Chlorophyll content, photosynthetíc oxygen evolution and the activities of ATP- sulphurylase (ATP-s) (E.C. 2.7,7.4.), OAS-sulphydrylase (OAS-s) (E.C, 4.2.99.8.), nitráte reductase (NR) (E.C. 1.6.6.1.) and glutamine synthetase (GS) (E.C. 6.3.1.2), were determined in leaves of Zea mays L. Dekalb cv. Sponsor plants grown in the presence of 0, 10, 100, 250 pM Cd in order to evaluate the effect of this metal on sulphate and nitráte assimilation pathway. Cd induced a slight decrease of photosynthetíc oxygen evolution (-21 % of the control at 250 pM), whereas the enzyme activities were differently influenced: OAS-s and GS increased up to 40 and 25 % of the control, respectively, at 250 pM Cd; NR showed a 20 % stimulation at 100 pM Cd, and ATP-s was slightly inhibited. The results stress the importance of experimental conditions adopted on the response of enzyme activities to Cd and suggest that the observed increases of enzyme activities are related to a defence mechanism.
In order to evaluate the effect of leaf age on enzymes of carbon and sulphur metabolism, chlorophyll (Chl) and protein contents and the activities of RuBP- carboxylase (RuBPC), ATP-sulphurylase (ATPS), and 0-acetylserinesulphydrylase (OASS) were determined in leaves of different ages from luceme (Medicago sativa L. cv. Lodi and Sewa) plants grown outdoors. The enzymes of sulphur and carbon metabolism were differently regulated throughout development of luceme leaves, as ATPS reached the highest activity at very early stages of leaf growth, when RuBPC activity was negligible. OASS activity inereased 2-fold during leaf expansion, and deereased with leaf senescence. The two cultivars showed rather similar trends of development in Chl, protein content and enzyme activities. The cultivars differed also in the absolute level of enzyme activities and other biochemical parameters.