RETENTION AND THE KINETICS OF UPTAKE AND EXPORT OF FOLIAGE-APPLIED, LABELED BORON BY APPLE, PEAR, PRUNE, AND SWEET CHERRY LEAVES

Leaf retention, uptake kinetics, total uptake (per unit leaf area), ex port kinetics, and the total export of foliage-applied, labeled B (B-1 0-enriched boric acid) were determined for apple (Malus domestica Bork h.), pear (Pyrus communis L.), prune (Prunus domestica L.), and sweet cherry (P. avium L.). Foliar uptake of labeled B by shoot leaves was 8 8% to 96% complete within 24 hours of application. More than 50% of th e B retained on shoot leaf surfaces following application was absorbed and exported within 6 hours of application. Genotypic differences in shoot leaf surface characteristics among the species tested greatly in fluenced the amount of solution retained per unit leaf area. Leaf rete ntion capacity was the primary determinant of the quantity of B absorb ed by and exported from shoot leaves following foliar application. On average, apple shoot leaves retained, absorbed, and exported at least twice as much labeled B per unit leaf area as prune and pear shoot lea ves and three to four times as much as sweet cherry shoot leaves. The sink demand of nearby, mature apples did not affect the export of labe led B when applied to adjacent spur leaves, but the fruit imported 16% of their total B from the applied solution during a 10-day period. De spite extensive documentation for the immobility of B accumulated by l eaves naturally (e.g., from the soil), the B accumulated by leaves fol lowing foliage application was highly mobile in all four species teste d.