Light-dependent changes of plant growth and development are known to be regulated by plant hormones.
In investigated, responsive species internode elongation is influenced by the R:FR-ratio through modulation of the levels of hormones, especially gibberellin (GA) and auxin.
In cowpea (Vigna sinensis) enhanced elongation in response to EOD-FR is apparently due to reduced inactivation of GA1 as a consequence of removal of the active phytochrome form Pfr
In hybrid aspen (Populus tremula × tremuloides) phytochrome A (PHYA) overexpression resulted in decreased shoot elongation due to reduced cell numbers and cell lengths, and this was correlated with reduced GA and IAA levels
However, after exposure of PHYAoverexpressing lines and wild type plants to EOD-FR light, the plant length, cell lengths and cell numbers as well as GA levels were similar in both genotypes, indicating that phyA is not involved in the response to EOD-FR
As shown in a number of species, responses to different R:FR and EOD treatments appear to depend on the light stable phytochromes phyB, phyD and phyE