Differential Effects of Short‐Term Exposure to Ultraviolet‐B Radiation upon Photosynthesis in Cotyledons of a Resistant and a Susceptible Species

Elevated exposure to ultraviolet‐B (UVB) radiation influences the growth and development of higher plants in a variety of ways, with different species exhibiting varying degrees of resistance and susceptibility. The effects of relatively short‐term high‐intensity exposure to UVB upon photosynthetic CO2 fixation were studied in cotyledons of cucumber (Cucumis sativus L. cv. Poinsett) and sunflower (Helianthus annuus L. cv. Gray Stripe). Treatment with 194 kJ m−2 of UVB radiation delivered over 16 h led to significantly reduced CO2 fixation rates in cucumber, while sunflower showed no inhibition or a slight increase. Other photosynthetic parameters were studied in order to determine the basis for the relative susceptibility and resistance of these two species. The concentrations of both chlorophyll a and chlorophyll b were unchanged in response to UVB treatment in cucumber but showed statistically significant increases in sunflower. Carotenoids did not change significantly in either species. Flavonoids (i.e., methanol extractable ultraviolet absorbing compounds) decreased in cucumber and were unchanged in sunflower. A decreased proportion of open stomata was observed in cucumber following UVB treatment, while sunflower remained unchanged. Chlorophyll fluorescence analysis showed UVB‐induced decreases in the ratio of variable to maximum fluorescence under both dark‐adapted and steady state photosynthesis conditions, and these decreases were more severe in cucumber than in sunflower. These results indicate that the differing abilities of cucumber and sunflower to withstand brief exposure to high‐irradiance UVB are based on a combination of factors, including but not limited to the content of photosynthetic and UV‐screening pigments, stomatal function, light‐harvesting efficiency, and electron transport capabilities.