Abstract:
An important part of developing sound water management strategies for crops is to identify phenological stages which are sensitive and insensitive to water stress in terms of yield and quality. This is particularly important for regions which are prone to droughts and where water resources are limited. As pecans are largely produced in semi-arid regions in South Africa and are reported to have very high annual water requirements, this information is particular important for this valuable crop. This study therefore attempted to evaluate how water stress at different phenological stages (flowering and nut set, nut sizing, nut filling and shuck dehiscence) impacts pecan (Carya illinoinensis Wangenh. K.Koch) yield and quality. A trial was conducted in a 12-year-old pecan orchard at Innovation Africa @UP over three consecutive seasons. Measurements included several plant physiological responses to quantify the level of plant stress throughout the growing season and yield and quality measurements at the end of each season. Water stress (midday stem water potential <-0.9 MPa) led to a decline in stomatal conductance, photosynthesis, transpiration, and growth (especially during bud break). At the flowering and nut set stage, water stress led to significant flower abortion and nut fall, which reduced the final number of nuts by 60%, which ultimately reduced the final yield compared to well-water control. Water stress at nut growth stage, did not necessarily reduce the final yield at this stage, but rather reduced the nut size (many smaller nuts), thereby compromising the nut quality and income. Water stress at the nut filling stage significantly reduced both the yield and nut quality, reducing the nut mass, due to a high percentage of wafers/air pockets as a result of poor nut filling. Stress during the final phenological stage, shuck dehiscence, did not have a major impact on final yield or quality, but there was an increase in the number of stick-tights. However, over the three seasons the increase in the number of stick tights did not always translate into a significant reduction in yield when compared to the control. Therefore, in seasons where water allocations are reduced, it may be possible to make some water savings during nut sizing and shuck dehiscence without compromising yield. However, some reduction in quality and income may result if the trees are stressed during these stages.
