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.

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Drought and heat tolerance tests that were developed for sorghum (Sorghum bicolor L. Moench) were adapted to and evaluated in field grown wheat (Triticum aestivum L. and T. durum Desf.) during 1977/1978 and 1978/1979.The drought tolerance test is based on the measurement of the electroconductivity of aqueous media containing leaf discs that were previously water stressed in vitro by exposure to a solution of polyethylene glycol‐6,000 (PEG). The heat tolerance test is similarly based on exposure of leaf discs to heating, in vitro, to 44 C.Drought tolerance of wheat leaves decreased with plant age. For a given plant growth stage, some variation was revealed in drought tolerance, according to leaf position. Maximal separation of wheat cultivars in drought tolerance was obtained with 40% (w/v) solution of PEG, when plants were grown under conditions of favorable moisture regime and sampled during the late jointing growth stage.Wheat was more drought tolerant than maize, sorghum or millet, based on published data.When plants were sampled during a period of water stress, they were more drought tolerant than well‐watered plants, indicating adjustment of cell membrane stability to drought stress. Wheat cultivars varied in their ability to adjust, in this respect.Unlike in sorghum, drought and heat tolerance were not correlated in wheat.The inclusion of a limited number of barley (Hordeum vulgare) and triticale (X Triticosecale Wittmark) genotypes in this study indicated that the methods discussed work equally well with these crop plants.

), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer soft-ware, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) In memory of our daughter Orly who combined science with love for every living thing.vii In my previous book entitled Plant Breeding for Stress Environments (Blum 1988) plant breeding for water-limited environments was only one chapter together with other chapters on heat, cold, mineral deficiency, mineral toxicity and salinity stress. Since that publication several major developments took place regarding plant breeding for stress environments.Firstly, plant molecular biology emerged as a major avenue of research in plant biology and crop science. Plant molecular biology and genomics research towards plant environmental stress has grown exponentially since 1988. The number of published scientific papers specifically on the molecular biology of drought stress and drought resistance increased five-fold from about 10 per year in 1990 to at least 53 per year in 2009. The major dilemma now is how to apply molecular biology and genomics to breeding for stress conditions and specifically for water limited environments. While molecular biologists underline the potential of their discipline and whereas plant breeders underline their needs, there is too much fog lying between the two -which must be cleared up. This book offers to help.Secondly, the plant breeding community realizes now that plant stress must be addressed in plant breeding programs in a dedicated and specific manner. Selecting for yield in diverse environments is not sufficient anymore for coping with stress problems. This approach also became too costly. There is now a growing understanding that breeding for stress conditions and specifically for water limited environments requires specific components within the general breeding program to the same extent that biotic stress resistance is approached. Qualified information on how to design and perform a breeding program component for water limited environments is in great demand.Thirdly, concerns about the agricultural implications of climate change grew exponentially since the early 1990s to the extent that the phenomenon is now being aggressively addressed in agricultural research. Even the optimists agree that climate change cannot be completely reversed and at best it could only be slowed. The impact on crop production can already be seen by increased aridity and warmer temperatures in some regions and vicious storms and floods elsewhere. Global warming is an additional serious engine of plant str...

Osmotic adjustment (OA) and cellular compatible solute accumulation are widely recognized to have a role in plant adaptation to dehydration mainly through turgor maintenance and the protection of specific cellular functions by defined solutes. At the same time, there has been an ongoing trickle of skepticism in the literature about the role of OA in supporting crop yield under drought stress. Contrarian reviews argued that OA did not sustain turgor or that it served mainly for plant survival rather than productivity. This critical review examined 26 published studies where OA was compared with yield under drought stress in variable genotypes of 12 crops, namely, barley, wheat, maize, sorghum, chickpea, pea, pigeon pea, soybean, canola, mustard, castor bean and sunflower. Over all crops a positive and significant association between OA and yield under drought stress were found in 24 out of 26 cases. Considering that it is generally difficult to find a singular plant trait responsible for yield advantage of numerous crops under different drought stress conditions, this evidence is no less than remarkable as proof that OA sustains crop yield under drought stress.

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