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Chenopodium album seedlings were grown in light environments in which supplementary far-red light was mixed with white fluorescent light during various parts of the photoperiod. Both the logarithmic rate constant of stem extension and the leaf dry weight: stem dry weight ratio were linearly related to estimated phytochrome photoequilibrium (ϕ) in each treatment regime. These data are taken to be indicative of a functional link between phytochrome and development in the green plant. A layer of chlorophyllous tissue only affected the linearity between calculated ϕ and the logarithmic stem extension rate at high chlorophyll concentrations, whilst even low concentrations-equivalent to the levels found in stem tissue-caused a significant shift in measured ϕ. End-of-day supplementary far-red (FR) light induced between 0-35 per cent of the response elicited by all-day supplementary FR, whilst daytime supplementary FR (with a white fluorescent light end-of-day treatment) induced approximately 90 per cent. The ecological significance of this difference is discussed with respect to shade detection.

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SUMMARYIn nature, fluence rate and the red .far-red ratio are reduced in shadelight beneath vegetation; both show similar exponential decays with increasing canopy density. An assessment of the degree to which each of these factors controls development in natural shade was made by comparing Chenopodium album plants grown under vegetational shade with those grown under the controlled environment light treatments of low fluence rate, and combined low fluence rate and low red .far-red ratio. The observations of stem extension and specific leaf area from vegetational shade were entirely consistent with the eflFects of the controlled environment light treatments. The increase of specific leaf area in natural shade was probably due to the low fluence rate; the initial rapid stem extension in natural shade was probably due to the reduced red: far-red ratio. There is good evidence to suggest that the latter is a phytochrome-controlled phenomenon.