Physiological and Ecological Studies in the Analysis of Plant EnvironmentVI. The Constancy for Different Species of a Logarithmic Relationship between Net Assimilation Rate and Light Intensity and its Ecological Significance

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Abstract
In this further study of light as an environmental factor the effects of shading on the growth of Helianthus annuus, Fagopyrum esculentum, Trifolium subterraneum, Tropaeolum majus, Lycopersicum esculentum, Vicia faba, Pisum sativum, Hordeum vulgare, Solanum dulcamara, and Geum urbanum have been investigated. It has been established that the reductions in the net assimilation caused by shading are similar for all ten species. The net assimilation rate during the season of active growth is linearly related to the logarithm of the light intensity. When similar experiments are conducted late in the autumn and the relative growth rate is very low, the logarithmic relationship no longer holds. From the data it is possible to obtain precise estimates of the compensation point. The mean values of the compensation point ranged from 0·06 to 0·09 of daylight for eight species, while for V. faba and H. vulgare somewhat higher figures were obtained—0·14 and 0·18 of daylight. Flctuations in the net assimilation rate in full daylight showed no correlation with variations in the value of the compensation point. From these results it is concluded that species cannot be grouped into ‘sun’ or ‘shade’ plants, either on the basis of differences in the value of the compensation point or on the grounds that there are large variations in the effects of shading on net assimilation rate.

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