UVB-induced cyclobutyl pyrimidine dimer and photorepair with progress of growth and leaf age in rice

We have examined the susceptibility to cyclobutyl pyrimidine dimer (CPD) induction by UVB radiation, the ability to photorepair cyclobutyl pyrimidine dimers (CPDs), and the levels of ultraviolet (UV)-absorbing compounds in various developing stages of leaves in UVB-resistant (Sasanishiki) and -sensitive (Norin 1) rices (Oryza sativa L.). UVB produces more CPDs in the second, third and fourth fully expanded leaves than in the fifth and sixth fully expanded leaves. In leaves in which the levels of UV-absorbing compounds are lower, more CPDs are produced. The ability to photorepair CPDs in fully expanded leaves of different stages of each cultivar remains approximately constant, although the photorepair capacity in the fourth leaf rapidly increases from leaf emergence until full expansion. The photorepair capacity is lower in Norin 1 than that in Sasanishiki in all the leaf stages or different aged leaves. These results therefore suggest that lice plants could be more susceptible to CPD induction at the stage when the second, third or fourth leaves are developing, and UVB sensitivity correlates with low CPD photorepair.