How plants respond to low quantities of light could help in understanding plant development and ecology in habitats with changing light levels. Most previous studies have investigated shading by other plants, where spectral quality of light is changed. In this study, by contrast, several aspects of growth and reproduction were measured in Ageratina altissima and Rudbeckia laciniata (both grassland forbs in Asteraceae) grown for six weeks in a greenhouse where light was reduced to 10%, 50%, and 100% of ambient levels without changing spectral quality. Ageratina altissima was more responsive to light than was R. laciniata. In response to low light quantity, A. altissima displayed traits that were related to shade avoidance, common in grassland species, such as decreased chlorophyll content and biomass, fewer branches, leaves, and ramets, and decreased reproductive output. However, A. altissima also displayed traits that were consistent with shade tolerance, common in forest species, such as reduced elongation of stems, higher chlorophyll a/b ratio, reduced root and shoot biomass, and fewer and thinner leaves. Rudbeckia laciniata was less responsive to light treatments, only showing effects in leaf thickness and the SPAD chlorophyll index. There was no effect of light on leaf area, petiole length, or several chlorophyll fluorescence parameters in either species. Responses to low light quantity might involve a combination of shade avoidance and shade tolerance strategies in responsive species, potentially helpful as lighting conditions vary.
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