This review focuses on the effects of water deficits on photosynthesis and partitioning of assimilates at the leaf level. It is now established that the rate of C02 assimilation in the leaves is depressed at moderate water deficits, mostly as a consequence of stomatal closure. In fact, depending on the species and on the nature of dehydration, carbon assimilation may diminish to values close to zero without any significant decline in mesophyll photosynthetic capacity. This remarkable resistance of the photosynthetic apparatus to water deficits became apparent after the measurement of photosynthesis at saturating C02 concentrations was made possible. Whenever light or heat stress are superimposed a decline in mesophyll photosynthesis may occur as a result of a 'down-regulation' process, which seems to vary among genotypes. A major secondary effect of dehydration on photosynthetic carbon metabolism is the change in partitioning of recently fixed carbon towards sucrose, which occurs in a number of species in parallel to the increase in starch breakdown. This increase in compounds of low molecular weight may contribute to an osmotic adjustment. Controlling mechanisms involved in this process deserve further investigation.

Effects of Water Deficits on Carbon Assimilation

Traits that enable plants to exploit low-resource environments (e.g., slow tissue turnover, low transpiration rate, high root: shoot ratio, and high concentrations of plant defenses against pathogens and herbivores) are physiologically linked to key growth-related traits (low rates of photosynthesis, nutrient uptake, and growth). Similar suites of traits occur as both phenotypically plastic and evolutionary responses to stress. We suggest that a genetic change in a switch or underlying trait that turns on this stress resistance syndrome (SRS), which causes it to be expressed over a wider range of environmental circumstances, would effectively convert a high-resource genotype into one that is more stress-tolerant. Because of physiological linkages between growth-related traits and the SRS, any heritable change in a key growth-related trait will pleiotropically affect the SRS. Therefore, heritable changes in these key growth-related traits could be accompanied by evolution of the entire SRS. Evidence for this hypothesis comes from single-gene mutants that differ in many stress-related traits, rapid evolution of metal- tolerant populations that are broadly stress-resistant, and consistent patterns of traits in species along gradients in resource availability. Similar evolutionary patterns occur in many animal taxa, which suggests that it is a general evolutionary phenomenon. We suggest that rapid evolution in response to changing environmental stress may allow many short-lived species to respond to human-induced environmental change and provide opportunities to develop stress-resistant crops. However, the time lag between generations of long-lived species that dominate most natural vegetation may not allow mature individuals of these species to keep pace with rapid global change.

https://www.jstor.org/stable/info/2462710

Evolution of suites of traits in response to environmental stress

Publisher Summary This chapter reviews the characteristics identified for improved drought resistance in two frameworks outlined in Turner (2000) and evaluate the potential of these characteristics to improve breeding efficiency. The first framework, the “Drought-Resistance Framework,” identifies the specific physiological and biochemical characteristics that lead to improved yields in drought-prone environments through drought escape, dehydration postponement, and dehydration tolerance. The second framework “Yield Component Framework” considers yield variation in terms of characteristics affecting water use, water use efficiency, and harvest index. Both frameworks are required if progress is to be made in breeding for drought-prone areas using physiological approaches. In recent years, the development of molecular approaches or methods to identify physiological and biochemical traits in breeding populations potentially has major implications for breeding crops targeted for drought-prone environments, and progress in this area of research is reviewed. However, the focus of this chapter is grain legumes, but it is necessary to draw on studies with cereals and other species to enable critical evaluation of traits and strategies.

Adaptation of grain legumes (pulses) to water-limited environments

Publisher Summary This chapter describes the various aspects of crop water deficits. The water status of a crop plant is usually defined in terms of its water content, water potential, or the components of water potential. The simplicity of measuring water content led to its early adoption, but the diurnal and seasonal changes in dry weight make comparisons of water content at different times of day or during the season unsatisfactory. Water deficits develop inevitably as a consequence of water loss from the leaf as the stomata open to allow the uptake of carbon dioxide from the atmosphere for photosynthesis. There have been developments in both direct and indirect methods of measuring water deficits that have resulted in considerable progress in the field. The use of infrared thermometry for the measurement of crop water deficits and the use of in situ psychrometers for the measurement of water potential respectively is elaborated. It is found that the identification of the root as the site of sensing soil water deficits does not eliminate the role of turgor pressure as the transducer of water deficits, but moves the emphasis from leaf to the root. It is observed that at the whole crop level, the water use efficiency will depend not only on the transpiration efficiency of the leaves, but also on the water loss from the soil and the optimization of yield per unit of water used.

Crop Water Deficits: A Decade of Progress

Mapping QTLs Regulating Morpho‐physiological Traits and Yield: Case Studies, Shortcomings and Perspectives in Drought‐stressed Maize

Selection for high-(H) and low-(L) drought-induced abscisic acid (ABA) accumulation in spring wheat was made from a cross between parents which contrasted in the character. By the F5 generation H and L selections were homozygous for ABA accumulation, which differed between the classes by 80%. The selections were evaluated in field experiments in which the amount of available water was varied.In one experiment, in which plots were automatically sheltered from rain, H selections outyielded L selections by an average 0·36 t/ha both when fully irrigated and when water was withheld between anthesis and maturity. The extra grain yield of the H selections was not produced at the expense of additional water requirement in either treatment. When water was withheld during the 4 weeks preceding anthesis, the grain yields of H and L selections were similar, but H selections used less water between 1 May and maturity. Averaged over all treatments the ratio of grain yield to water use was greater for H selections than for L selections by 0·12 t/ha/100 mm. Withholding water caused a reduction in water use and a corresponding reduction in biomass production in both sets of selections.In two trials in which plots were either irrigated weekly or received natural rainfall only, the ratio of grain yield to water use was again greater for H than for L selections, by an average 0·10 t/ha/100 mm.It is concluded that in spring wheat, selection for high levels of ABA accumulation in response to water stress may provide an opportunity of increasing grain yield and water-use efficiency.

P. Innes

Papers

Some effects of genetic variation in drought-induced abscisic acid accumulation on the yield and water use of spring wheat