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, photosynthetic function, and some mineral concentrations in hydroponically-grown Lolium 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 protect itself against toxic zinc concentrations, requiring a large amount of ZnSO4 (20 mmol/L) to inhibit growth. In the leaves, the increase in zinc content 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 Fv/Fm ratio and ΦPSII due to zinc stress, and thus granting zinc resistance to ryegrass. The net photosynthesis rate was significantly 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 μmol (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.