Significance of Fluence‐response Data in Phytochrome‐initiated Seed Germination

A model was developed to describe changes in fluence‐response kinetics in terms of total phytochrome level (Ptot), level of the factor X interacting with active phytochrome (Pfr), PfrX equilibrium constant, seed sensitivity to Pfr‐X interaction, and variation in phytochrome sensitivity within the seed population. Under conditions of stable X levels and stable population variation, the model predicted that a change in any of the other components will result in a parallel fluence‐response curve on a probit‐logarithmic plot. The linearity of the subsaturation plot is dependent on the ratio of Ptot to X concentrations. The model showed that changes in threshold response fluences can result from many causes other than changes in total phytochrome [Ptot>]. Changes in response‐saturating fluences when maximal germination is less than 100% are predicted to be due to limiting levels of X. Changes in slope of fluence‐response curves can be explained by changes in seed population variation by this model. Rumex crispus L. fluence‐response data for germination is best explained by this model in terms of neither changes in Ptot nor X levels altering kinetics.