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

The kinetic parameters of nitrate uptake (I-max, Km and C-min) were evaluat ed in young seedlings of Triticum durum L., cv. Appulo, exposed to nitrate and/or to soil-extracted humic acids (HAs) of different molecular weight. T he uptake was enhanced after induction at low levels of nitrate (50 mu M KN O3) while it was inhibited after induction at higher concentrations (2000 m u M). The kinetic parameters of uptake were selectively influenced by pretr eatment with HAs: total (TE) and, at a greater extent, low (LMS, < 3500 Da) molecular size humic fraction increased either the nitrate uptake rate (I- max) and the efficiency of the whole transport system (low Km and C-min), w hile an opposite result was evidenced in high molecular size (HMS, >3500 Da )-treated plants. An additive effect was shown when nitrate and humic subst ances were provided simultaneously: the uptake rate was enhanced in TE- and LMS-treated plants, but was strongly delayed in HR IS-treated plants. Remo val 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 a ctivity in root and leaf tissues. Inhibitors of protein synthesis p-fluorop henylalanine 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 sy stem.