Physiological and Ecological Studies in the Analysis of Plant Environment: VII. An Analysis of the Differential Effects of Light Intensity on the Net Assimilation Rate, Leaf-Area Ratio, and Relative Growth Rate of Different Species |
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Authors: | BLACKMAN, G. E. WILSON, G. L. |
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Affiliation: | Department of Agriculture Oxford |
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Abstract: | Since relative growth rate is the product of net assimilationrate and leaf-area ratio (leaf area/plant weight), it followsthat if the effects of shading on both net assimilation rateand leaf-area ratio can be expressed mathematically, then therelationship between light intensity and relative growth ratecan be derived from the product of the two mathematical expressions. For all the ten species investigated in field and pot cultureexperiments, it has been found that during the early vegetativephase both the changes in leaf-area ratio and net assimilationrate, over the range of 0·1 to full daylight, are linearlyrelated to the logarithm of the light intensity. In consequence,the relationship between relative growth rate and the logarithmof light intensitybeing the product of the two linearregressionsis curvilinear. For species of shady habitats (Geum urbanum, Solamun dulcamara)neither the levels of assimilation rate nor the compensation-pointvalues are very different from those of the eight species fromopen situations (e.g. Hordeum vulgare, Pisum sativum, Fagopyrumesculentum). Nevertheless the intensity at which growth rateis maximal varies between species: it is 05 for G. urbanum,07 for H. annuus, full daylight for F. esculentum, whilefor Trifolium subterraneum the calculated value is 1·8daylight. Such specific differences can be largely accountedfor in terms of the differences in leaf-area ratio at the differentlight levels. On the basis of this analysis of the light factor, a shadeplant is best redefined as a species in which a reduction ofthe light intensity causes a rapid rise in the leaf-area ratiofrom an initial low value in full daylight: for a sunplant the converse definition holds. |
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