EFFECT OF HUMIC SUBSTANCES ON PHOTOSYNTHETIC SULFATE ASSIMILATION IN MAIZE SEEDLINGS

Maize plants were grown in nutrient solution containing 0, 15, 30, 50 mg organic carbon L-1 of humic substances (HS) (M.W. < 12 KDa) extracted from a cambisol. After 14 d of growth the second and third leaves were collected and analyzed for chlorophyll and protein content and for the activities of ATP sulphurylase (ATP-s) and O-acetylserine sulphydrylase (OAS-s) to evaluate the effect of HS on maize sulphate assimilation pathway. Humic substances induced significant decreases in chlorophyll content, whereas the protein level slightly increased up to 30 mg L-1 of HS. ATP-s and OAS-s were positively affected by HS. The stimulations were different according to the enzyme and leaf investigated. Our results suggest that HS might play an important role in the efficiency of maize plants to assimilate sulphate.