Crop yield response to water - Cra

ox 15 Evolution of soil water under deficit irrigation.If irrigation is applied at rates below the ET c , soil water deficits develop. Such deficitsmay not be detrimental unless they reach levels that negatively impact the orchard orvineyard. When microirrigation is applied with a fixed rate, water budget schedulingkeeps track of the soil water reservoir and facilitates the safe extraction of part of thisreservoir, making the best use of the stored soil water. However, note that, in areas oryears where seasonal rainfall is insufficient to wet the potential root zone, the soil waterreservoir with microirrigation may be much smaller than under full-coverage irrigation.Thus, deficit irrigation will manifest crop water stress sooner with microirrigation underthose specific conditions. On the other hand, since irrigation can be applied frequently,it is easier to overcome the onset of detrimental stress with frequent applications undermicroirrigation, provided the system has enough capacity. The graph below shows theseasonal evolution of soil water under deficit irrigation, with both conventional andmicroirrigation systems. In both cases, the applied water is less than the ET c and the soilwater reservoir is being depleted, but the threshold is not reached.100IrrigIrrigIrrigIrrigTimeRoot zonewater content(relative)IrrigConventional systemMicroirrigationsystemAD threshold0develops, any organ that may be expanding at that time, be it a leaf, a fruit, a branch, or thetrunk, slows its rate of growth. This is because high rates of expansion require high internalpressure inside the growing cells, or turgor, which is directly dependent on the water statusof the tissue. The high sensitivity of shoot growth to water stress has important implicationsfor tree irrigation; on the one hand, it may be desirable in some cases to reduce the growth ofvegetative shoots relative to their potential growth under unlimited water supply. On the otherhand, if large fruit size is an important factor in determining growers’ revenues, water stressmust be completely avoided during the period of fast fruit expansion. There is evidence fordifferent species that the various growth processes, shoot initiation, shoot extension, leaf andfruit growth, and trunk growth all have differential sensitivity to water stress within a generallyYield Response to Water of Fruit Trees and Vines 281