PRODUCTION OF WASHINGTON NAVEL ORANGE IN A DESERT CLIMATE.

Yields of Washington Navel orange [Citrus sinensis (L.) Osbeck] under Arizona desert conditions are low, due mostly to a low number of harvested fruit and secondly to small sized fruits. To determine at which development stage yield reduction occurred, flowering branches were tagged at mid-bloom and the leaves and flowers were counted. The percent flowers shed at certain times was then recorded. In addition, 27 years yield data for the Arizona-California and Central California production regions were examined in relation to temperature and relative humidity to identify factors causing low yields in the desert climate. It was found that a low number of fruit was due to a low percentage fruit set, while fruit shedding during June drop contributed somewhat less to reducing the number of harvested fruits. A lower percentage of fruits were lost from Arizona Navel orange trees during and after June drop than reported for trees in Florida and California. This resulted in a higher percentage of initial flowers becoming mature fruit, even though there were fewer harvested fruits per tree in Arizona. It was concluded that Navel orange trees growing in Arizona have relatively fewer flowering branches. A correlation between yields and temperatures for 27 years showed that mean and maximum temperatures during the period from June prior to flowering through June after flowering had a highly significant, adverse effect on yields of Arizona grown trees, but not on yields of California grown trees. Relative humidity had no observable effect on yield. In addition, low yields of Navel orange trees in Arizona was disassociated with soil salinity. Low leaf to flower ratios (L/F) had a significant effect on fruit set only if L/F was 1.0 or less. However, the effects of L/F were insignificant to the number of harvested fruits. Foliar applications of potassium nitrate (KNO₃) for two successive years significantly increased yield. Gibberellic acid (GA₃) did not increase fruit set and harvested yields. Benzyladenine (BA) combined with overhead sprinklers failed to reduce the number of fruit shed during June drop. Transpiration and photosynthesis rates for both Navel and Valencia orange trees were low under Arizona conditions