Fruit Set of ‘Italian’ Prune Following Fall Foliar and Spring Boron Sprays1

Abstract

A pre- or postharvest foliar B application was found to increase fruit set of ‘Italian’ prune (Prunus domestica L.). A prebloom B spray failed to increase set. Neither fall nor spring applications influenced the amount of fruit lost in the midsummer or “blue” drop. All trees involved in the experiment had adequate B by the standard index of tree nutrition, August mid-shoot leaf analysis. Incipient B deficiency did not appear to be involved. Fall foliar B increased B levels in dormant bud and spur tissues and in flower buds and flowers. A prebloom B spray increased B levels of floral tissues to a lesser degree. The highest B concentration was found in the ovary. Boron concentration in flower buds in April following a fall B spray was as much as five times the amount found in mid-shoot leaves in August. August mid-shoot leaf analysis revealed higher levels in leaves from trees treated the previous fall in only one of the 2 years. Several morphological and physiological effects of the fall B spray were observed. Among these were a slight delay in the time of bud break, a decrease in the size of flower buds and mature flowers accompanied by reduction of style and pedicel length, and a decrease in pollen germinability. B level of pistil and pollen had no effect on in vivo pollen tube growth rate.

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

“…The 30% increase in fruit set induced by B in 1983 was smaller than the response seen in other work using similar fall B sprays (4,5). The improvement in set was first seen in late May, indicating that B affected a process of fruit set or early fruit development and not mid-summer fruit drop.…”

“…Since flowers on excised branches forced indoors accumulated as much B (Table 4) as flowers developing on intact trees (Table 3), the B content of prune flowers is likely derived from adjacent branch tissues. Callan et al (4) suggested that dormant flower buds and adjacent spurs may supply the B required for spring growth and fruit set. Factors which affect the accumulation of B reserves in branches also may affect the amount available for flower development the next spring.…”

“…Flowers were tagged at anthesis, and 5 pistils were collected from each tree at 4, 7, 10, 15, and 19 days after anthesis. The length of the longest pollen tube in each style was measured at described by Callan et al (4). Composite tissue samples were collected from each tree for B analysis, including dormant buds and spur tissue (28 Jan. 1983), flowers and pistils (1 Apr.…”

Foliar-applied Boron Increases Fruit Set in ‘Barcelona’ Hazelnut

“…Foliar boron (B) applications have been reported to increase fruit set in some species of fruits (4)(5)(6)9). Fruit set of 'Italian' prune trees that were not considered B-deficient according to leaf analysis standards was increased when B sprays were applied (7,8). In a 2-year study of 'Italian' prune trees, also not considered B-deficient, Hanson and Breen (12) reported inconsistent fruit set response from sprays applied in September.…”

“…The timing of B application may be important. In prunes, prebloom B sprays were ineffective, whereas positive responses to postharvest sprays were observed (7,8). The stage of flower development at which B was applied was critical for inducing a higher set in 'Leconte' pears (1) and in apples (10).…”