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 a gents in the resistance of Cucumis sativus L. to UV-B radiation was in vestigated by exposing young leaves at a defined developmental stage f rom 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 incr eases in UVAP) immediately following UV-B were compared to damage in t he same tissue (increased specific leaf weight or chlorosis) determine d 72 h after the start of UV-B. Poinsett was more sensitive to both fo rms 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 re sponsible for genetic differences in sensitivity to UV-B for the follo wing reasons: 1) the kinetics of UVAP increase were similar to that fo r 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 Poin sett when given simultaneously but had relatively small stimulatory ef fects on rapid UVAP accumulation. These results do not rule out a gene ral role for optical screening by UVAP nor do they exclude the possibi lity that qualitative differences in UVAP (e.g., as antioxidants) are the basis for cultivar differences.