WINTER IRRIGATION INCREASED YIELD OF WASHINGTON NAVEL ORANGE (CITRUS-SINENSIS L OSBECK)

The present study aimed to quantify the effect of withholding irrigati on during the winter season in southern California on the productivity of 'Washington' navel orange and to determine whether the additional expense of irrigating navel orange trees during the winter is a cost-e ffective production management strategy. Yield and fruit size were qua ntified for 30 year old 'Washington' navel orange scions on Troyer cit range rootstock which were rain-fed from October 1 to March 1 in each of three successive years and for another set of trees which were irri gated during the winter. Supplementing winter rainfall with irrigation significantly increased the weight of fruit per tree in all three yea rs of the study (45 +/- 17 kg fruit tree(-1), n = 3 years) and number of fruit per tree in the two years in which the rain-fed, -winter irri gation trees had significantly lower predawn water potentials than the +winter irrigation trees. Despite the yield increases which resulted from supplementing winter rain with irrigation in each year of the stu dy, there was no reduction in the number of commercially valuable frui t with tranverse diameters between 7.0 to 8.0 cm in any year (P less t han or equal to 0.05). Trees receiving winter irrigation to supplement rainfall were less affected by a preharvest freeze: compare a 50% red uction in yield from the previous year for the +winter irrigation trea tment to a 93% reduction for the -winter irrigation trees. Irrigation treatment did not affect tree nitrogen status. Even in the year when t he lowest net increase in yield was obtained with the greatest amount of irrigation water, valued at a high cost for California, winter irri gation was a cost-effective management strategy for the production of 'Washington' navel orange.