The term "transpiration ratio" (TR) was introduced  to quantify the relation between crop transpiration and crop dry-matter production. TR is defined as the mass ratio of crop transpiration to crop dry-matter production. This term was later called the transpiration coefficient. The TR concept has been expanded and improved by normalizing the transpiration by the mean daily potential evaporation during the growing season .
The analysis of photosynthesis and transpiration based on physiology allows a more clear comprehension of those processes . Water evaporates from the inside of the stomatal cavity and passes to air through the stomatal opening. During photosynthesis, CO2 transfers in the opposite direction. The transpiration rate, tl , per unit of leaf area is determined by the vapor-pressure difference between the evaporative surface within the leaf (eo) and air (ea), and by the resistance to the water-vapor transfer:
where rs and ra are stomatal resistance and layer (aerodynamic) resistance to the diffusion of water vapor, p is the humid-air density, £ is the ratio of molecular weight of water vapor in relation to that of the air, and p is the air pressure. The photosynthesis rate, NL , per unit of leaf area, is determined by the difference between the concentration of CO2 within the leaf (co) and the outside air (ca) and the resistance to the CO2 transfer:
rst + r'a where r'st and r'a are the stomatal resistance and the boundary-layer resistance, respectively, to the CO2 transfer. Thus, the photosynthesis transpiration ratio is expressed as
It has been noticed that, because the daily average temperature of a leaf is normally very close to the air temperature, (eo — ea) will be almost equal to the vapor-pressure deficit (es — ea), where es is the saturation vapor pressure and ea is the actual vapor pressure . Equation (5.174) can be inferred by using the former estimates, which are derived by considering the behavior of an isolated leaf, as well as assuming that crops may be similarly described:
T es — ea where k is a crop-specific constant. The review by Tanner and Sinclair  suggests that k is a very stable term within the crop system and that transpiration efficiency, N/ T, depends on (es — ea).
WUE is defined differently. The photosynthetic WUE is defined as the ratio of leaf net assimilation to leaf transpiration, A/ T . When WUE is enhanced, more carbon is gained per unit of water used by plants .
The biomass WUE of field crops is defined as follows:
where ET is plant or canopy evapotranspiration and t0 and t f are initial and final time limits for the integration. The timescales are medium (week) and long term (season). When soil evaporation in ET is assumed to be negligible, biomass WUE equals biomass water ratio (BWR) .
Agronomists have preferred the ratio of usable or marketable yield (which is expressed as fresh weight) to unit mass of ET. The yield WUE is defined as the product of aboveground consumptive biomass WUE (biomass WUE) times the harvest index (HI). In this case, the timescale is the whole crop cycle. HI is the ratio of usable reproductive-organ yield to total dry-matter production.
The two main factors that directly affect the WUE of a crop are crop species and genotypes (through regulation of internal CO2 concentration by the photosynthesis pathway) and environmental conditions (mainly through the atmospheric humidity, radiative flux, temperature, precipitation, and evaporative demand).
Biomass WUE may differ between species. Large differences in biomass WUE occur when species are categorized by CO2 fixation pathways. It is now accepted that the biomass WUE of C4 species, so-called because of the foul-carbon malic or aspartic acid present in the basic photosynthesis, is generally higher than that of C3 species, so-called because of the three-carbon phosphoglyceric acid (PGA) present in the basic photosynthesis reaction. Earlier field data for biomass WUE , when regrouped into C3 and C4 species, illustrate a two-fold increase for C4 species. Stanhill  reported values of BWR of 2.9 to 3.7 kgm-3 and 1.2 to 2.7 kgm-3 for the C4 and C3 species, respectively. Differences between C3 and C4 species increase as the temperature rises from 20°C to 35°C. The factors contributing to the higher biomass WUE of C4 species include higher photosynthesis and growth rates under high light and temperature, and more stomatal resistance .
The major environmental factor influencing the WUE of a crop is vapor-pressure deficit (VPD). Monteith  questioned whether transpiration or photosynthesis was the independent process in the TR. Because the internal CO2 concentration is regulated by the plant species through its photosynthesis pathway, he argued that plants dynamically balance their leaf conductance to maintain this CO2 concentration, thereby indirectly regulating transpiration.
