Effect of humic substances on nitrogen uptake and assimilation in two species of pinus

The effects of a humic fraction, with a high molecular weight (Hsp) extract ed from a soil with a Fagus sylvatica vegetal cover were compared on ammoni um (NH4+) and nitrate (NO3-) uptake in two different coniferous species (Pi nus pinaster and Pinus laricio). The activity of malate dehydrogenase (MDH; EC 1.1.1.37), glutamate dehydrogenase (GDH; EC 1.4.1.3) and phosphoenolpyr uvate carboxylase (PEPC; EC 4.1.1.31) key enzymes involved in nitrogen meta bolism were tested. The data obtained indicate that the humic fractions, at all concentrations, increased ammonium uptake by both species, while nitra te uptake was unaffected. In P. laricio seedlings this fraction increased o nly leaf malate dehydrogenase activity, while in P. pinaster seedlings an i ncrease of glutamate dehydrogenase in leaves, and of phosphoenolpyruvate ca rboxylase activity in both roots and leaves was observed. These results sho w that humic substances stimulate only ammonium uptake and differently infl uence ammonium assimilation in the two species of Pinus. This diverse behav ior might be related to the different environmental conditions where these two species live.