Boron uptake and distribution by oilseed rape (Brassica napus L.) as affected by different nitrogen forms under low and high boron supply

Ammonium (NH4+) and nitrate (NO3-) conversely alter pH of the rooting medium, and thus differentially affect the equilibrium between boric acid and borate in soil solution. This can alter boron (B) uptake by plants, which is passive under high, but facilitated (boric acid) or active (borate) under low B supply. Therefore, the effect of NH4+ and NO3- forms was investigated on the growth, B-10 uptake rate and accumulation, and expression of B transporters in Brassica napus grown with low (1 mu M) or high (100 mu M) B-10 for five days in the nutrient solution. At the low B-10 level, NO3--fed plants had the same specific B-10 uptake rate, B-10 accumulation and xylem B-10 concentration as NH4NO3-fed plants but these attributes were reduced at the high B-10 level. BnaBOR1;2 and BnaNIP5;1 were upregulated in roots of NO(3)(-)fed plants at low B-10 supply. NH4+-fed plants had substantially lower dry matters; due to nutrient solution acidification (2.0 units)-induced deficiency of nitrogen, potassium, magnesium, and iron in plant shoots. Reduced transpiration rates resulted in lower B-10 uptake rate and accumulation in the roots and shoots of NH4+-fed plants. BnaNIP5;1 in roots, while both BnaBOR1;2 and BnaNIP5;1 in shoots were upregulated in NH4+-fed plants at low B-10 level. Collectively, NH4+-induced acidity and consequent lowering of B-10 uptake induced the upregulation of B transport mechanisms, even at marginal B-10 concentrations, while NO3--induced alkalinization resulted in altered B distribution between roots and shoots due to restricted B transport, especially at higher B-10 supply.