LAUSR

Abstract Plant responses to the red-to-far-red ratio (R:FR) of light have been investigated, but interaction between R:FR and other factors is less known. Here, we investigated the effects of R:FR × vapor-pressure deficit (VPD) on stem elongation and leaf expansion. Cucumber (Cucumis sativus) seedlings were grown in growth chambers at a VPD of 0.4 or 3.0 kPa, an air temperature of 28 °C, and an R:FR of 1.0 or 10 for 7 days after germination. Lower R:FR plus lower VPD synergistically stimulated stem elongation, likely owing to an interaction between cell-wall extensibility and turgor pressure, which (respectively) respond to changes in the proportion of active phytochrome and in evaporative demand. Leaf expansion was also stimulated at lower R:FR plus lower VPD, but unlike stem elongation, lower VPD reduced the effect of R:FR on leaf expansion. This may be explained by resource competition between stems and leaves: photosynthate allocation to leaves decreased under lower R:FR plus lower VPD. This may limit leaf expansion and thereby moderate the R:FR effect on leaf expansion, although additional factors may stimulate cell extension.