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 10 B 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 10 B 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 10 B was rapidly translocated out of leaves, only a small percentage of the 11 B 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.