Fruit set is the major determinant of productivity in almond [Prunus dulcis (Mill D,A, Webb)] where seed is the commercial product, Boron influences flowering and fruit set in a number of crops, but little has been reported on this subject in almond, Here, we investigated the effect of a fall foliar application of B on fruit set and tissue B concentration in open pollinated 'Butte' and 'Mono' almond over a 2-year period, Early fall B application significantly increased the vegetative, floral, and fruit tissue B concentration in the subsequent year, The greatest increase in organ B concentration was observed in flower buds, flowers, and hulls, Recent work has demonstrated that B forms a B-sorbitol complex in Prunus species, This B-sorbitol complex is phloem mobile and is transported to sink organs, Here we demonstrate that fall-applied B is absorbed by the leaf and is subsequently transported (presumably as the B-sorbitol complex) to floral buds where it is available to flowers and, hence, influences fruit set and yield, It is concluded that fall foliar-applied B is a useful fertilization strategy that can be used to optimize tissue B concentration in species in which B is phloem mobile, Boron applied at 245 and/or 490 ppm significantly increased fruit set in 'Butte' and 'Mono' and increased yield of 'Butte' in 1991 on open-pollinated trees (yield was not determined in 1993), The highest initial and second fruit set was associated with either the 245 or 490 ppm B treatments in both cultivars and both years, This increased fruit set resulted in yield increases of 53% and 4%, respectively, for 'Butte' and 'Mono', Of the two cultivars,'Butte', which had lower tissue B concentration before B application, responded more significantly in yield to B application, Application of 735 ppm B was less effective than either 245 and 490 ppm B; however, no visual symptoms of excess B mere observed.
