Is boron transport to avocado flowers regulated by carbohydrate supply

Low carbohydrate reserves have been cited as a cause of alternate bearing in avocado, and boron deficiency has been associated with poor tree growth and low productivity. Our recent work has demonstrated that perseitol, an important transport sugar in avocado, can complex with boron to make boron phloem mobile. This suggests that boron transport from avocado leaves could be regulated by carbohydrate supply. We tested this hypothesis using branch girdling treatments to manipulate the supply of carbohydrates from leaves to flowers and examined if these treatments would also affect the supply of boron from leaves to flowers. Branch girdling treatments were applied at different phenological stages to ‘Hass’ avocado trees with high (ON) and low (OFF) crop loads. Girdling treatments applied to ON cropping trees increased the rate of boron loss from leaves during winter and increased the boron content of flowers at mid-bloom in spring. Leaf D-Mannoheptulose concentration, the storage form of perseitol, declined during winter at a similar timing to that of leaf boron. These results are consistent with boron supply to flowers being linked to carbohydrate flows to inflorescences. Few differences were found between leaf carbohydrate concentrations in ON and OFF cropping trees. However, leaf boron concentrations were consistently lower in ON cropping trees, which suggests that in ON cropping years fruit are a major competing sink for boron and could result in less boron being available to support flower and early fruit development in the following spring.