Relationship between PSII activity, CO2 fixation, and Zn, Mn and Mg contents of Lolium perenne under zinc stress

The effect of zinc toxicity on photosystem II (PSII) activities, photosynth etic function, and some mineral concentrations in hydroponically-grown Loli um perenne was studied at four ZnSO4 concentrations (1, 5, 10, and 20 mmol/ L) In the nutrient solution. Ryegrass showed a very strong capacity to prot ect itself against toxic zinc concentrations, requiring a large amount of Z nSO4 (20 mmol/L) to inhibit growth. In the leaves, the Increase in zinc con tent was accompanied by an increase in manganese concentration, which never reached a toxic level. This Increase in manganese concentration may limit the decrease In the F-v/F-m ratio and Phi PSII due to zinc stress, and thus granting zinc resistance to ryegrass. The net photosynthesis rate was sign ificantly reduced, and after 15 days, with a zinc concentration of 2.7 mg g (-1) DW in the leaves, the carboxylase activity was nearly nil. At the same time, the oxygenase activity of RuBisCO was unaffected or even increased, another possible contribution. to the tolerance of ryegrass to excess zinc, The carboxylase and oxygenase activities of RuBisCO were Identical (93 mu mol (e(-)) m(-2) s(-1)) With a zinc concentration in the leaves of 1.6 mg g (-1) DW after 8 days and 1.4 mg g(-1) DW after 15 days, corresponding to 5 mmol/L zinc and 1 mmol/L zinc in the growth medium, respectively.