Photomorphogenic responses triggered by low fluence rates of ultraviolet B radiation (UV-B; 280-315 nm) are mediated by the UV-B photoreceptor UV RESISTANCE LOCUS8 (UVR8). Beyond our understanding of the molecular mechanisms of UV-B perception by UVR8, there is still limited information on how the UVR8 pathway functions under natural sunlight. Here, wildtype Arabidopsis (Arabidopsis thaliana) and the uvr8-2 mutant were used in an experiment outdoors where UV-A (315-400 nm) and UV-B irradiances were attenuated using plastic films. Gene expression, PYRIDOXINE BIOSYNTHESIS1 (PDX1) accumulation, and leaf metabolite signatures were analyzed. The results show that UVR8 is required for transcript accumulation of genes involved in UV protection, oxidative stress, hormone signal transduction, and defense against herbivores under solar UV. Under natural UV-A irradiance, UVR8 is likely to interact with UV-A/blue light signaling pathways to moderate UV-B-driven transcript and PDX1 accumulation. UVR8 both positively and negatively affects UV-Aregulated gene expression and metabolite accumulation but is required for the UV-B induction of phenolics. Moreover, UVR8-dependent UV-B acclimation during the early stages of plant development may enhance normal growth under long-term exposure to solar UV.

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The objective of the work reported was to answer the following questions; (1) Do stomata respond to both humidity and temperature? (2) Do these responses interact in sueh a way that relative humidity at the leaf surface is a more appropriate variable than water vapour saturation deficit at the leaf surface and yields a simpler description of the cotnpound response? To answer these questions, we tneasured the response of leaf conductance to humidity under constant leaf temperature, and the response to increasing leaf temperature under constant relative humidity and under constant water vapour saturation deficit. We found that, in Hedera helix subsp, canarietisis (Willd.) Coutinho, there was a reversible response to humidity under constant temperature, and that there was also a response to temperature under constant relative humidity. The relationship between leaf conductance and relative humidity was different when rneasured at the same temperature rather than at diffefent temperatures. An inversely proportional response was consistently obtained when stomatal conductance was expressed in relation to water vapour sa'turation deficit. The interaction between the effects of leaf temperature and water vapour saturation deficit was not compatible with a mechanism of response to humidity and temperature based solely on relative humidity. From these data, we conclude that water vapour saturation deficit is a more appropriate variable for describing stomatal responses to humidity. Key-words: Hedera helix; ivy; gas exehange; stomatal conductance; relative humidity; water vapour saturation deficit; leaf temperature.Abbreviations: A = CO2 flux density (net CO2 assimilation rate); k = proportionality coefficient; h = relative humidity; Ih = relative humidity at the leaf surfaee -assuming that the air at the leaf surfaee is at the same temperature as the leaf; Ti -leaf temperature; x,, = mole fraetion of water vapour in the air outside the boundary layer; x, = mole fraction of water vapour at the leaf surface; x* = x*{Ti) = saturated water vapour molar fraction at T/; D,, = x* -x"; /)., = A' J -xf, gg = stomatal conductance; gi, = boundary layer conductance; C, = CO2 molar fraetion at the leaf surfaee; C, = CO2 molar fraetion in the intercellular air space; / = photon flux density, A = 4t)t)-700nm,

Ultraviolet (UV) radiation is an important environmental factor for plant communities; however, plant responses to solar UV are not fully understood. Here, we report differential effects of solar UV-A and UV-B radiation on the expression of flavonoid pathway genes and phenolic accumulation in leaves of Betula pendula Roth (silver birch) seedlings grown outdoors. Plants were exposed for 30 days to six UV treatments created using three types of plastic film. Epidermal flavonoids measured in vivo decreased when UV-B was excluded. In addition, the concentrations of six flavonoids determined by high-performance liquid chromatography-mass spectrometry declined linearly with UV-B exclusion, and transcripts of PAL and HYH measured by quantitative real-time polymerase chain reaction were expressed at lower levels. UV-A linearly regulated the accumulation of quercetin-3-galactoside and quercetin-3-arabinopyranoside and had a quadratic effect on HYH expression. Furthermore, there were strong positive correlations between PAL expression and accumulation of four flavonols under the UV treatments. Our findings in silver birch contribute to a more detailed understanding of plant responses to solar UV radiation at both molecular and metabolite levels.

The natural variation in quantity and quality of light modifies plant morphology, growth rate and concentration of biochemicals. The aim of two growth-room experiments was to study the combined effects of red (R) and far-red (FR) light and ultraviolet-B (UV-B) radiation on the concentrations of leaf phenolics and growth and morphology of silver birch ( Betula pendula Roth) seedlings. Analysis by high-performance liquid chromatography showed that the leaves exposed to supplemental FR relative to R contained higher concentrations of total chlorogenic acids and a cinnamic acid derivative than the leaves treated with supplemental R relative to FR. In contrast, concentration of a flavonoid, quercetin 3-galactoside, was higher in the R + + + + UV-B leaves than in the FR + + + + UV-B leaves. The UV-B induced production of kaempferols, chlorogenic acids and most quercetins were not modified by the R : FR ratio. Growth measurements showed that the leaf petioles and stems of FR seedlings were clearly longer than those of R seedlings, but leaf area was reduced by UV-B radiation. Results of these experiments show that exposure of silver birch seedlings to supplemental FR compared to R leads to fast elongation growth and accumulation of phenolic acids in the leaves.