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.