Phloem mobility of boron is species dependent: Evidence for phloem mobility in sorbitol-rich species

Abstract

Boron is generally considered to be phloem immobile or to have only limited phloem mobility in higher plants. Evidence suggests, however, that B may be mobile in some species within the Pyrus, Malus and Prunus genera. These genera utilize sorbitol as a primary translocated photosynthate and it has been clearly demonstrated that B forms stable complexes with sorbitol in vitro. In the research presented here we demonstrate, further, that B is freely phloem mobile in Pyrus, Malus and Prunus species and suggest that this is mediated by the formation and transport of B-sorbitol complexes. The pattern of B distribution within shoot organs and the translocation of foliar-applied, isotopically-enriched 10B was studied in six tree species. Results demonstrate that in species in which sorbitol is a major sugar (sorbitol-rich), B is freely mobile while in species that produce little or no sorbitol (sorbitol-poor) B is largely immobile. The sorbitol- rich species used here were almond [Prunus amygdalus B. syn. P. dulcis (Mill.)], apple (Malus domestica B.) and nectarine (Prunus persica L. B. var. nectarina M.), sorbitol-poor species included fig (Ficus carica L.), pistachio (Pistacia vera L.) and walnut (Juglans regia L.). In sorbitol-rich species foliar applied 10B was transported from the treated leaves to adjacent fruit and specifically to the fruit tissues (hull, shell or kernel) that developed during the experimental period. Whereas, foliar-applied 10B was rapidly translocated out of leaves, only a small percentage of the 11B present in the leaf at the time of foliar application was retranslocated. In sorbitol- rich species, B concentrations differed only slightly between old and young leaves while fruit tissue had significantly greater B concentrations than leaves. In contrast, sorbitol-poor species had significantly higher B concentrations in older leaves than young leaves while fruit tissue had the lowest B concentration. This occurred irrespective the source of plant B (soil solution or foliar-applied). In a subsequent experiment the growth of apple trees in solutions free of applied B was maintained solely by foliar applications of B to mature leaves. These results indicate that B is mobile in species that produce significant amounts of sorbitol. We propose that the mobility of B in these species is mediated by the formation of B- sorbitol complexes.