Effects of Fall Boron Sprays and Environmental Factors on Fruit Set and Boron Accumulation in ‘Italian’ Prune Flowers

Foliar B sprays (500 ppm) applied in the fall to ‘Italian’ prune ( Prunus domestica L.) trees had no effect on fruit set in a warm spring when set was high (average, 12.2%), but increased fruit set by 32% in a cool spring when set was low (3.2%). Fall B sprays increased flower B levels and decreased pistil length, but had no effect on the rate of pollen tube growth through styles in the cool spring. Calcium sprays (0.5 m ) applied in the fall prior to the cool spring did not affect fruit set, pistil length, pollen tube growth or flower Ca or B levels. Flowers on cut branches forced indoors accumulated as much B as those on intact trees, indicating that flowers are supplied B from reserves in nearby branches. Simulated rain did not leach B from flower buds readily. Flowers blooming on excised branches in high relative humidity (86%) contained 13% less B per flower than flowers in low humidity (29%). The amount of set and B concentrations in flowers in a given year may influence the response to fall applied B.