Effect of selenium on CO2 and NO3 - assimilation under low and adequate nitrogen supply in wheat ( Triticum aestivum L.)

In order to study the mechanisms of Se-mediated growth improvement as related to carbon (C) and nitrogen (N) metabolism, wheat plants were cultivated hydroponically with adequate (4 mM, N-a) or low (1 mM, N-d) N supply and treated with 10 and 50 mu M Na2SeO4 for six weeks. The Se supplementation enhanced plant biomass; it was significant for shoots of N-a plants at 50 mu M Se. Chlorophyll fluorescence parameters were significantly lowered under N-d conditions but restored completely by Se addition reaching values of those in N-a plants. Net CO2 assimilation rate (P (N)) decreased only slightly by limited N availability, but it enhanced significantly in both N-d and N-a plants equally by 10 and 50 mu M Se. Effect of Se on P (N) in the N-a plants occurred mainly due to the stomata opening, while it was related to both stomatal and nonstomatal mechanisms in the N-d plants. The Se treatment resulted in enhancement of nitrate reductase (NR) activity in both N-a and N-d plants with an optimal response at 10 mu M Se. Negative correlations between nitrate concentration and NR activity indicated a partial nitrate depletion in the roots following by elevated NR activity in N-d plants. In contrast, nitrite concentrations were higher in the Se treated plants. Higher amino acids and protein concentrations in the Se-treated plants might be an indication of a general upregulation of N metabolism. However, in N-a plants, the stimulation of N metabolism was not observed at 50 mu M Se which could not be attributed to lesser availability of C skeletons because of maintaning higher CO2 fixation under these conditions. It implies the function of some regulatory mechanisms that are responsible for coordination of C and N metabolism in whole plant.