Photosynthesis performance in sweet almond [Prunus dulcis (Mill) D. Webb] exposed to supplemental UV-B radiation

Due to anthropogenic influences, solar UV-B irradiance at the earth's surface is increasing. To determine the effects of enhanced UV-B radiation on photosynthetic characteristics of Prunus dulcis, two-year-old seedlings of the species were submitted to four levels of UV-B stress, namely 0 (UV-B-c), 4.42 (UV-B-1), 7.32 (UV-B-2) and 9.36 (UV-B-3) kJ m(-2) d(-1). Effects of UV-B stress on a range of chlorophyll (Chl) fluorescence parameters (FPs), Chl contents and photosynthetic gas-exchange parameters were investigated. UV-B stress promoted an increase in minimal fluorescence of dark-adapted state (F-0) and F-0/F-m, and a decrease in variable fluorescence (F-v, F-v/F-m, F-v/F-0 and F-0/F-m) due to its adverse effects on photosystem II (PSII) activity. No significant change was observed for maximal fluorescence of dark-adapted state (F-m). Enhanced UV-B radiation caused a significant inhibition of net photosynthetic rate (P-N) at UV-B-2 and UV-B-3 levels and this was accompanied by a reduction in stomatal conductance (g(s)) and transpiration rate (E). The contents of Chl a, b, and total Chl content (a+b) were also significantly reduced at increased UV-B stress. In general, adverse UV-B effects became significant at the highest tested radiation dose 9.36 kJ m(-2) d(-1). The most sensitive indicators for UV-B stress were F-v/F-0, Chl a content and P-N. Significant P < 0.05 alteration in these parameters was found indicating the drastic effect of UV-B radiation on P. dulcis.