Boron Phloem Mobility and Boron Levels in Leaves and Fruit Are Altered in Transgenic Apple Trees with Decreased Sorbitol Synthesis

It was previously demonstrated in tobacco that introducing sorbitol synthesis even at a very low capacity enables the plants to transport boron in the phloem via the formation of sorbitol-boron-sorbitol complexes in the phloem, but the effect of reducing sorbitol synthesis in species where sorbitol is a major end product of photosynthesis and translocated form of carbohydrates remained unclear. In this study, we used transgenic apple trees with approximately 1/2 or 1/3 of the sorbitol biosynthetic capacity of the untransformed control to determine the effect of decreased sorbitol synthesis and transport on phloem mobility of boron and boron levels in leaves and fruit. Five-year-old trees of the two transgenic lines along with the untransformed control were grown in pots and provided with complete nutrient solution via fertigation.  Compared with the untransformed control, the transgenic lines accumulated only 36% to 40% of the control boron levels in fruit, but about 2.8 times higher boron levels in leaves by fruit harvest. When ¹⁰B was applied to bourse shoot leaves in mid-season, more ¹⁰B was retained in the leaves and less ¹⁰B was translocated to the fruit on the same fruiting spur in the two transgenic lines than in the untransformed control by the end of a 12-day period. These results indicate that decreasing sorbitol synthesis (and its corresponding translocation in the phloem) reduces boron phloem mobility, leading to lower boron levels in fruit but higher boron levels in leaves of apple trees.