Increasing atmospheric CO2 concentrations generally escalates photosynthetic rates and the plant biomass production. A doubling of atmospheric CO2 concentrations increases biomass production by an average of 33% in several vegetal species . This buildup in biomass production, coupled with a reduction in ET, results in a significant increase in plant WUE. Both forest and agricultural species have been shown to double WUE under a doubling of atmospheric CO2 concentration . More than 500 studies analyzing the effects of increasing atmospheric CO2 concentration have reported an acceleration in crop yield, biomass production, leaf area, and photosynthetic rates, as well as decrease in plant water-use requirements . An increase in biomass and leaf area implies that plants can transpire more water. However, the reduction in transpiration caused by an increase in stomatal resistance may result in a cumulative decrease in evapotranspiration . The photosynthetic reactions of C3 plants are more sensitive than those of C4 plants to increased CO2 concentrations, resulting in a larger escalation in biomass production in C3 plants.
Air temperature usually operates through its effect on VPD, which is the major environmental factor influencing WUE of a crop species.
Other environmental factors that influence WUE are radiative flux and soil water availability. Diurnal changes in biomass WUE occur in response to radiative flux and VPD. Seasonal changes may be related to different temperature or growth stages. For maximum biomass WUE, there is an optimum radiation that is usually lower than the radiation incident on a leaf oriented normal to the sun .
Biomass WUE generally is increased by drought because of reduced evaporation from the soil surface, whereas yield WUE is generally less because of adverse effects on reproductive growth. These effects depend on intensity and duration of the stress, along with the phenological stage of the plant at which stress is occurring. In dryland cropping, where water deficits cannot be controlled by irrigation, a severe deficit can considerably lower yield WUE.
In subhumid regions, because of the low VPD, crop production and BWR may be substantially higher than in arid regions .
Other factors interact with these two main factors, including soil infiltrability, waterholding capacity and water depth, soil salinity, soil fertility, weed competition, pests and diseases, cultivation techniques (seeding rates, seeding date, row spacing, and tillage), and institutional and socioeconomic factors.
Improved crop management and plant breeding have led to substantial gains in WUE. Most of these gains are derived from increased leaf area production, larger water availability due to deeper roots and/or better water extraction, and an increase in HI.
WUE can be increased  by increasing the HI; reducing TR; reducing the root dry matter; and increasing the transpiration component relative to the other water-balance components, in particular reducing the evaporation of soil surface, the drainage below the root zone, and the runoff.
Three options exist for improving the TR for dryland crop production systems , which also apply to irrigated crop production: plant manipulation to increase the photosynthesis rate; utilization of the growing season with the lowest atmospheric VPD; and manipulations of crop nutrition so that less dry matter is partitioned into the roots without limiting their water extraction capacity.
WUE can be enhanced by modifying cultural practices, such as land preparation, irrigation, cultivation, and crop selection or genetic modification of the plant . The latter provides short-term solutions, whereas the former is more of a long-term solution. Five options for improving the WUE are biochemical modifications, control of stomatal physiology, HI improvement; crop microenvironment changes, and increase in the fraction of transpired water out of the total amount of water used during the growing season . Among these options, the first two are in the domain of plant breeding and genetics and the third is partly so.
WUE is not the same as drought resistance. Most of the research on WUE has been oriented toward attaining high yield WUE while maintaining high productivity. In drought resistance research, emphasis often is 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.
