Crop yield response to water - Cra

high sensitivity level, as discussed in the different crop sections. It should be emphasized thatduring the first years of the orchard or vineyard, canopy expansion is the most importantprocess leading to high productivity. As the plantation reaches maturity, the importance ofwater deficits that affect vegetative growth is reduced. Another important response to waterdeficits is accelerated leaf senescence. In some species such as almonds, this response is quitemarked, while in others it requires severe water deficits to be detectable, as for olives.Carbon assimilation of plants depends on CO 2 uptake through stomata, which responds towater deficits by partially closing. The behaviour of stomata in many fruit trees follows apattern that maximizes CO 2 uptake per unit water loss. However, stomatal regulation may alsoinvolve trade-offs between maximizing CO 2 uptake per unit water loss (which may requirestomatal closure at times of high evaporative demand) and tolerance to heat stress (whichrequires high stomatal conductance to allow for evaporative cooling). Different patterns ofstomatal behaviour are shown in Box 16 for several tree species. Stomatal conductance ishighest during the morning when the vapour pressure deficit is low and declines to a plateauat midday when VPD is high (Box 16, C). This pattern has been observed in many fruit treespecies but is different in others, such as apple and in grapevines (Box16, A) where stomatastay wide open for most of the day if the plant is well supplied with water.It has been thoroughly documented that stomatal conductance and the rate of CO 2 assimilationper unit leaf area of the major field crops are not as sensitive as is vegetative growth to waterdeficits. Significant water deficits are required in most annual crops to reduce stomatalbox 16 Examples of diurnal patterns of stomatal conductanceThe graph below shows A) wide open stomata with a small midday depression, typical ofapple or grapevines under ample water supply; B) continuous decline of stomatal openingfollowing the increase in VPD, typical of citrus; C) Morning peak with substantial middaydepression, typical of many deciduous fruit species, and of the olive; D) same pattern asin C but when the trees are under significant water stress. The relative scale may differfor the different patterns.RelativestomatalconductanceBACDHour of day282crop yield response to water