Abstract

The effects of solar UV-B radiation, in combination with elevated temperature (4 °C) and CO2 (680 μL L-1) concentration, on sunflower and maize seedlings were studied from May to August in 1991 at the research station Quinta de Sao Pedro in Portugal (38.7°N). The ambient solar radiation of Portugal was reduced to levels of Central European latitudes by using the ozone filter technique. This radiation served as control, while the ambient solar radiation of Portugal was to simulate intense UV-B treatment (+30%). All plants were grown up to 18 days in 4 climate controlled growth chambers simulating a daily course of temperature with T max=28 °C or 32 °C, resp., and ambient CO2 concentrations (340 μL L-1); in one chamber the CO2 concentration was twice as high (680 μL L-1). Under intense UV-B and at 28 °C (T max) all growth parameters (height, leaf area, fresh and dry weight, stem elongation rate, relative growth rate) of sunflower and maize seedlings were reduced down to 35% as compared to controls. An increase in growing temperature by 4 °C, alone or in combination with doubled CO2, compensated or even overcompensated the UV-B effect so that the treated plants were comparable to controls. Chlorophyll content, on a leaf area basis, increased under intense UV-B radiation. This increase was compensated by lower leaf areas, resulting in comparable chlorophyll contents. Similar to growth, also the net photosynthetic rates of sunflower and maize seedlings were reduced down to 29% by intense UV-B calculated on a chlorophyll basis. This reduction was compensated by an increased temperature. Doubling of CO2 concentration had effects only on sunflower seedlings in which the photosynthetic rates were higher than in the controls. Dark respiration rates of the seedlings were not influenced by any experimental condition. Transpiration and water use efficiency (wue) were not influenced by intense UV-B. Higher temperatures led to higher transpiration rates and lower water use efficiencies, resp. Doubling of CO2 reduced the transpiration rate drastically while for wue maximum values were recorded.

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