Effect of nitrate and humic substances of different molecular size on kinetics parameters of nitrate uptake in wheat seedlings

The kinetic parameters of nitrate uptake (Imax, Km and Cmin) were evaluated in young seedlings of Triticum durum L., cv. Appulo, exposed to nitrate and/or to soil-extracted humic acids (HAs) of different molecular weight. The uptake was enhanced after induction at low levels of nitrate (50 μM KNO3), while it was inhibited after induction at higher concentrations (2000 μM). The kinetic parameters of uptake were selectively influenced by pretreatment with HAs: total (TE) and, at a greater extent, low (LMS, < 3500 Da) molecular size humic fraction increased either the nitrate uptake rate (Imax) and the efficiency of the whole transport system (low Km and Cmin), while an opposite result was evidenced in high molecular size (HMS, >3500 Da)-treated plants. An additive effect was shown when nitrate and humic substances were provided simultaneously: the uptake rate was enhanced in TE- and LMS-treated plants, but was strongly delayed in HMS-treated plants. Removal of nitrate and/or humic fractions de-induced the system and NO3- uptake rate decreased. Exposure to HAs was not able to induce nitrate reductase activity in root and leaf tissues. Inhibitors of protein synthesis p-fluorophenylalanine and cycloheximide reversed the positive effect of LMS fraction on nitrate uptake. This would support the hypothesis of a promoting effect of HAs on the molecular expression of proteins of the nitrate transport system.