As noted in textbook Chapter 16, phytochrome is encoded by members of a multigene family, and it is possible that the effects of red and far-red light described in the previous section are mediated by different types of phytochrome (A and B). In LDPs such as Arabidopsis, pea (Pisum sativum), and sorghum (Sorghum bicolor), phytochrome B (phyB) appears to be an inhibitor of flowering, since mutations in the gene PHYB that eliminate or reduce the amount of the phyB protein cause more rapid flowering (Reed et al. 1994).

The promotion of flowering by far-red light may result from a decrease in the amount of the Pfr form of phyB, which would have the same effect on the phyB Pfr levels as the mutations that reduce the total amount of phyB. As already discussed, red-light interruptions of a dark period prevent flowering in SDPs; therefore, the Pfr form of phyB may be an inhibitor of flowering in SDPs as well.

In the LDPs Arabidopsis and pea, mutations in the PHYA gene interfere with the promotion of flowering by long days (Weller et al. 1997); thus the Pfr form of phyA may promote flowering in LDPs. The effect of the phyA mutation is quite strong in pea (the mutant plants are essentially unable to respond to inductive photoperiods) whereas the phyA mutation in Arabidopsis has only a slight effect on the ability to respond to inductive photoperiods. The species difference in the effect of phyA mutations is probably due to the fact that pea relies entirely on phyA to sense inductive photoperiods, whereas Arabidopsis also uses a blue light photoreceptor for this purpose.