Rapid fluence-dependent responses to ultraviolet-B radiation in cucumber leaves: The role of UV-absorbing pigments in damage protection

The role of foliar UV-absorbing pigments (UVAP) as optical screening agents in the resistance of Cucumis sativus L. to UV-B radiation was investigated by exposing young leaves at a defined developmental stage from sensitive (cv Poinsett) and insensitive (cv Ashley) lines to brief UV-B treatments varying between 4 and 10 h. The amount of blue light (BL) or UV-A radiation during W-B exposure was also varied. Rapid increases in UVAP) immediately following UV-B were compared to damage in the same tissue (increased specific leaf weight or chlorosis) determined 72 h after the start of UV-B. Poinsett was more sensitive to both forms of UV-B damage than Ashley under conditions where the response to UV-B was not saturated. Although UVAP increased rapidly in response to UV-B, it is unlikely that optical screening by these compounds was responsible for genetic differences in sensitivity to UV-B for the following reasons: 1) the kinetics of UVAP increase were similar to that for induction of damage; 2) increases in UVAP in the UV-sensitive line (Poinsett) were similar to those in the resistant line (Ashley); and 3) BL and UV-A radiation significantly reduced damage by UV-B in cv Poinsett when given simultaneously but had relatively small stimulatory effects on rapid UVAP accumulation. These results do not rule out a general role for optical screening by UVAP nor do they exclude the possibility that qualitative differences in UVAP (e.g., as antioxidants) are the basis for cultivar differences.