LIGHT QUALITY AND STEM GROWTH IN IMPATIENS PARVIFLORA DC.

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Abstract
Summary: Internode and hypocotyl elongation were measured in Impatiens parviflora DC. in light environments in which extra far‐red light was mixed with white light from fluorescent tubes during the photoperiod and at the end of the photoperiod. Night breaks of varying spectral composition were also given. The logarithmic rate constant of internode elongation was found to be linearly related to the estimated proportion of phytochrome existing as the far‐red absorbing form. Supplementary far‐red light at the end of the day caused no significant increase in the elongation rate whereas night break treatments significantly increased the rate of internode elongation. Hypocotyl elongation was not related to the estimated phytochrome photo‐equilibrium and appeared to be under the control of both blue and red light. The results are discussed in relation to the hypothesis that the control of stem elongation by phytochrome is an adaptation for detecting and avoiding shade.