S A Tr Rsi5170

where ^ is the hydric potential of the leaf, is the hydric potential of the soil, A^g is the difference in gravitational potential between the soil and the leaf, Tr is the transpiration flux, and Rsl is flow resistance from soil to leaf.

The hydric status of a plant is determined by an entire series of environmental and physiological factors represented in diagram form in Fig. 5.37.

Four main factors control the plant's water balance [4]:

1. soil water potential and factors involved at its level, such as rain, irrigation water, crop water extraction, soil texture, structure, and depth, and root distribution and dynamics.

2. evapotranspiration rate, which implies several environmental factors (e.g., radiation, air humidity, temperature, and wind) and physiological factors (leaf area and exposition, plant canopy, stomatal conductance).

3. water conductance in roots, stems, and leaves, which depends on the physicochem-ical characteristics of the plant tissues and affects the water-movement velocity through the plant and the water equilibrium among the plant organs.

Figure 5.37. Diagram of factors controlling hydric status of a plant: ® = hydric potential; P = turgidity; and E = evaporation Source: Adapted from [4].

4. relation between the hydric and other water-status measures (e.g., turgor pressure, osmotic potential, or water content), which might be affected by factors such as osmotic regulation, cell-size alterations, and cell-wall elasticity.

The parameters commonly used as indicators of the plant's hydric status are the relative water content (RWC); the hydric potential of a tissue or organ, ty; and the hydric potential components, tys and ty p. A comprehensive review [5] of methods for measuring water deficit will provide newcomers to the field with an array of direct and indirect techniques for measuring water status of a plant.

An indicator of water status is tissue water content, which can be measured easily with high accuracy, on either a fresh-weight or a dry-weight basis. However, earlier works [6] have shown that the water-holding capacity of different organs or tissues of a plant can vary widely, because of variation in the dry-matter content of the tissue. Consequently, the parameter used is normalized RWC, which is the water content relative to that when the tissue is saturated with water and ty is zero. As implied in its definition, RWC is a measurement of tissue volume maintenance and water holding and is linked closely with plant functions. This parameter and its possible errors have been reviewed [7].

Many methods have been developed to determine ty within tissues. These methods have been widely reviewed [5,8]. At present, methods based on thermocouple psychrom-etry and the pressure chamber method are the most commonly used and are considered to be accurate.

Technical bases and management of psycrometers and hygrometers have been discussed [7-11]. These methods determine ty by measuring air humidity in equilibrium with the tissue sample. They are based on the fact that ty is the same throughout the water phases when there is equilibrium. Then, ty is determined in the liquid phase from ty of the vapor phase. With the psychrometer technique, thermocouples function like wet and dry bulbs to measure humidity. With the hygrometric technique, humidity is determined by using thermocouples to measure the dew point of the air.

Measures of ty using psycrometric techniques are subject to several errors [7], for example, from heat produced by respiration of the tissue sample, incomplete equilibrium because of resistance of leaf tissues to the vapor transfer, or calibration errors.

The pressure chamber is the device more commonly employed for estimating water potential of leaves and shoots. The development of this technique is essentially due to Scholander et al. [12], although other authors [13-14] worked on the same bases before them. There is experimental evidence that leaf ty measured with the pressure chamber is quite similar (numerically) to its potential measured by using psycrometry techniques.

The solute or osmotic, tys, and the pressure, typ, components of ty can be measured separately, but the most common practice is to measure ty and tys, and then to calculate typ as the difference between them. The solute potential is measured directly from the cell liquid extracted from the tissue after destroying the cell walls by freezing (immersion in liquid nitrogen). The measuring methods are refractometry, based on the refraction change; cryoscopy, based on determination of the decline in the freezing point of water in a sample of cell liquid; and thermocouple psycrometry in which tys is measured after eliminating the turgor pressure [7].

Several nondestructive methods have been developed for the study of the water content changes within plant organs [7]. The change of leaf water content is detected from ¡3 particles absorbed by a leaf irradiated with these particles. In trunks and shoots, the diameter size allows estimation of changes in water content. Dendrometers allow detection of both stem contractions and dilations associated with the day cycle of the plant's water content [15].

Water stress affects stomata behavior, and so, the stomatal conductance can be considered as a plant-stress indicator. In addition, stomatal conductance is well correlated with the rate of photosynthesis, which also depends on water status. Stomatal conductance usually is measured with a diffusion porometer, which allows precise measurement in situ. There are two kinds of porometers [11]: carrier porometers, where the leaf is in contact with a chamber equipped with sensors capable of responding to changes in humidity, thus changing electrical resistance; and steady-state diffusion porometers, where measurement of the dry-air flow that is necessary to balance water transpired by the leaf within the chamber offers an estimate of stomatal resistance, by keeping the vapor density constant within the chamber.

When stomata close partially or fully under water stress, the energy balance of the crop canopy is altered. Hence, increasing its values also alters the canopy temperature. The transpiration process leads to a cooling effect on the leaf in relation to air temperature. Differences between air and leaf temperature are related to the leaf's hydric potential [16].

In stressed plants, ^ is low and the water tension within the xylem is very high, which suddenly induces discontinuities in the liquid continuum. As a result, spaces are formed and filled with air or water vapor and the water flow is disabled. This process is known as cavitation. The greater the water deficit, the greater is the number of cavitated spaces [17]. The explosion frequency might be considered as a specific index of the hydric status for a crop species [7].

