AMMONIA AND/OR ITS METABOLITES INFLUENCE FLOWERING, FRUIT SET, AND YIELD OF THE `WASHINGTON' NAVEL ORANGE

Abstract

A cause and effect relationship between tree ammonia status and floral intensity was established for Citrus sinensis L. cv. Washington navel orange. Artificially raising tree ammonia content by foliar application of low-biuret urea during or at the end of a minimal stress treatment increased leaf ammonia content and both the number of floral shoots and flowers per shoot, but did not increase vegetative shoot production. Apical flowers initiated in response to stress exhibited maximum tissue concentrations of ammonia and putrescine and activity of the de novo arginine biosynthetic pathway one week after the end of the stress treatment. All three criteria decreased in parallel as flowers developed through petal fall. Foliar-applications of low-biuret urea during early bloom increased ovary putrescine content and fruit set. Winter foliar applications of low-biuret urea to commercially-producing, nitrogen-sufficient `Washington' navel orange trees just prior to or during flower initiation increased yield by approximately one packing carton (17 kg) per tree in three successive years without a reduction in fruit size. Winter application of low-biuret urea was cost effective.