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Abstract: Reflection by waxy or resinous surface structures and hairs, repair reactions of biomolecules and induction of different sheltering components provide the means of plant protection from harmful solar UV-B radiation. Secondary products, especially flavonoids and phenolic acids as defense components are also important in plant tolerance to UV-B, fulfilling the dual role as screens that reduce UV-B penetration in plant tissues, and as antioxidants protecting from damage by reactive oxidant species. Plants are sensitive to UV-B radiation, and this sensitivity can be even more clone-specific than species-specific. The results available in the literature for deciduous trees and shrubs indicate that UV-B radiation may affect several directions in the interaction of woody species with biotic (herbivores) and abiotic (CO2 and nutrition) factors depending on the specific interaction in question. These multilevel interactions should have moderate ecological significance via the overall changed performance of woody species and shrubs. [Copyright &y& Elsevier]

Five-year-old Norway spruce (Picea abies (L.) Karst.) seedlings were subjected to three simulated growing seasons in controlled environment chambers. Plants were acclimated to a soil temperature of 16 °C during the first and third growing seasons, but were allocated at random to soil temperature treatments of 9, 13, 18 and 21 °C during the second growing season. Low soil temperature during the second growing season depressed stomatal conductance and photosynthetic rate (A) per unit of projected leaf area, although intercellular CO2 concentrations did not differ significantly between treatments. At all soil temperatures, total chlorophyll concentration first decreased and then increased, although the rate of increase and the final concentration increased with soil temperature, which may explain the effect of soil temperature on A. Neither chlorophyll a/b ratio nor leaf nitrogen concentration was significantly affected by soil temperature. Treatment differences disappeared during the third simulated growing season when plants were again acclimated to a soil temperature of 16 °C. [ABSTRACT FROM PUBLISHER]

The response of Fuchsia magellanica Lam. -- a shade-tolerant perennial -- to white light photon flux density and end-of-day red and far-red light irradiation was studied to evaluate the role that different morphological and functional alterations play in whole-plant acclimation to shade. The estimated relative growth rate of plants growing under a photon flux density of 30 μmol m-2 s-1 was 71% of that of those growing under 450 μmol m-2 s-1. The proportion of total dry weight allocated to shoots was higher in low light plants, but the proportion of shoot dry weight allocated to leaf blades was not affected by any of the treatments. The estimated relative growth rate, specific leaf area and leaf conductance were not affected by end-of-day irradiation. In low light plants the specific leaf area was higher, and at saturating photon flux density photosynthesis was lower on a leaf area basis, but was not on a leaf dry weight basis. Dark respiration was higher in high light plants when expressed on an area basis, but not when expressed on a dry weight basis. During the normal photoperiod, leaf conductance measured in darkness was higher in high light plants than in low light ones, which is consistent with the higher stomatal density observed in these plants. The cross-sectional area of the petioles was slightly reduced in low light in spite of a large increase in the area of individual leaves. Low irradiance affected the display of leaves, decreased leaf thickness and increased chlorophyll content per unit dry weight. End-of-day far-red resulted in plants with more erect branches. Both low photon flux density and far-red enhanced internode elongation. The effect of end-of-day irradiations was bigger in low light than in high light, which is the opposite to what is observed in shade-avoiders. The relatively small difference in mean growth rate can be explained by changes in specific leaf area, shoot : root dry weight partition ratio, and by the shape of the photosynthetic light response curve. However when comparing photosynthetic rates on a dry weight basis low light plants do not seem to be at a disadvantage under high light. [ABSTRACT FROM AUTHOR]

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO2 and water vapour concentrations, responses of stomatal conductance (gsw) to boundary layer thickness have two components, one resulting from changes in intercellular CO2 concentration (χic) and another from changes in leaf surface water vapour saturation deficit (Dsw). The experiments and simulations also show that the boundary layer conductance (gbw) can significantly alter the apparent response of gsw) to ambient air CO2 mole fraction (χac) and water vapour mole fraction (χ aw). Because of the feedback loop involved the responses of gsw) for χac) and χaw) each include responses to both χic) and Dsw), The boundary layer alters the state of the variables sensed by the guard cells-i.e. χic) and Dsw)-and so it is a source of feedback, Thus, when scaling up from responses of stomata to the response of gsw) for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of gsw) to χic)and Dsw), the apparent responses of gsw) to Daw and χac) depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods. [ABSTRACT FROM AUTHOR]

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 such a way that relative humidity at the leaf surface is a more appropriate variable than water vapour saturation deficit at tile leaf surface and yields a simpler description of the compound response? To answer these questions, we measured the response of leaf conductance to humidity under constant leaf temperature, and the response to increasing leaf temperature tinder constant relative humidity and under constant water vapour saturation deficit. We found that, in Hedera helix subsp. canariensis (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 measured at the same temperature rather than at different temperatures. An inversely proportional response was consistently obtained when stomatal conductance was expressed in relation to water vapour saturation 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 lot describing stomatal responses to humidity. [ABSTRACT FROM AUTHOR]

We studied the effects of light quality and nutrient supply on growth and nitrogen accumulation in silver birch (Betula pendula Roth) seedlings to test three hypotheses: (1) growth of birch seedlings is sensitive to changes in light quality; (2) the response of birch seedling growth to light quality depends on nutrient supply; and (3) assimilation and allocation of nitrogen by birch seedlings are affected by light quality. The two light regimes simulated the spectral quality of sunlight and shadelight, but did not differ in photosynthetic photon flux density, and the two nutrient supply regimes differed in the rate of supply, but not in the composition, of mineral nutrients. [ABSTRACT FROM PUBLISHER]