UV-B strengthens antioxidant responses to drought in Nicotiana benthamiana leaves not only as supplementary irradiation but also as pre-treatment

Potentials of UV-B (280-315 nm) radiation to alleviate effects of water deficit were studied using Nicotiana benthamiana plants in growth chambers. 10-days of limited watering resulted in 40% loss of soil water content as compared to well-watered controls. This drought was applied in three different ways: (i) in itself, (ii) after 4-days exposure of 6.9 kJ m(-2) d(-1) biologically effective supplementary UV-B radiation as pre-treatment, or (iii) in parallel with 6.9 kJ m(-2) d(-1) biologically effective supplementary UV-B. Responses were examined in two leaf groups: fully developed mature leaves (ML) and young leaves emerging during the 10-day treatment (YL). ML responded to UV-B or drought as single factor treatments with 7-14% loss of photochemical yield, while YL photochemistry was not decreased under the same conditions. The parallel two-factor treatment had no aggravating effect but alleviated drought-induced loss of leaf photochemistry in ML. Several positive single factor effects of drought or UV-B on antioxidants remained significant in the two-factor treatment both in ML and YL. Effects of the two factors applied in parallel were additive (equal to the sum of the effects caused by single factors separately) on total antioxidant capacities and singlet oxygen neutralizing; and synergistic (larger than the sum of single factor effects) on the flavonoid index in ML. A sequential application of UV-B and drought had additive positive effects on antioxidant capacity and flavonoid index of ML suggesting lasting effects of UV-B pre-treatment.