In O.xalis, epidermal cells on both the adaxial and abaxial surface of the leaf concentrated light within lhe leaf by a lens mechanism. Focal lenglhs of epidermal cells were estimated using two iTiethods: they were calculated from radius of curvature measurements taken from individual epidermal cells, and were measured directly in agarosc replicas of the leaf surface. In the three species of Oxalis examined, light that was incident upon the adaxial leaf surface was concentrated within the palisade, whereas light that was incident upon the abaxial leaf surface was concentrated within the spongy mesophyll. Using scnsiomctric analysis, theoretically maximal focal intcnsilications were measured in leaf replicas at the focal maximum and at intermediate positions corresponding to the mid-region of the palisade and spongy mesophyll tissues. Focal intensifications ranged from 2.2 to 10.4 limes incident light at the focal maximum, and 1.3 to 4.5 in the palisade or spongy mesophyll layers. Elimination of epidermal focussing, by covering the leaf surface with a thin layer of mineral oil, strongly alTcctcd chlorophyll fluorescence induction curves resulting in a decrease of 10-40% in the initial (FJ and variable fluorescence {¥,). These results arc consistent with the interprclation thai the chloroplasts were adapted to their light microcnvironment within the leaf and that focussing by the epidermis channelled light to a population of chloroplasts thai were adapted to high light.

Light that exceeds the photosynthetic capacity of a plant can impair the ability of photosystem II to oxidize water. The light-induced inhibition is initiated by inopportune electron transport reactions that create damaging redox states. There is evidence that secondary electron transport pathways within the photosystem II reaction center can protect against potentially damaging redox states. Experiments using thylakoid membranes poised at different ambient redox potentials demonstrate that light-induced damage to photosystem II can be controlled by a redox component within the reaction center [Nedbal, L., et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 7929-7933]. The rate of photoinhibition is slow when the redox component is oxidized, but increases by more than 10-fold when the redox component is reduced. Here, using spinach thylakoid membranes, we provide evidence that the redox component is cytochrome b559, an intrinsic heme protein of the photosystem II reaction center. The results support a model in which the low-potential (LP) form of cytochrome b559 protects photosystem II by deactivating a rarely formed, but hazardous redox state of photosystem II, namely, P680/Pheo-/ QA-. Cytochrome b559LP is proposed to deactivate this potentially lethal redox state by accepting electrons from reduced pheophytin. The key observations supporting this proposal are as follows: (1) The oxidation-reduction potential of cytochrome b559LP is in the range predicted by redox titrations of photoinhibition. (2) If cytochrome b559LP is reduced prior to illumination, the rate of photoinhibition is fast, whereas if the cytochrome is oxidized prior to illumination, the rate of photoinhibition is slow.(ABSTRACT TRUNCATED AT 250 WORDS)

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Arabidopsis thaliana grown in a light regime that included ultraviolet-B (UV-B) radiation (6 kJ m(-2) d(-1)) had similar light-saturated photosynthetic rates but up to 50% lower stomatal conductance rates, as compared to plants grown without UV-B radiation. Growth responses of Arabidopsis to UV-B radiation included lower leaf area (25%) and biomass (10%) and higher UV-B absorbing compounds (30%) and chlorophyll content (52%). Lower stomatal conductance rates for plants grown with UV-B radiation were, in part, due to lower stomatal density on the adaxial surface. Plants grown with UV-B radiation had more capacity to down regulate photochemical efficiency of photosystem II (PSII) as shown by up to 25% lower phi(PSII) and 30% higher non-photochemical quenching of chlorophyll fluorescence under saturating light. These contributed to a smaller reduction in the maximum photochemical efficiency of PSII (F (v)/F (m)), greater dark-recovery of F (v)/F (m), and higher light-saturated carbon assimilation and stomatal conductance and transpiration rates after a four-hour high light treatment for plants grown with UV-B radiation. Plants grown with UV-B were more tolerant to a 12 day drought treatment than plants grown without UV-B as indicated by two times higher photosynthetic rates and 12% higher relative water content. UV-B-grown plants also had three times higher proline content. Higher tolerance to drought stress for Arabidopsis plants grown under UV-B radiation may be attributed to both increased proline content and decreased stomatal conductance. Growth of Arabidopsis in a UV-B-enhanced light regime increased tolerance to high light exposure and drought stress.

We investigated the effects of an ambient dose of ultraviolet-B (UV-B) radiation on chamber-grown Pseudotsuga menziesii var. glauca (Beissn.) Franco (Douglas-fir) seedlings, to determine if the presence of UV-B radiation in the growth light regime induces tolerance to environmental stresses such as high light and drought. Douglas-fir seedlings were grown without UV-B radiation or with 6 kJ m-2 day-1 of biologically effective UV-B, which is ambient for the intermountain regions of Idaho. Non-stressed seedlings grown with UV-B radiation had 35% lower seedling dry mass, 36% higher concentrations of UV-B absorbing compounds per unit leaf area, 30% lower stomatal frequencies, 25% lower light-saturated photochemical efficiencies of Photosystem II and 45% lower light-saturated stomatal conductance than non-stressed seedlings grown without UV-B radiation. After 4 days of high-light stress, seedlings grown with UV-B radiation had 32% higher light-saturated carbon assimilation rates (A(CO2)) than seedlings grown without UV-B radiation. After water was withheld from the seedlings for up to 15 days, seedlings grown with UV-B radiation had 50% higher A(CO2) and 40% higher seedling water potentials than seedlings grown without UV-B radiation. The results support the hypothesis that UV-B radiation can act as an environmental signal to induce tolerance to high-light and drought stress in Douglas-fir seedlings. Possible mechanisms for the enhanced stress tolerance are discussed.