Vol. 87 No: 1

Title:
A possible mechanism for phloem transport of boron in 'Hass' avocado (Persea americana Mill.) trees

Authors:
P.E.H. MINCHIN, T.G. THORP, H.L. BOLDINGH, N. GOULD, J.M. COONEY, F.B. NEGM, E. FOCHT, M.L. ARPAIA, H. HU and P. BROWN

pp: 23-28

Abstract:
A 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.

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