Drought resistance can take the form of tolerance or avoidance. Avoidance is accomplished by reducing the length of the season or by increasing rooting depth and water extraction from the lower profile. Drought tolerance refers to plant adaptation to deficits through physiological and morphological processes that limit transpiration losses and/or condition plants to withstand lowered water potentials. The seasonal progression of temperature, the distribution and intensity of rainfall, and the availability of soil moisture largely govern which attributes of the plant might be beneficially altered to enhance WUE. These attributes might include deep rooting to exploit stored soil moisture during long droughts, stomata that close quickly at threshold water potentials, waxy leaves to increase reflection of radiation, reduced stomatal density to limit transpiration, leaf rolling to reduce light interception, survival of protracted drought, premature leaf
Table 5.17. Drought resistance mechanism and traits for plant breeding strategies
Rapid phenological development Development plasticity
Reduction of water losses stomatal resistance(+)
Maintenance of water extraction root depth and density(+)
liquid- phase conductance(+) Drought tolerance
Maintenance of turgor osmotic adjustment cellular elasticity(-) cell size(-)
Tissue water capacitance
Accumulation of solutes
Short biological cycle
Branching/tillering and variation in flower, floret, and panicle
Size, number, and opening of stomata Leaf rolling, smaller and fewer leaves, senescence Leaf pubescence and leaf orientation Thicker and tighter cuticules
More extensive and intensive rooting More or larger xylems in roots and stems
Water potential kinetics
Cell membranes Cell size
Favorable water potential kinetics
Photoplasmic and chloroplast conditions Abscisic acid, ethylene, proline, betaine
Lower total water demand
Lower reduction in seed numbers
Smalter loss surface and less radiation absorbed Higher reflectivity and less radiation Lower transpiration, higher resistance to desiccation Lower transpiration, higher resistance to dissecation
Lower root and soil resistances Lower resistance to water fluxes
Decrease osmotic potential in response to stress Larger changes in volume Increased bound water fraction
(in cell wall) Ability to maintain the daily water balance Maintenance of photosynthesis
Regulation of senescence and abscission
senescence, or maintenance of extension growth under severe stress. These processes have been reviewed . Table 5.17 summarizes this information from the literature from which plant breeders can better plan their strategies.
Genetic variation in WUE has been detected in crop species [33, 51-53], but most crop improvement programs do not emphasize WUE, even though this should be an important trait in water-limited environments. The lack of simple, rapid, and reliable screening criteria and measurement techniques for WUE has greatly restricted progress in this critical area of crop improvement.
Plant characteristics are needed for breeding programs that are associated with WUE and are easier to measure, and thus, more suited to selection .
Highly significant genotype and drought effects on specific leaf weight (SLW = dry weight/unit projected leaf area) have been observed . However, genotype differences in SLW were not associated with differences in WUE. Under drought, the root/shoot ratio was 32% higher than under wet conditions, but genotype differences were not also associated in this ratio with WUE classes.
WUE increases under dry conditions because of decreased water use due to reduced stomatal conductance , whereas the ratio of total plant biomass to leaf area (la) increases because of drought-induced reductions in the rate of leaf expansion and an increase in allocation of carbohydrates to roots. Virgona et al.  reported a similar association between la and WUE for sunflower genotypes under well-watered conditions, and suggested that they both might be positively associated with photosynthetic capacity.
Techniques involving stable C isotopes may provide an efficient method for estimating integrated WUE in tissue of C3 plants . During photosynthetic C assimilation, plants discriminate against 13C, fixing a relatively larger portion of CO2 composed of 12C. Variation in 13C discrimination in C3 plants depends on leaf intercellular CO2 concentration (co) . Data supporting this theoretical relationship have been provided for a broad range of C3 plants and have been recently reviewed . That variation has been shown to be negatively related to WUE in several C3 crop species and has been proposed as a criterion to select for improved WUE in plant breeding programs .
Nonetheless, although such manipulation of plant characteristics remains a goal for plant breeders, the most immediate and dramatic increase in WUE will be achieved through improved crop management.
Comparisons between dryland and irrigated conditions indicate that the yield reduction under dryland conditions can be attributed to a decrease in aboveground biomass production and to a reduction in HI. During the past 100 years, HI of new cultivars has been increased through plant breeding. Essentially, the same total dry matter is produced by new cultivars and, if the growing season has not been shortened, ET remains about the same. The resulting increases in WUE with new cultivars have been mainly from increases in HI. For many crops, HI is not totally controlled genetically but is influenced by environmental factors. This allows the farm manager some responsibility for efficient water use.
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