Nitrate and nitrite transport into cereals affected by low pH and different temperatures

Abstract

The effects of pH and temperature on the uptake of nitrate (NO3−) and nitrite (NO2−) of wheat Triticum aestivum L. cv. GK Othalom) and rice (Oryza saliva L. cv. Oryzella) seedlings were studied in hydroponic cultures in short-term (3–4 h) and long-term (12–24 h) experiments with use of stable (15N) isotope labelled compounds. In short-term experiments the nitrate uptake of the roots of wheat was lower at pH 6.5 than at pH 4. However, in the long-term tests this changed for the opposite, the uptake of nitrate was definitely higher at pH 6.5. The nitrite uptake at pH 4 hardly increased with time. At pH 6.5 its amount was comparable to nitrate. Nitrate and nitrite was translocated towards the shoot. In long-term experiments this value for nitrite was definitely higher than that of nitrate at pH 6. NO3− and NO2− uptake data show that K+, the accompanying ion, compared to Na+, increased the absorption and translocation of NO3− and NO2− in seedlings. The growth of seedlings, under the same experimental conditions, show the same tendency. The temperature coefficient (Q10) for nitrite confirmed the active character of its uptake, and its toxic effect at pH 4. Our results clearly show, that at least in a limited range of concentration (<1mM), at non-acidic pH, plants can utilise the nitrite taken up by roots. However, at low pH HNO2 is formed, generally considered as strong oxidant and toxic substance.