Interactive effects of PAR and UV radiation on the physiology, morphology and leaf optical properties of two barley varieties

Abstract

The interactive effects of photosynthetically active radiation (PAR) on plants’ acclimation to ultraviolet (UV) radiation were examined under field conditions in two barley varieties (Barke, Bonus). Plants, pre-treated under UV exclusion and low PAR intensities, were subsequently exposed to four radiation treatments representing the combination of low [−] and high [+] UV and PAR intensities. Selective UV and PAR filters were used for UV exclusion and reduction of PAR to ca 25% of ambient irradiance. A system of modulated lamps was used to enhance UV to ca 200% of ambient. Changes in flavonol and chlorophyll content, chlorophyll fluorescence, gas-exchange and leaf hyperspectral reflectance were studied during seven days of acclimation to the new treatments. At the end of this period morphological analysis of aboveground biomass was carried out. The [UV+PAR−] treatment significantly reduced the photosynthetic activity of barley leaves; the reduction was more pronounced in old than young leaves and greater in the variety Barke than Bonus. Whereas, [PAR+] treatment triggered photoprotective mechanisms which partially ameliorated the UV effects on photochemistry and carbon assimilation. The [PAR+] treatment induced accumulation of flavonols, mainly in young leaves, whereas in old leaves UV-induced accumulation was more pronounced. An inverse proportion was found between flavonol content and specific leaf area irrespective of barley variety and UV/PAR treatment. Enhanced UV radiation reduced the final leaf length, particularly in [PAR−] plants, in young leaves and in variety Barke. However, [PAR+] mitigated the morphological effects induced by the [UV+] treatment, particularly changes in SLA.