A possible mechanism for phloem transport of boron in ‘Hass’ avocado (Persea americana Mill.) trees

Abstract

SummaryA continuous supply of boron (B) is vital for the formation of new cells, with reproductive growth being particularly sensitive to boron deficiency. An inadequate B supply therefore has the potential to effect fruit set, which is a major problem in avocado (Persea americana Mill.) cultivation. B is not phloem mobile in many plant species, but when sorbitol or mannitol (both polyols) are present in phloem sap, then B forms a complex with the polyol, enabling transport in the phloem. Phloem mobility of B has never been demonstrated in avocado. Avocados are unusual in that they produce the 7-carbon sugar, D-mannoheptulose, and its polyol form, perseitol. In this work, we demonstrate that B forms a complex with perseitol in vitro, that both B and perseitol are found in avocado phloem sap, that B is found in higher concentrations within immature leaves than in mature leaves, and that 10B is exported from mature leaves into inflorescences and new leaves. All these findings are consistent with B being transported as a B-perseitol complex in the phloem sap of avocado trees.