KINETICS OF NITRATE UPTAKE BY CITRUS SEEDLINGS AND INHIBITORY EFFECTSOF SALINITY

Nitrate absorption by roots of both Troyer citrange (hybrid of Citrus sinensis x Poncirus trifoliata) and Cleopatra mandarin (C. reshni) see dlings was studied. The ion depletion in the external nutrient solutio ns was used to estimate NO3- net uptake. In both rootstocks the kineti cs of nitrate uptake showed a biphasic pattern in response to increasi ng external NO3- concentrations up to 10 mol m(-3). The low concentrat ion transport system followed Michaelis-Menten kinetics reaching satur ation at 0.5 mol m(-3) NO3- in both rootstocks. The apparent K(m)s of nitrate uptake were 282 and 281 mmol m(-3) respectively, for Troyer ci trange and Cleopatra mandarin. A linear system was also found at highe r external NO3- concentrations (between 1-10 mol m(-3)). Double recipr ocal plots showed that Cl- inhibited NO3- uptake competitively in the low concentration system. In contrast, the high concentration system w as insensitive to external Cl-, but was inhibited when plants were pre treated with salt solutions. Net NO3- uptake in both systems was sever ely affected by Cl- ions more in Troyer citrange than in Cleopatra man darin. Since Cl- accumulation in tissues was higher in Troyer citrange than in Cleopatra mandarin, a relationship between the ability to abs orb Cl- and the inhibition of NO3- uptake is suggested for these plant s. (C) 1997 Elsevier Science Ireland Ltd.