Photochemical and Nonphotochemical Reactions of Phytochrome in vivo.

The nonphotochemical reactions of phytochrome in the coleoptiles of dark-grown corn seedlings were studied at 3 temperatures: 14 degrees , 24 degrees , and 34 degrees . The data obtained show that the destruction of P(fr) is the only measurable reaction occurring; reversion of P(fr) to P(r) was not found. The Q(10)'s (2.7 and 3.5) and zero order kinetics found for the destruction reaction are consistent with the hypothesis that the reaction is enzyme-mediated.In vivo action spectra for phytochrome transformation in the coleoptiles of darkgrown corn seedlings were obtained which agree qualitatively with those obtained by other workers for phytochrome-mediated physiological responses and in vitro action spectra. In vivo conversion of phytochrome by blue light, as determined from spectrophotometric measurements of phytochrome itself, is reported. Action peaks for P(r) were found at 667 mmu and in the blue in the region of 400 mmu, with a broad shoulder from 590 mmu to 640 mmu. Action peaks for P(fr) were found at 725 mmu and in the blue in the region of 400 mmu with a minor peak at 670 mmu, and a broad shoulder from 590 mmu to 640 mmu. The ratio of the quantum efficiencies of P(r) at 667 mmu and P(fr) at 725 mmu (Phi(r667)/Phi(fr725)) was estimated to be 1.0.