Plant Growth, Nitrate Content and Ca Signaling in Wheat (Triticum aestivum L.) Roots under Different Nitrate Supply

Nitrate (NO₃^-) is the main source of inorganic nitrogen for plants in aerobic soil conditions. Nutrient solution experiments were conducted with two winter wheat (Triticum aestivum L.) cultivars, ‘Shimai 15’ and ‘Hengguan 35’, as the tested crops. The objective was to investigate the changes in Ca²⁺-CaM signaling and root growth of wheat under different concentrations of nitrate supply. Net Ca²⁺ ion fluxes in different root zones were measured non-invasively using the scanning ion-selective electrode technique. Calmodulin (CaM) content in the roots was determined by enzyme-linked immunosorbent assay. Results showed that shoot fresh weight, nitrate content in shoots, and lateral root numbers of both wheat cultivars were reduced without nitrate supply compared with those under 2.5 mmol/L nitrate treatment. High levels of nitrate (50.0 mmol/L) significantly increased nitrate content in shoots, but reduced the number of lateral roots in ‘Shimai 15’. The CaM content in roots declined under conditions of no nitrate or excess nitrate (50.0 mmol/L), and were more drastically reduced in ‘Hengguan 35’. The roots without nitrate supply showed net Ca²⁺ ion efflux. However, the roots with 50.0 mmol/L nitrate supply showed net Ca²⁺ ion influx, the speed of which was significantly slower than that with 2.5 mmol/L nitrate supply. These results suggested that when wheat seedlings were grown under the stress of nitrate deficiency or excess, the influx or efflux of Ca²⁺ became slower and the content of CaM declined, which might inhibit the root growth of wheat.