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

Abstract

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, ¹⁰B uptake rate and accumulation, and expression of B transporters in Brassica napus grown with low (1 μM) or high (100 μM) ¹⁰B for five days in the nutrient solution. At the low ¹⁰B level, NO₃^--fed plants had the same specific ¹⁰B uptake rate, ¹⁰B accumulation and xylem ¹⁰B concentration as NH₄NO₃-fed plants but these attributes were reduced at the high ¹⁰B level. BnaBOR1;2 and BnaNIP5;1 were upregulated in roots of NO₃-fed plants at low ¹⁰B 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 ¹⁰B 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 ¹⁰B level. Collectively, NH₄⁺-induced acidity and consequent lowering of ¹⁰B uptake induced the upregulation of B transport mechanisms, even at marginal ¹⁰B concentrations, while NO₃^--induced alkalinization resulted in altered B distribution between roots and shoots due to restricted B transport, especially at higher ¹⁰B supply.

Keywords: Ammonium; Boron isotopes; Brassica napus; Nitrate; Soil pH.