Annals of Botany 77: 497-506, 1996
© 1996 Annals of Botany Company
Phloem Mobility of Boron is Species Dependent: Evidence for Phloem Mobility in Sorbitol-rich Species
Department of Pomology,, University of California—Davis, Davis, CA, 95616, U.S.A.
August 25, 1995 ; November 28, 1995
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 thePyrus, Malus andPrunusgenera. These genera utilize sorbitol as a primary translocated photosynthate and it has been clearly demonstrated that B forms stable complexes with sorbitolin vitro.In the research presented here we demonstrate, further, that B is freely phloem mobile inPyrus, MalusandPrunusspecies 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-enriched10B 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 amygdalusB. syn.P. dulcis(Mill.)], apple (Malus domesticaB.) and nectarine (Prunus persicaL. B. var.nectarinaM.), sorbitol-poor species included fig (Ficus caricaL.), pistachio (Pistacia veraL.) and walnut (Juglans regiaL.). In sorbitol-rich species foliar applied10B 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-applied10B was rapidly translocated out of leaves, only a small percentage of the11B 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.
Almond; Prunus amygdalus ; apple; Malus domestica; nectarine; Prunus persica; fig; Ficus carica; pistachio; Pistacia vera; walnut; Juglans regia; boron; phloem mobility; deficiency; toxicity; inductively coupled plasma-mass; spectrometer
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