A number of species of sun and shade plants in the vegetative phase were grown in different light intensities, different light qualities (r/fr ratio) and different combinations of light intensity and nutrient supply. Sun and shade species were also grown at various plant densities and in interspecific competition in different light intensities and qualities. All the species examined responded to light intensity strongly, and in very much the same way. Sun species generally responded differently than shade species to a low red/far-red ratio: their stem extension increased markedly and their dark respiration rate was higher. The shade species generally responded similarly, but to a lesser degree. Interactions were recorded between the effects of light intensity and the effects of nutrient supply when nitrate supply was limiting and also when phosphate supply was limiting. To ensure that its limiting effect did not depend on plant size, the nitrate, or phosphate, was supplied in a high concentration intermittently and therefore exponential growth occurred in all combinations of light intensity and nutrient supply. When competing with shade species in higher light intensities, the sun species definitely had greater competitive abilities than their competitors. In lower light intensities the competitive ability of a species seemed to depend more on its weight at the beginning of the experiment. The formation of weaker stems in sun species, however, could be an important disadvantage for these species when competing in lower light intensities, especially when the red/far-red ratio is also low, as occurs in natural shade. It can be concluded that the responses to the red/far-red ratio are crucial in the explanation of the habitat preferences of sun and shade species. Responses to the light intensity may play a supplementary role, but systematic differences between sun and shade species in this respect were not observed.
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