NITRATE REDUCTASE-ACTIVITY IN WHEAT SEEDLINGS AS AFFECTED BY NO(3-) NH(4+) RATIO AND SALINITY/

Nitrate reductase is considered to be a limiting factor for growth, de velopment and protein production in plants. This enzyme is influenced by external conditions. The purpose of this study was to investigate t he effect of different NO3-/NH4+ ratios and salinity on NRA (nitrate r eductase activity) in shoots and roots of wheat (Triticum aestivum L. cv. Betres) seedlings. Plants were grown in a growth chamber under con trolled conditions. Nutrient solution contained 4 mM N, applied as NO3 - and NH4+ in different ratios, and saline treatments consisted in tow levels of NaCl:1 mM and 60 mM. Wheat seedlings had most of their NRA in shoots. When NH4+ was present in the nutrient solution NRA was almo st completely inhibited in roots and decreased in shoots. Inhibition o f NRA by NH4+ seems to be a direct effect on synthesis and activity of NR rather than through inhibition of NO3- uptake and thus by lack of substrate. Salinity significantly decreased NRA in shoots, this effect being influenced by NO3- in the and by NO3-/Cl- ratio in shoots. The effect on RNA may be due to the alteration of NO3- uptake, to an exces sive Cl- shoot content which may affect NO3- transport from vacuoles t o cytoplasm and to the alteration of NO3- flux from root to shoot.