Crop phenology modifies wheat responses to increased UV-B radiation

Ozone layer depletion increases the level of ultraviolet radiation reaching the earth's surface affecting both natural and agricultural ecosystems, especially in the Southern Hemisphere. Considering the harmful effects UV-B radiation has on plant growth the future productivity of wheat crops in Southern Chile could be challenged by both (i) the forthcoming level of UV-B increase and (ii) the sensitivity of this crop to higher UV-B radiation. In this study the effect of increased UV-B radiation at different phenophases on a spring wheat cultivar (Pandora) was investigated in two experiments at plant and crop levels under out-door conditions. The experiments consisted of controls, increased UV-B radiation at specific phenophases (from 3 leaf stage to booting 3L-Bo, and from booting to maturity Bo-PM), and increased UV-B radiation for the majority of the crop cycle (from 3 leaf stage to maturity). UV-B radiation was increased by Q panel UV-313 lamps set in plastic framed structures. Control plants were grown either without frames or below the same framed structures as those which received increased UV-B treatments. Phenology, above-ground biomass, grain yield, components, grain protein concentration, leaf area index (LAI), Fv/Fm and pigments were measured at booting and/or at harvest. Above-ground biomass and yield decreased by 11-19 and 12-20%, respectively, when UV-B radiation was increased at the 3L-Bo phase, while no effect was observed when irradiation was applied later in the crop cycle (Bo-PM). No additional UV-B effects to those observed at booting were detected in plants irradiated during the majority of the entire crop cycle (3L-PM). Biomass variation was strongly associated (r =0.99; P <0.01) with UV-B/PAR ratio in the sensitive treatments to UV-B increases (3 L-Bo) of both experiments. Flour protein was not affected by UV-B increases at any phenophase evaluated in this study. In both experiments, leaf green area and weight were negatively affected by increased UV-B radiation and no effect on specific leaf area (SLA) was found. Lower Fv/Fm, chlorophyll, carotenoid concentration and carotenoid:chlorophyll ratio were found at crop level (experiment 2) under higher UV-B in the 3L-Bo and 3L-PM treatments. The flavonoid concentration responded differently in the two experiments, probably due to the optimum responses these pigments had to expose UV-B doses.