Characterization of stomatal closure caused by ultraviolet-B radiation

The effects of ultraviolet-B (UV-B) radiation on stomatal conductance (g(s)) in pea (Pisum sativum L.), commelina (Commelina communis L.), and oilseed rape (Brassica napus L.) plants were investigated. Plants were grown in a greenhouse either with three different high ratios of UV-B to photosynthetically active radiation or with no UV-B radiation. Pea plants grown in the highest UV-B radiation (0.63 W m(-2)) exhibited a substantial decrease of adaxial and abaxial g(s) (approximately 80% and 40%, respectively). With growth in 0.30 W m(-2) of UV-B adaxial g(s) was decreased by 23%, with no effect on abaxial g(s), and lower UV-B irradiance of 0.21 W m(-2) had no effect on either surface. Although abaxial g(s) increased when leaves were turned over in control plants, it did not in plants grown with the highest UV-B. Adaxial g(s) in commelina and oilseed rape also decreased on exposure to high UV-B (0.63 W m(-2)). For previously unexposed pea plants the time course of the effect of UV-B on g(s) was slow, with a lag of approximately 4 h, and a time constant of approximately 3 h. We conclude that there is a direct effect of UV-B on stomata in addition to that caused by changes in mesophyll photosynthesis.

Does free-Air carbon dioxide enrichment affect photochemical energy use by evergreen trees in different Seasons? A chlorophyll fluorescence study of mature loblolly pine

Previous studies of the effects of growth at elevated CO(2) on energy partitioning in the photosynthetic apparatus have produced conflicting results. The hypothesis was developed and tested that elevated CO(2) increases photochemical energy use when there is a high demand for assimilates and decreases usage when demand is low. Modulated chlorophyll a fluorescence and leaf gas exchange were measured on needles at the top of a mature, 12-m loblolly pine (Pinus taeda L.) forest. Trees were exposed to ambient CO(2) or ambient plus 20 Pa CO(2) using free-air CO(2) enrichment. During April and August, periods of shoot growth, light-saturated photosynthesis and linear electron transport were increased by elevated CO(2). In November, when growth had ceased but temperatures were still moderate, CO(2) treatment had no significant effect on linear electron transport. In February, when low temperatures were likely to inhibit translocation, CO(2) treatment caused a significant decrease in linear electron transport. This coincided with a slower recovery of the maximum photosystem II efficiency on transfer of needles to the shade, indicating that growth in elevated CO(2) induced a more persistent photoinhibition. Both the summer increase and the winter decrease in linear electron transport in elevated CO(2) resulted from a change in photochemical quenching, not in the efficiency of energy transfer within the photosystem II antenna. There was no evidence of any effect of CO(2) on photochemical energy sinks other than carbon metabolism. Our results suggest that elevated CO(2) may increase the effects of winter stress on evergreen foliage.

Ultraviolet-B radiation effects on water relations, leaf development, and photosynthesis in droughted pea plants

The effects of ultraviolet-B (UV-B) radiation on water relations, leaf development, and gas-exchange characteristics in pea (Pisum sativum L. cv Meteor) plants subjected to drought were investigated. Plants grown throughout their development under a high irradiance of UV-B radiation (0.63 W m-2) were compared with those grown without UV-B radiation, and after 12 d one-half of the plants were subjected to 24 d of drought that resulted in mild water stress. UV-B radiation resulted in a decrease of adaxial stomatal conductance by approximately 65%, increasing stomatal limitation of CO2 uptake by 10 to 15%. However, there was no loss of mesophyll light-saturated photosynthetic activity. Growth in UV-B radiation resulted in large reductions of leaf area and plant biomass, which were associated with a decline in leaf cell numbers and cell division. UV-B radiation also inhibited epidermal cell expansion of the exposed surface of leaves. There was an interaction between UV-B radiation and drought treatments: UV-B radiation both delayed and reduced the severity of drought stress through reductions in plant water-loss rates, stomatal conductance, and leaf area.

Relationship between CO2 Assimilation, Photosynthetic Electron Transport, and Active O2 Metabolism in Leaves of Maize in the Field during Periods of Low Temperature

Measurements of the quantum efficiencies of photosynthetic electron transport through photosystem II (phiPSII) and CO2 assimilation (phiCO2) were made simultaneously on leaves of maize (Zea mays) crops in the United Kingdom during the early growing season, when chilling conditions were experienced. The activities of a range of enzymes involved with scavenging active O2 species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures during development, the ratio of phiPSII/phiCO2 was elevated, indicating the operation of an alternative sink to CO2 for photosynthetic reducing equivalents. The activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and superoxide dismutase and the levels of ascorbate and alpha-tocopherol were also elevated during chilling periods. This supports the hypothesis that the relative flux of photosynthetic reducing equivalents to O2 via the Mehler reaction is higher when leaves develop under chilling conditions. Lipoxygenase activity and lipid peroxidation were also increased during low temperatures, suggesting that lipoxygenase-mediated peroxidation of membrane lipids contributes to the oxidative damage occurring in chill-stressed leaves.