Effects of Temperature and Aeration on Phytochrome Transformations in Pastinaca sativa Root Tissue

Phytochrome reversion from the Pfr form to the Pr form in Pastinaca sativa L. tissue occurs very rapidly during the first 1½ hr after exposure to red light and then more slowly; some Pfr is still present after 12 hr at 26 C. Phytochrome destruction, on the other hand, is initially very slow and does not become evident until after the period of rapid reversion. The rate of reversion is reduced by low temperatures but not by low levels of oxygen. Phytochrome destruction in parsnip tissue briefly exposed to red light is also unaffected by low levels of oxygen, suggesting that it may differ in mechanism from destruction in etiolated seedlings. However, reduced oxygen levels inhibit phytochrome loss in tissues held in darkness. Phytochrome in cauliflower tissue proved to be fully light-stable, confirming earlier observations.