As the result of intensive research and breeding efforts over the last 20 years, the yield potential and yield quality of cereals have been greatly improved. Nowadays, yield safety has gained more importance because of the forecasted climatic changes. Drought and high temperature are especially considered as key stress factors with high potential impact on crop yield. Yield safety can only be improved if future breeding attempts will be based on the valuable new knowledge acquired on the processes determining plant development and its responses to stress. Plant stress responses are very complex. Interactions between plant structure, function and the environment need to be investigated at various phases of plant development at the organismal, cellular as well as molecular levels in order to obtain a full picture. The results achieved so far in this field indicate that various plant organs, in a definite hierarchy and in interaction with each other, are involved in determining crop yield under stress. Here we attempt to summarize the currently available information on cereal reproduction under drought and heat stress and to give an outlook towards potential strategies to improve yield safety in cereals.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2007.01727.x

The effect of drought and heat stress on reproductive processes in cereals.

Publisher Summary This chapter evaluates traits for improving crop yield in water-limited environments. The chapter describes the components of yield and the determinants of survival against which the proposed and demonstrated contributions by traits are critically assessed. Both modern and subsistence agriculture are differentiated mainly by the degree of risk that can be tolerated. Many traits have been proposed for improving the performance of drought-affected crops. Putative traits are more effective in enhancing drought resistance and contributing to grain yield in water-limited environments. The potential value of a trait depends upon the crop and the moisture environment in which it is grown. The chapter presents simulation models, which are very powerful tools for critically assessing the value of putative traits. However, more work is needed in the development and testing of suitable models, and in their application for this purpose. Use of near-isogenic populations as opposed to isogenic lines also appears to hold great promise.

A critical evaluation of traits for improving crop yields in water-limited environments.

This presentation is a concept review paper dealing with a central dilemma in understanding, designing, and acting upon crop plant improvement programs for drought conditions. The association among yield potential (YP), drought resistance (DR), and water-use efficiency (WUE) is often misunderstood, which in turn can lead to conceptual oversight and wrong decisions in implementing breeding programs for drought-prone environments. Although high YP is the target of most crop breeding programs, it might not be compatible with superior DR. On the other hand, high YP can contribute to yield in moderate stress environments. Plant production in water-limited environments is very often affected by constitutive plant traits that allow maintenance of a high plant water status (dehydration avoidance). Osmotic adjustment (OA) is a major cellular stress adaptive response in certain crop plants that enhances dehydration avoidance and supports yield under stress. Despite past voiced speculations, there is no proof that OA entails a cost in terms of reduced YP. WUE for yield is often equated in a simplistic manner with DR. The large accumulation of knowledge on crop WUE as derived from research on carbon isotope discrimination allows some conclusions on the relations between WUE on the one hand, and DR and YP on the other, to be made. Briefly, apparent genotypic variations in WUE are normally expressed mainly due to variations in water use (WU; the denominator). Reduced WU, which is reflected in higher WUE, is generally achieved by plant traits and environmental responses that reduce YP. Improved WUE on the basis of reduced WU is expressed in improved yield under water-limited conditions only when there is need to balance crop water use against a limited and known soil moisture reserve. However, under most dryland situations where crops depend on unpredictable seasonal rainfall, the maximisation of soil moisture use is a crucial component of drought resistance (avoidance), which is generally expressed in lower WUE. It is concluded that the effect of a single 'drought adaptive' gene on crop performance in water-limited environments can be assessed only when the whole system is considered in terms of YP, DR, and WUE.

Drought resistance, water-use efficiency, and yield potential-are they compatible, dissonant, or mutually exclusive?

