Boron Uptake, Translocation and Distribution in Oilseed rape (Brassica napus L.) : Effect of Application Methods, Boron Status of Plants and Nitrogen Forms
Boron (B) is an essential micronutrient for vascular plants and its plant availability is reduced by soil liming, high soil pH, and drought periods during growth stages. This work, consisting of three independent hydroponics experiments, aimed at investigating the growth and physiological response of oilseed rape (Brassica napus L.) plants to B supply. In the first experiment, the influence of B supply on the growth and distribution of B in oilseed rape was investigated. The results showed that oilseed rape plants supplied with 0.25 µM B in the nutrient solution showed severe symptoms of B deficiency, which were not seen in the plants supplied with 1.0 µM B. The increasing B supply in the nutrient solution significantly increased the translocation factor, and hence increased B shoot to root ratio in oilseed rape plants. In the second experiment, the effects of continuous B resupply via roots and one-time foliar application leaves on growth and physiological response, the relative expression of specific genes involved in B absorption and transport in B-deficient oilseed rape were evaluated. It was found that continuous root resupply and one-time foliar resupply B to B-deficient plants partially retrieved B concentration in the leaves and dry matter production, with a corresponding increase in the net carbon assimilation, transpiration rate and sugar accumulation. The finding from this work provided evidence that the relative expression of BnaNIP5;1 and BnaBOR1;2 strongly upregulated in the roots and leaves of B-deficient oilseed rape plants. However, gene expression of BnaNIP5;1 and BnaBOR1;2 was upregulated only in roots, which could be attributed to the low B concentration in roots in the foliar-applied B treatments. The effects of foliar B resupply on the growth of oilseed rape suggest that dual resupply via roots and foliage is needed to achieve a complete growth recovery of the B deficiency in oilseed rape. In the third experiment, the mechanisms underlying the uptake and distribution of B in oilseed rape under different nitrogen forms were elucidated. The concentration of B in the oilseed rape was not affected by the N forms, whereas it depended on the levels of B supplied. Marginal B supply significantly decreased specific B uptake rates as well as B concentrations and accumulation in roots and shoots as compared to the high B supply. The NO3--fed plants showed reduced xylem B concentrations and accumulated relatively more B in roots than that recorded with other N forms. The relative expression of BnaNIP5;1 was upregulated in roots, while both BnaBOR1;2 and BnaNIP5;1 were upregulated in shoots in the NH4+-fed plants under marginal B supply. Nitrate nutrition led to up-regulation of BnaBOR1;2 and BnaNIP5;1 in roots only. Nitrate-induced alkalinization results in altered B distribution between roots and shoots due to a restriction of B transport, while NH4+-induced acidity can induce upregulation of B transport mechanisms, even though B concentrations do not indicate B deficiency.
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