Stomata play a pivotal role in regulating gas exchange between terrestrial plants and the atmosphere controlling water and carbon cycles at organismal, ecosystem and global levels. Accordingly, our objective was to investigate the impact of ultraviolet-B radiation, a neglected environmental factor varying with ongoing global change, on stomatal morphology and function by means of a comprehensive meta-analysis. We found 45 peer-reviewed publications containing altogether 143 case studies for analysis. The overall UV effect at the leaf level is to decrease stomatal conductance, stomatal aperture and stomatal size, although stomatal density was increased. The significant decline in conductance is marked in short-term experiments, with more modest decreases noted in long-term UV studies. We found that short-term experiments in growth chambers are not representative of long-term field UV effects on stomatal conductance. Further, we found a stronger UV effect in grasses than in herbs, while the reduction of stomatal conductance was insignificant in trees. It is hypothesised that these alterations in stomatal function have important potential consequences for plant life. In the short term, UV-mediated stomatal closure may reduce transpiration and alleviate drought stress. However, in the long term more complex changes in stomatal aperture, size and density may reduce carbon sink capacity, and enhance leaf and surface warming, potentially exacerbating the negative effects of drought and/or heatwaves on plant ecosystems and endangering long-term plant survival.

Since water deficit (WD) and ultraviolet radiation (UV) trigger similar protective mechanisms in plants, we tested the hypothesis that UV modulates grassland acclimation to WD, mainly through changes in the root/shoot (R/S) ratio, enhances the ability of grassland to acquire water from the soil and hence affects its productivity. We also tested the potential of spectral reflectance and thermal imaging for monitoring the impacts of WD and UV on grassland production parameters. The experimental plots were manipulated by lamellar shelters allowing precipitation to pass through or to exclude it. The lamellas were either transmitting or blocking the UV. The results show that WD resulted in a significant decrease in above-gound biomass (AB). In contrast, below-ground biomass (BB), R/S ratio and total biomass (TB) increased significantly in response to WD, especially in UV exclusion treatment. UV exposure had a significant effect on AB and BB, but only in the last year of the experiment. The differences in the effect of WD between years show that the effect of precipitation removal is largely influenced by the potential evapotranspiration (PET) in a given year and hence mainly by air temperatures, while the resulting effect on production parameters is best correlated with the water balance given by the difference between precipitation and PET. Canopy temperature and selected spectral reflectance indices showed a significant response to WD and also significant relationships with morphological (AB, R/S) and biochemical (C/N ratio) parameters. In particular, the vegetation indices NDVI and RDVI provided the best correlations of biomass changes caused by WD and thus the highest potential to remotely sens drought effects on terrestrial vegetation.

Barley (Hordeum vulgare) accumulates phenolic compounds (PhCs), which play a key role in tolerance to environmental stress. The influence of irradiance and atmospheric CO2 concentration ([CO2]) on the accumulation and localization of PhCs in barley leaves was examined for two varieties with different tolerances to oxidative stress. PhC localization was visualized using Naturstoff reagent A and fluorescence microscopy. Close relationships were found between fluorescence-determined localization of PhCs in barley leaves and PhC content estimated using liquid chromatography coupled with mass spectroscopy detection. High intensity light had the strongest effect on the accumulation of PhCs, but total PhC content converged at elevated [CO2], minimizing the differences between high and low irradiance. PhCs localized preferentially near the surfaces of leaves, but under low light, increasing allocation of PhCs deeper mesophyll layers was observed. The PhC profile was very different between barley varieties. Our research presents novel evidence that [CO2] modulates the accumulation of PhCs accumulation in barley leaves. Mesophyll cells, rather than epidermal cells, were most responsive to environmental stimuli in terms of PhC accumulation. The relatively tolerant variety accumulated significantly more hydroxycinnamic acids, indicating these PhCs may play a more prominent role in oxidative stress prevention.