Crop yield response to water - Cra

size as affected by soil water deficit, and stomatal control plays a minor role. After anthesis,the leaves are fully grown and consequently the control of transpiration is more dependentupon stomatal closure and the extent of canopy senescence. Periods of water deficit at anygrowth stage can cause canopy senescence with subsequent reduction in seed yield. Thereis genetic variability in the response of sunflower genotypes to water deficits. Long-seasongenotypes have greater canopy cover and produce more biomass under drought conditions,because of their ability to extract more water from the subsoil (Gimenez and Fereres, 1986).Also, it has been observed that sunflower cultivars adapted to an arid climate are less sensitiveto water stress than the cultivars developed for humid climates.Under water stress, the time between planting and flowering remains relatively constant, andinflorescence initiation is also relatively insensitive to water stress. Subsequent developmentof the inflorescence is, however, affected by water deficits and water stress reduces thenumber of flowers. The entire flowering period is thus the most sensitive to water deficits asthe number of seeds may be negatively affected. Seed filling following flowering is the nextmost sensitive period to water deficits that reduce both seed weight, seed number (because ofabortion) and oil content. Therefore, the reproductive stages (flowering and ripening stages)are more sensitive to water stress than the vegetative stages. Maintenance of green leaf areaand photosynthesis after anthesis is key, as seed weight and oil content are mostly influencedby intercepted radiation and carbon assimilation during seed filling. During this phase, bothstomatal closure and leaf senescence play a role in the control of plant water status. On theother hand, there is the associated cost of reduced CO 2 assimilation. The contribution thatcarbon fixed before anthesis makes to grain-filling of sunflower may be important underwater stress after anthesis. Nevertheless, the harvest index of sunflower is often negativelyaffected by water deficits during the reproductive phase.Genotypes having a gradual response to water stress may be most suited to environmentswith severe water deficits. In environments with short, frequent and moderate soil waterdeficits alternating with well-watered periods, maintaining organ expansion and biomassproduction would result in better agronomic performance. Several studies of sunflower havedemonstrated that there is a close relationship between canopy size at flowering and cropseed yield. The plasticity of the crop, in terms of adapting leaf area development to wateravailability, is well known.Sunflower is moderately tolerant to salinity, being unaffected by soil salinity up to 4.8 dS/m. Itis currently cultivated in dry areas where salinity can be a threat. Thus, it could be adapted tosalt-affected soils, provided irrigation management is adequate and salt leaching is practised.In all cases, leaching is important to limit salt accumulation in the root zone, avoiding thereduction of seed and oil yield under salt stress. It would be feasible to use moderately salinewater to irrigate sunflower. Nevertheless it is necessary to maintain high soil water content inthe root zone all the time, and to minimize salinity built up by leaching.The nutrient content extracted by a sunflower crop producing around 1 tonne of seed yieldper hectare includes around: 50 kg/ha of N, 15 kg/ha of P 2 O 5 , and 35 kg/ha of K 2 O. Fertilizerapplication depends on the expected yields and the residual nutrients, varying from 20 to 140kg/ha of N, 15 to 70 kg/ha of P 2 O 5 , and 15 to 150 kg/ha of K 2 O.168crop yield response to water