Combination effects of UV-B radiation and temperature on sunflower (Helianthus annuus L, cv Polstar) and maize (Zea mays L, cv Zenit 2000) seedlings

Sunflower (Helianthus annuus L., cv. Polstar) and maize (Zea mays L., cv. Zenit 2000) seedlings were grown for 23 days in climate controlled growth chambers under solar radiation. Using UV-transmitting filters as covers (ozone filter technique) a difference in UV-B irradiance of 20 to 29 % was simulated. This difference corresponds to a decrease in stratospheric ozone of about 12 % based on ambient ozone levels at Lisbon (Portugal). During the experimental season in 1990 the daily course of temperature ranged from T-min = 13.5 degrees C at night to T-max = 28 degrees C around midday, and in 1991 from T-min = 17.5 to T-max = 32 degrees C simulating an average diurnal temperature increase, which might be expected in the 21st century as a result of the greenhouse effect. Generally, sunflower and maize seedlings grown under T-max = 32 degrees C had increased growth parameters compared to seedlings grown under T-max = 28 degrees C. In both plant species fresh and dry weight as well as plant size and leaf area were increased as a result of reduced UV-B radiation at T-max 28 degrees C, but, with very few exceptions, not at T-max = 32 degrees C. Thus, the increase in temperature by 4 degrees C mainly compensated the reduced growth caused by increased UV-B radiation. The net photosynthetic rates of sunflower and maize seedlings based on leaf area exhibited higher or similar rates under enhanced UV-B radiation in comparison to the control plants. However, on a plant basis significant UV-B dependent decreases were observed. Increased growing temperature resulted in higher net photosynthetic rates per plant. Neither UV-B nor temperature had an influence on the chlorophyll content per plant. Respiration and transpiration of sunflower and maize plants related to the entire plant did not show any UV-B dependent differences. Increased temperature resulted in higher respiration rates in maize only, whereas transpiration increased in both species.