Phytochrome photoconversion in vivo. Effect of the initial Pfr/Ptot ratio

The equation for phytochrome photoconversion, derived from photoconversion kinetics of purified phytochrome, predicts that the rates of photoconversions starting from low and high Pfr/Ptot0, (initial Pfr/Ptot) should be the same for light of the same quality and fluence rate. The situation might be different in vivo. Phytochrome photoconversion rates were measured in excised cotyledons of Cucurbita pepo L. exposed to BL (blue; ϕBL= 0.39; ϕ, Pfr/Ptot at photoequilibrium) and RI (mixture of red and far red; ϕRI= 0.46) after saturating preirradiations with red and far‐red to establish high (0.78) and low (0.02) Pfr/Ptot0, respectively. Under BL, the rate of photoconversion is faster when starting from a high than a low Pfr/Ptot0; under RI, the rate of photoconversion is faster when starting from a low than a high Pfr/Ptot0., No effects of Pfr/Ptot0, on photoconversion rates were found in phytochrome solutions exposed to BL and RI. These data provide another indication of the discrepancies between phytochrome photconversion kinetics in vivo and in vitro.