Higher amounts of anthocyanins and UV-absorbing compounds effectively lowered CPD photorepair in purple rice (Oryza sativa L.)

Growth of a near-isogenic line (NIL) for the purple leaf gene Pl of rice with a genetic background of Taichung 65 (T-65) rice was significantly retarded by supplementary ultraviolet-B radiation (UV-B), despite the fact that the amounts of UV-absorbing compounds and anthocyanins in NIL were significantly higher than those in T-65. In order to understand the role of flavonoids in UV-B induced damage protection in T-65 and the NIL, both the (1) relationships between changes in the steady state of cyclobutane pyrimidine dimer (CPD) levels and changes in accumulation of anthocyanins and UV-absorbing compounds in leaves with leaf age, and (2) the susceptibility to CPD induction by UV-B radiation and the ability to photorepair CPD were examined. Although supplementary UV-B elevated the steady state of CPD levels in leaves in both strains, the level in the leaf of the NIL was higher than that in T-65 at any time. The susceptibility to CPD induction by short-term (challenge) UV-B exposure was lower in the NIL than in T-65. On the other hand, the CPD photorepair was also lower in the leaves of the NIL than in those of T-65. The decrease in CPD-photorepair in the NIL was due to a lowering of the leaf-penetrating blue/UV-A radiation, which is effective for photoreactivation by photolyase, by anthocyanins. Thus, accumulation of anthocyanins and UV-absorbing compounds did not effectively function as screening against damage caused by elevated UV-B radiation in the NIL, and the retardation of growth in the NIL resulted from its lower ability to photorepair CPD by higher amounts of anthocyanins.