Crop yield response to water - Cra

Stage III was the most stress sensitive period of the season for pistachio (Goldhamer et al., 1991).Of the numerous pistachio yield components, it is remarkable that tree nut load was unaffectedby any of the nine deficit irrigation treatments imposed, including dryland conditions duringStage I, Stage II, Stage III, and postharvest and the various Stage II and postharvest stresscombinations (Goldhamer and Beede, 2004). When averaged over the last two years of theirfour-year study, tree nut load ranged from 10 900 to 12 300 with the fully irrigated treesaveraging 11 500 nuts per tree (Goldhamer and Beede, 2004). This suggests that there wasenough preformed shoot growth very early in the season, even with Stage I dryland conditions,to produce the number of nodes (fruiting positions) necessary to support a full crop and thatstored winter rainfall (200 mm per year) was sufficient to support this growth. This ability ofthe pistachio tree to produce equal fruit loads under a variety of stress regimes highlights theimportance of the preformed shoot growth from mid-April to mid-May; a period when treeswould normally rely on stored winter rainfall rather than irrigation. Indeed, the early work ofSpiegel-Roy et al. (1977) found that 54 to 163 mm of annual precipitation was sufficient fordryland trees to differentiate enough flower buds to obtain appreciable yields.Effects on alternate bearingKanber and others observed that a long duration of water stress aggravated alternatebearing and suggested that irrigation could alter periodicity, presumably by making morecarbohydrates available during peak carbon demand periods (Kermani and Salehi, 2006).Goldhamer found that Stage I stress during an ‘on’ year (shown as 2004 in Figure 4) resultedin more than a three-fold increase in fruit load the following season (the subsequent ‘off’year) relative to fully irrigated trees. The mechanisms of why this happened are unknown andit should be emphasized that the early season stress was possible only because winter rainfallwas abnormally low. The following season, the winter rainfall eliminated any Stage I stress butthe fruit loads of this RDI regime were 25 percent lower than the fully irrigated trees (Figure4). This pattern continued in the succeeding season when this RDI regime had a fruit load 25percent higher than those under full irrigation. It appears that regardless of why there arehigher yields in a normally ‘off’ year, the one time higher yields can alter the alternate bearingpattern for the following years.Crop load also influences the impact of deficit irrigation on the various yield components ofpistachio. This was observed when dryland conditions with Kerman on Atlantica rootstock wereimposed in both ‘off’ and ‘on’ alternate bearing years (Figure 5). All the yield components, withthe exception of harvestability, were more negatively impacted in the ‘on’ year. Harvestabilitywas higher in the ‘on’ year only because entire rachises, rather than individual nuts, wereremoved from the tree with the mechanical shaking. Thus, growers can anticipate greaternegative impacts of serious droughts during ‘on’ versus ‘off’ alternate bearing years. In fact, apossible management strategy, with very limited water supplies under microirrigation, wouldbe to cutoff irrigation to the trees with low fruit loads (those in the ‘off’ year), making thatwater available for the trees in the orchard with the high fruit loads.Indicators of tree water statusThe established method to quantify water stress for pistachio is to measure water potentialwith a pressure chamber. Although the standard method is to measure stem-water potentialPISTACHIO 423