DEPENDENCE OF Pfr/Ptot-RATIOS ON LIGHT QUALITY and LIGHT QUANTITY

Abstract— Not only the spectral distribution of the light source determines the relative proportion of phytochrome in the P_fr(P_r) form, the P_fr/P_tot-ratio also depends strongly on the fluence rate of the irradiation. This dependence has been observed in the cotyledons of etiolated mustard seedlings for blue light of fluence rates below 20 Wm_-2. It has also been observed for white light and seems to be a characteristic of the phytochrome system resulting from the involvement of phytochrome thermal reactions as well as P_r P_fr photoconversions. The fluence rate dependence of P_fr/P_tot-ratios can be used to analyze the characteristic transformations of the phytochrome system. Phototransformations together with a fast thermal transformation (τ_½⋍ 3min) are consistent with the results obtained for blue and white light.