Nutrient Effect

7.1. Nitrogen availability

Carbohydrate and nitrogen metabolism are closely related. Nitrogen nutrition affects the level of sugar alcohols in sugar alcohol accumulating plants. By applying supplemental ammonium nitrate, it was shown that leaf size was significantly correlated to sorbitol levels in the wood and bark of apple trees [147], The effect of applying different amounts of nitrogen on the accumulation of nonstructural carbohydrates, including sorbitol, was studied in potted peach trees [148]. The sorbitol and sucrose contents were higher in leaves of trees grown with sufficient nitrogen than in leaves of trees deprived of nitrogen. Similar results were reported in apple trees [149].

7.2. Nutrient stress (boron)

Boron is said to be immobile in the phloem or to have only limited phloem mobility in higher plants. However it was shown that in species where sorbitol is a major sugar (almond, apple, peach), boron was freely mobile, while in those that produce little or no sorbitol (fig, pistachio, walnut), boron was largely immobile [150]. In sorbitol-rich species, boron was transported from treated leaves to adjacent fruits. It has been suggested that the mobility of boron in sorbitol-rich species is mediated by the formation of boron-sorbitol complexes [150]. Recently, tobacco (Nicotiana tabucum) was genetically engineered to synthesise sorbitol, in order to study the resultant effect on phloem mobility of boron. Using transgenic plants transformed with the antisense gene construct for sorbitol-6-phosphate dehydrogenase or the sorbitol-synthesising sense construct compared with those of the wild-type tobacco, the authors concluded that transgenic tobacco plants containing sorbitol exhibited a marked increase in within-plant boron mobility and a resultant increase in plant growth and yield when grown with a limited boron supply [151], It has been suggested that the selection or transgenic generation of cultivars with an increased sugar alcohol content could result in increased boron uptake, without apparent negative effects on growth [152],

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