The growth of leaves and branches is undoubtedly the process most susceptible to water stress [7]. Several studies have shown that leaf ^ depletion under a threshold (obtained with nonstressed plants) reduces organ growth [11]. This suggests that the water status of plants might be inferred from measures of leaf and branch growth.

The most obvious general effects of water stress are the reduction in plant size, leaf area, and crop yield. Water deficits affect the major growth and metabolic processes of plants. Plants can maintain their turgor potential by osmotic balance. However, species and cultivars vary in amount of osmotic balance. Organs of the same plant differ in the amount of osmotic balance, which may be important for short-term survival under drought, but not for long-term survival. Betaine and proline are two osmolytes that accumulate under drought and permit a plant to lower its osmotic potential without damaging metabolic functions. Accumulation of osmolytes differs among species and cultivars and may be subject to genetic control.

The quantity and quality of plant growth depend on cell division, enlargement, and differentiation, and all are affected by water deficits, but not necessarily to the same extent. In addition to affecting leaf expansion, water deficits can affect leaf area by senescence and death of leaves during all phases of growth. Water deficits also reduce tilling and increase the death rate of tillers in multitilled species, thereby influencing the leaf area of a plant canopy. The effect of stress during the vegetative stage is the development of smaller leaves, which reduces the leaf area index (LAI) at maturity and results in less light interception by the crop.

The effects of water stress on reproductive differentiation are even greater. Spike elongation and spikelet formation of cereals are inhibited by water stress. If this occurs at early anthesis, flowers are injured and the number and size of seeds are reduced.

Water deficits are sufficient to close stomata and reduce photosynthesis and also to decrease dark respiration. The decrease in the rate of dark respiration is less than the decrease in photosynthesis. A review of the literature [18] concluded that photorespiration was unaffected by short-term stress in crop species but that, ultimately, photorespiration decreased as the substrates for photorespiration were depleted.

Generally, mild water stress for brief periods results in reductions in protein synthesis, hydrolysis of proteins, and accumulation of amino acids, especially proline. Severe prolonged stress is necessary for major damage to occur. Under moderate to severe stress conditions, the amino acid proline increases in larger concentration 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 of the cytoplasm [19]. At extreme levels of stress, respiration, CO2 assimilation, assimilate translocation, and xylem transport rapidly decrease to lower levels whereas the activity of hydrolytic enzymes increases. Chlorophyll synthesis is inhibited at higher water deficits.

The effects of water deficits on the actual distribution of assimilates to the various plant organs depends on the stage of development of the plant, the degree of and previous periods under stress, and the degree of sensitivity to stress of the various plant organs.

With reduced water potentials, plant hormones also change in concentration. For example, abscisic acid (ABA) undoubtedly increases in stressed leaves and fruits, leading to stomatal closure. Several studies indicate that drought-resistant cultivars accumulate more ABA than do drought-sensitive cultivars. Increased ethylene production is a general response of plant tissues to environmental stress, including water stress. Cytokinin (CK) content decreases in tissues under conditions of water stress. Leaf senescence associated with water stress could be due to a reduced supply of CK. Both CK and ABA can modify the rate of ethylene synthesis in water-stressed leaves, emphasizing the importance of considering hormonal interaction. Evidence exists that water stress causes a moderate reduction in indoleacetic acid.

The effect of water stress on production has been the most relevant aspect from an agronomic point of view. The amount of injury caused by water stress depends to a considerable extent on the stage of plant development at which it occurs. Reproductive stages of growth are often the most sensitive to drought. The effect of soil water deficit on flowering and fruit formation depends on the timing and severity of the water deficit.

Flowering is affected most severely by water stress at or just before peak flowering. Possible causes include reduced photosynthate supply, reduced turgor, and low relative humidity. The effect of water deficit during fruit formation is primarily to reduce the number of fruits formed while scarcely affecting weight per fruit. The effect of drought depends on the timing of water deficit relative to achieving a sufficient number of fruits of that minimum size and on availability of photoassimilates to fill the existing fruits. Grain yield is determined more by the total photosynthesis taking place during the entire season than by photosynthesis occurring during the grain-filling period alone.

Water deficits during seed filling have been reported to reduce weight per seed and weight per fruit. Even different stages of grain filling are differentially sensitive to drought. Kernel growth is decreased at lower water potential than photosynthesis.

For seed yield, the timing of water stress may be as important as the degree of stress. In the case of several species, such as maize, a severe four-day stress at certain stages of the reproductive cycle might be critical [20]. Pollination (silking) and the two weeks following are the periods most sensitive to water stress; the number of kernels per ear is the yield component most drastically affected. Three weeks after pollination, water stress no longer affects kernel number but does decrease kernel weight. A similar pattern also exists for wheat, another determinate species [21].

Indeterminate crop species that have the potential to flower over a longer period of time may be less sensitive to water stress. Short-term severe water stress during early flowering of soybeans causes little reduction in seed yield because the plant has time to germinate more flowers after stress is removed [22]. However, flowers produced late in the flowering period are less likely to produce mature pods. The yield component most influenced by water stress at flowering is the number of pods per plant. The stage most sensitive to water stress is late pod development and midbean filling.

Root elongation and dry weight are not affected by water deficit as much as leaf area, stem elongation, and dry weight of tops. Roots extend into areas where available water is not depleted, resulting in less reduction of cell elongation. Thus, root-shoot ratios generally are increased by water stress.

Water stress is not always injurious. Although it reduces vegetative growth, it sometimes improves the quality of plant products. For example, moderate water stress is said to improve the quality of apples, peaches, pears, and prunes, and to increase the protein content of hard wheat [3].

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