The effects of UV-B radiation on the gas exchange rates, chlorophyll fluorescence, pigmentation, soluble sugars, starch, soluble proteins and carboxylating enzyme activities of maize (Zea mays L.) were investigated at four levels of applied nitrogen (0, 100, 200 and 300 kg ha−1 of N) under Mediterranean field conditions. The experiment simulated a 20% stratospheric ozone depletion over Portugal. Supplementary UV-B radiation and N deficiency reduced photosynthetic and transpiration rate, stomatal conductance, total chlorophyll, total carotenoids, soluble sugars and UV-B absorbing compounds concentration, as well as the pool size of the electron acceptors in PSII and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) and phosphoenolpyruvate carboxylase (PEPCase) activities. Enhanced UV-B and N deficiency increased minimal and maximal chlorophyll fluorescence. The effect of additional UV-B on maize photosynthesis was dependent on the plant nutritional status, since the lower reduction of net photosynthetic rate occurred in N-stressed plants. At the same time, photosynthesis was less affected by N nutrition under high UV-B. Similar results were obtained for RuBisCO and PEPCase activities. N-starved plants had higher amounts of soluble proteins at high UV-B as compared to ambient UV-B. The starch concentration of N-deprived plants at ambient UV-B was higher than all other treatments. The underlying mechanisms for these results are discussed.
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