Many open-habitat species are capable of perceiving and reacting to the reduced red: far-red ratio of vegetational shade. A well-documented example is the arable weed Chenopodium album, which responds most noticeably by a dramatic increase in height. Here this characteristic response is considered in greater detail, together with other morphogenic modifications elicited by shadelight quality. To enable such modifications to be interpreted in terms of differences in light quality, particular attention was paid to the minimization of fluence-rate differences between simulated daylight and simulated shadelight treatments. It was found that the greater stem extension in simulated shadelight results mainly from an increase in cell elongation. Individual internodes also showed an acceleration of the extension growth phase, and a reduction in the time taken for completion of their growth, in simulated shadelight. There was no evidence to suggest that the rate of leaf formation was affected; however, apical dominance was enhanced resulting in the suppression of axillary outgrowth.
Total leaf area was relatively unaffected by the different light quality regimes; however, leaves of higher insertion expanded more rapidly in simulated shadelight. Leaves which developed in this treatment had reduced stomatal frequencies on both surfaces, together with a reduction in photosynthetic pigment content on a leaf area basis. The possibility that these modifications are part of a coordinated response to shadelight, and the site of light quality perception, are discussed.