Combination Effects of UV-B Radiation and Temperature on Sunflower (Helianthus annuus L., cv. Polstar) and Maize (Zea mays L, cv. Zenit 2000) Seedlings

Summary 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 °C at night to T max = 28 °C around midday, and in 1991 from T min = 17.5 to T max = 32 °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 °C had increased growth parameters compared to seedlings grown under T max = 28 °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 °C, but, with very few exceptions, not at T max = 32 °C. Thus, the increase in temperature by 4 °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.