Nitrate reductase activity and yield of dry mass and protein content in cucumber seedlings supplied with nitrates and ammonium

Abstract

The presence of NH₄⁺ ions in a nutrient solution containing NH₄NO₃ with the K+ ions removed, caused an inhibition of nitrate reductase (NR) activity in cucumber leaves. The lack of K⁺ in a NaNO₃ medium also decreased the NR activity. Addition of K⁺ to these media suppressed the inhibitory effect of NH₄⁺ and enhanced the NR activity in the leaves of plants growing in NaNO₃ nutrient solution. The results suggest that K⁺ is essential for NO₃^- reduction, Na⁺ is less effective in this process, whereas NH₄⁺ ions markedly inhibit NO₃^- reduction. The protein content and increment of dry mass of cucumber plants grown 10 days with NH₄-N as the sole source of nitrogen was significantly lower as compared with NO₃-N supplied plants feed with plants feed with both forms of mineral nitrogen (NH₄NO₃). The results show that cucumber prefers the nitrates, although it can utilize the ammonium form of nitrogen.