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

Water scarcity imposes huge reductions in crop yield and isone of the greatest limitations to crop expansion outside present-dayagriculture areas. Because the scenarios for global environmentalchange suggest a future increase in aridity and in the frequencyof extreme events in many areas of the earth (IPCC, 2001Go), irrigationand the use of appropriate crops is an important issue worldwide.Nowadays, approximately 70% of the global available water isemployed in agriculture and 40% of the world food is producedin irrigated soils. Some irrigation (around 10%) uses waterfrom aquifers, leading to many underground water tables beingexploited unsustainably (Somerville and Briscoe, 2001Go).

It is now recognized that fine-tuning irrigation can improvecrop water-use efficiency, allowing a more precise use of waterand, at the same time, having a positive impact on the qualityof the products. Similarly, modern biotechnology offers newtools for agricultural improvement and sustainability. Whereasthe main advances in agriculture during the 1960s were designedfor favourable environments, today, crop performance for sub-optimalenvironments and marginal lands which were bypassed by the ‘greenrevolution’ are also being addressed. In recent decades,physiological and molecular bases for plant responses to drought,and concurrent stresses, such as high temperature and irradiance,have been the subject of intense research (see reviews by Chaveset al., 2003Go; Flexas et al., 2004aGo).

Plant water deficits may occur as a consequence of a seasonaldecline in soil water availability, developing in the long term,or may result from drought spells. An increased evaporativedemand of the atmosphere, occurring mostly on a daily basis,affects total carbon gain by the crops, even irrigated ones.The timing, intensity and duration of stress episodes are pivotalto determine the effects produced by drought. Plant strategiesto control water status and resist drought are numerous (Schulze,1986Go). In general, genotypes native from climates with markedseasonality are able to acclimate to the fluctuating environmentalconditions, enhancing their efficiency for those conditions(Pereira and Chaves, 1993Go, 1995Go). In the case of slowly developingwater deficits, plants may also escape dehydration by shorteningtheir life cycle. In the case of rapid dehydration, oxidativestress developing as a secondary effect is potentially verydamaging to the photosynthetic machinery (Ort, 2001Go). The capacityfor energy dissipation (Flexas et al., 2002Go) and metabolic protection(induced or constitutive) against the damaging effects of reactiveoxygen species (Foyer and Noctor, 2003Go) is a key element forthe success of plants under drought. Tissue tolerance to severedehydration is not common in most higher plants, including crops,but do arise in species native from extremely dry environments(Ingram and Bartels, 1996Go). Understanding the mechanisms underlyingthose different responses can support the design of new managementtools and genotypes for modern precision agriculture.

It is well known that a major effect of decreased water availabilityis diminished leaf carbon fixation (A) due to stomatal closure,which may start at moderate plant water deficits. At the wholeplant level, total carbon uptake is further reduced due to theconcomitant or even earlier inhibition of growth. It has beenshown that cell division and expansion are directly inhibitedby water stress (Zhu, 2001aGo). Slower growth has been suggestedas an adaptive feature for plant survival under stress, becauseit allows plants to divert assimilates and energy, otherwiseused for shoot growth, into protective molecules to fight stress(Zhu, 2002Go) and/or to maintain root growth, improving wateracquisition (Chaves et al., 2003Go). This feature may be relevantfor crops intended for drought-prone areas, but inconvenientfor regions where only mild and sporadic stress is likely tooccur. On the other hand, the ability to accumulate (and lateron remobilize) stem reserves is likely to be an important characteristicto maintain reproductive growth under water deficits in variousspecies, like cereals and some legumes (Blum et al., 1994Go).

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