Development of UV defense mechanisms during growth of spinach seedlings.

Changes in UV defense mechanisms were studied during the growth periods of spinach seedlings grown under the white light, which did not contain UV-B light. DNA photolyase activity in the photorepair of cyclobutane pyrimidine dimers in spinach seedlings was high in the early growth phase (cell division phase) and declined thereafter, whereas UV-absorbing substances accumulated throughout the growth period acted as a major UV-defense mechanism in the cell expansion phase.