Drought is one of the greatest limitations to crop expansion outside the present-day agricultural areas. It will become increasingly important in regions of the globe where, in the past, the problem was negligible, due to the recognized changes in global climate. Today the concern is with improving cultural practices and crop genotypes for drought-prone areas; therefore, understanding the mechanisms behind drought resistance and the efficient use of water by the plants is fundamental for the achievement of those goals. In this paper, the major constraints to carbon assimilation and the metabolic regulations that play a role in plant responses to water deficits, acting in isolation or in conjunction with other stresses, is reviewed. The effects on carbon assimilation include increased resistance to diffusion by stomata and the mesophyll, as well as biochemical and photochemical adjustments. Oxidative stress is critical for crops that experience drought episodes. The role of detoxifying systems in preventing irreversible damage to photosynthetic machinery and of redox molecules as local or systemic signals is revised. Plant capacity to avoid or repair membrane damage during dehydration and rehydration processes is pivotal for the maintenance of membrane integrity, especially for those that embed functional proteins. Among such proteins are water transporters, whose role in the regulation of plant water status and transport of other metabolites is the subject of intense investigation. Long-distance chemical signalling, as an early response to drought, started to be unravelled more than a decade ago. The effects of those signals on carbon assimilation and partitioning of assimilates between reproductive and non-reproductive structures are revised and discussed in the context of novel management techniques. These applications are designed to combine increased crop water-use efficiency with sustained yield and improved quality of the products. Through an understanding of the mechanisms leading to successful adaptation to dehydration and rehydration, it has already been possible to identify key genes able to alter metabolism and increase plant tolerance to drought. An overview of the most important data on this topic, including engineering for osmotic adjustment or protection, water transporters, and C4 traits is presented in this paper. Emphasis is given to the most successful or promising cases of genetic engineering in crops, using functional or regulatory genes. as well as to promising technologies, such as the transfer of transcription factors.

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Mechanisms underlying plant resilience to water deficits: prospects for water-saving agriculture

Plant breeding and drought in C3 cereals: what should we breed for?

The grain filling of wheat (Triticum aestivum L.) is seriously impaired by heat stress due to reductions in current leaf and ear photosynthesis at high temperatures. An alternative source of carbon for grain filling is stored stem reserves. Two spring wheat cultivars (V5 and V2183) of very similar phenology and plant stature, which had previously been found to differ in grain shrivelling under drought and heat stress conditions in the field, were used to evaluate the hypothesis that the mobilisation of stored stem reserves into the growing grain is an important source of carbon for supporting grain filling under heat stress. In two experiments in Israel (1990 and 1991), the rates of stem dry matter (DM) and stem total non-structural carbohydrates (TNC) loss, grain growth and leaf senescence were monitored under optimal (control) and high (stressed) temperatures in the glasshouse (1990) and the growth chamber (1991). Cultivar V5 always sustained a smaller reduction in grain dry weight under heat stress, than V2183. Irrespective of temperature, V5 had a higher stem DM and TNC content at the onset of grain filling, greater depletion of stem dry matter (or TNC) during grain filling, and longer duration of grain filling, than V2183. During grain filling V5 generally exported about two to three times more DM from the stems than V2183, under both non-stressed and stressed conditions. On the other hand, V5 was more heat-susceptible than V2183 in terms of leaf longevity, in vivo chlorophyll stability and grain abortion under heat stress. In a third experiment (1992) five cultivars (including V5 and V2183) were subjected to chemical desiccation (0.3% potassium iodide) of the canopy in the field in order to destroy the photosynthetic source ofthe plant after anthesis. The same cultivars were subjected to heat stress (35/25oC) or non-stressed (25/15oC) conditions after anthesis in the growth chamber. It was found that grain dry weight reduction by chemical desiccation was highly correlated with grain dry weight reduction by heat stress (r2 = 0.89). Therefore, the superior capacity of V5 for grain filling from mobilised stem reserves is a consti- tutive trait which supports grain filling under heat stress which can be tested for by chemical desiccation of plants under non-stressed conditions.

G. Golan

Papers

Stem Reserve Mobilisation Supports Wheat-grain Filling Under Heat Stress

Yield stability and canopy temperature of wheat genotypes under drought-stress

Chemical desiccation of wheat plants as a simulator of post-anthesis stress: II. Relations to drought stress

I. Effects of translocation and kernel growth

The effect of dwarfing genes on sorghum grain filling from remobilized stem reserves, under stress