THE ROLE OF BORON IN AVOCADOS: THEORY, PRACTICE AND REALITY

Abstract

It has been found that Boron improves fruit set in various plant species, but very little information is available on the optimal Boron requirement for optimal fruit set. There is also very little information on the movement of boron in plants. By using boron enriched with the isotope 10 B, we have shown that the boron is indeed absorbed by the avocado leaves and then remobilised from the distally positioned mature leaves of the branch to the inflorescence. Therefore, the boron concentration of the leaves at the stem tip, directly below the axillary buds which are destined to produce the inflorescences, should be adjusted well in advance of the flowering period, to a level for optimal pollen tube growth during the flowering period. This level was determined to be between 60 mg.kg- 1 and 80 mg.kg 1 . It is important to distinguish between the free and the bound boron in leaves, as it is only the free boron which is available for remobilization to the inflorescences. The optimal level of the free boron is yet to be established. Increasing the boron concentration in avocado leaves can best be achieved by repetitive foliar spraying. Relatively little boron is absorbed by the avocado roots themselves due to the absence of root hairs; most of the minerals are provided to the plants by mycorrhiza.. Fungi however, do not require boron and usually do not take up boron naturally. Care should therefore be taken about the amount of boron sprayed onto a tree, for when the excess spray drips down from the leaves, it can cause a buildup of boron in the soil which can damage the mycorrhiza.