Growth and physiological responses of sunflower plants exposed to ultraviolet-B radiation

Abstract

The effects of UV-B radiation were studied in sunflower plants (Helianthus annuus L. cv. Catissol-01) growning in greenhouse under natural photoperiod conditions. The plants received approximately 0.60Wm-2 (control) or 4.0Wm-2 (+UV-B) of UV-B radiation for 7h d-1, centered around solar noon from 15 days after sowing. Compared to the control, plants exposed to high UV-B radiation for 12 or 21 days did not show any difference in shoot dry matter, specific leaf weight or UV-B absorbing compounds. Enhanced UV-B radiation caused a significant inhibition of photosynthesis (A) only in the first sampling and this was accompained by reduction in stomatal conductance (g s) and transpiration rate. The inhibition in A can not be fully explained by reduction in g s since intercellular CO2 concentration was not affected by UV-B radiation. In both samplings, the total chlorophyll content was not affected by enhanced UV-B radiation whereas in the first sampling, the chlorophyll a and the ratio of chlorophyll a/b were reduced. Enhanced UV-B radiation increased the minimal fluorescence yield, but did not alter the ratio of variable to maximal fluorescence yield of dark adapted leaves. Overall, this study suggests that the present level of solar UV-B radiation affects sunflower plants performance even though the shoot dry biomass may not be affected.