Crop yield response to water - Cra

Summer stressDuring this period of cell expansion in the fruit, deficit irrigation can reduce the fruit growth rate.In fact, severe stress can result in fruit shrinkage. However, reintroduction of full irrigation canincrease the fruit growth rate, such that harvest fruit size is unaffected (Goldhamer and Salinas,2000). This has been attributed to the maintenance of dry matter accumulation in the fruit duringthe stress period and/or full rehydration of the fruit upon reirrigation. Under the experimentalconditions on the east coast of Spain (Gonzalez-Altozano and Castel, 1999), summer stress hadno impact on clementine yield as long as predawn leaf water potential (LWP) was not reducedbelow -1.3 MPa. However, severe stress for a prolonged duration during the summer can reduceharvest fruit size, tree growth and increase sugars (Mantell, 1977). Harvest sugar concentrationcan increase because of summer stress, presumably the result of fruit dehydration. Of the twoprimary yield components, fruit size is more susceptible to reduction than fruit load as a resultof summer stress. Working with cv. Valencia, it was found that moderate stress in the summerand fall did not reduce yields in Arizona (Hilgeman and Sharp, 1970).It should be noted that the imposition of severe stress during the summer has been usedwith lemons to induce an off-season bloom and resulting summer harvest fruit. This is knownas the Forzatura technique or the Verdelli effect. In one study that withdrew irrigation fornine weeks starting in June, predawn LWP reached - 2.7 MPa (Barbera and Carimi, 1988).This regime yielded 30 kg/tree in each of the summer and winter harvests. They found thatinsufficient stress resulted in the lack of off-season flowering but that when stress was toosevere, it produced excessive leaf drop, higher flower abortion, winter fruit drop and reducedwinter harvest fruit quality (Barbera and Carimi, 1988).Autumn stressAs opposed to stress-induced reductions in fruit growth being overcome upon thereintroduction of full irrigation, reducing fruit growth in the autumn usually results in smallerfruit at harvest. Irrigation of clementines at 25 and 50 percent ET c during late summer/earlyfall reduced harvest fruit size by 11 and 25 percent, respectively (Gonzalez-Altozano andCastel, 1999) and produced more fruit peel creasing, as has been found with navel oranges(Goldhamer and Salinas, 2000). Both studies found no influence of autumn stress on fruit load.In addition to reducing fruit size, numerous studies have shown that autumn stress can increaseboth the sugar and acid content of the fruit as well as increase the peel thickness.Winter stressIn most citrus producing areas of the world, winter soil water deficits are unlikely to occurbecause of rainfall. However, in Florida, water restrictions applied to Valencia oranges, duringthe winter were used to modify the timing of flowering, delaying bloom dates by two to fourweeks (Melgar et al., 2010). The fruit were able to overcome a fruit size reduction duringthe following spring when irrigation was restored to full crop water needs. This was becauseValencia is a very late-season cultivar and fruit had three to four months to recover to theoptimum size. In this case, winter stress was used to reduce immature fruit drop for the nextseason’s crop during mechanical harvesting with trunk shakers.320crop yield response to water