET for different species, with water availability remaining high, were time of year (atmospheric demand) and rate of canopy development.
The consumptive use coefficient (k), which is calculated as follows-
Consumptive use coefficient (k) = actual evapotranspiration/potential evapotranspiration
It ranges from 0.65 to 0.87. It is primarily affected by the amount of ground covered by the crop canopy integrated over the growth period. Wheat has a low (k) because it is grown in the relatively cooler spring season and slowly develops leaf area from seeds.
Alfalfa has a high k because in the spring it develops leaf area rapidly from reserve carbohydrates; although harvested during the year, it recovers leaf area rapidly from reserve carbohydrates of the root and crown and maintains a ground cover for a longer time during the growing season. Sorghum and soybean have intermediate k values because they are grown in the warm spring and summer seasons but have relatively slow leaf area development from seeds.
The WUE values in Table 4.3 illustrate that the C4 species have an advantage, but not as great as that shown in Table 4.2. This may indicate that over a full season soil evaporation and atmospheric demand somewhat reduce the advantage one species has over another in WUE efficiency.
Improved crop management and plant breeding have led to substantial gains in WUE. Most of these gains are derived from increased leaf area production (which increases transpiration, reduces soil evaporation, and increases light interception for increased photosynthesis), greater water availability due to deeper roots and/or better water extraction, and (for economic yield) an increase in harvest index.
Conclusion:
Between 70 and 90% of an actively growing herbaceous crop is water, which is indispensable for most plant functions. The roots take up water from a moist soil and move it to plant tops where it is transpired into a dry atmosphere. Thus crop plants require a consistent source of water for consistent growth and development. The system used to describe the behavior of water movement in soils and plants is based on water potential (ψ), which is the sum of component potentials- matrix potential, solute potential (osmotic potential), pressure potential (turgor pressure), and gravitational potential.
Osmotic potential and turgor pressure exert the greatest influence on how plants respond to moisture stress. High turgor pressure is required for cell elongation; some plants can maintain high turgor pressure even at fairly low water potentials by increasing the osmotic potential through increased solute levels in cells, a process called osmotic adjustment. The plant’s ability to adjust osmotically is greatly influenced by its growth environment.
The amount of ET from a crop canopy is a function of the water potential gradient from within the soil out to the air and the resistance to flow through the plant or from soil surfaces. Solar radiation, temperature, relative humidity, and wind are the primary environmental factors affecting ET.
Stomatal closure, stomatal number and size, leaf amount, and leaf characteristics are the plant factors determining the resistance of water movement from soil to air. Potential ET is ET with a full crop canopy and abundant moisture (for minimum resistance). It is an indication of environmental effects on atmospheric demand and has both annual and diurnal fluctuations. It is estimated using open-pan evaporation.
Water deficits reduce vegetative development and yield through reduced leaf expansion and reduced leaf photosynthesis, resulting in reduced canopy photosynthesis. These reductions are primarily affected by the degree of stress. For seed yield, the timing of stress may be as important as the degree.
Water stress during floral initiation, pollination, or seed development may greatly reduce the number of seeds that develop. If water stress is alleviated during grain filling, the potential seed yield may be below that of potential photosynthate production.
WUE is the yield produced per unit of water used. Since crop yields have increased considerably in the last 40 years with little increase in seasonal ET, the WUE has been increased by reducing the limitations to crop growth. WUE is particularly important in areas where water is normally a major limitation to crop yields.
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