EFFECTS OF LIGHT ON STEM AND LEAF GROWTH

Abstract

OUR KNOWLEDGE of the effect of light on form or growth in general is in the same stage as the knowledge of phototropism before the work of Blaauw (1909). This means that qualitatively a number of effects are known, but that our knowledge of the quantitative relation between wave length, light intensity and total energy and their physiological effects is still shockingly deficient. This is clearly indicated in chapters 20-26 of Biological Effects of Radiation (Duggar, 1936) or by the statement (Kostytchew-Went, 1931, p. 293), An understanding of the relation between light effect and form has hardly been approached. The light-growth reactions, which are the immediate response of growing organs to relatively short exposures to light, represent the only case which has been studied extensively, but even here much remains to be done before the relations between light and growth are clearly understood. Some experiments concerning the lightgrowth reaction of Avena coleoptiles are described below. The light effects are not only interesting in themselves, but they also offer a tool for further investigation of the growth processes, by enabling a separation between successive reactions. As an example, the analysis of photosynthesis might be mentioned, where a separation between light and dark reactions was possible, each of which could then be studied separately. Furthermore, the absorption spectrum of the reaction indicates which pigments are involved. Finally, a separation between light induced processes can be accomplished if their absorption spectra are very different. Although no different principles are involved, still a separation between the photosynthetic carbon dioxide assimilation carried out in the leaves, and the activation and inactivation reactions which are affected by light and which may occur also in nongreen plant parts is advantageous. The light energy required to keep up the proper nutritional level is very large, and in most cases is so much greater than the other light-dependent reactions require, that the latter are not directly analyzable. Their existence can be inferred only by indirect means from green plants growing in sufficient light to support their continued development. For a study of light-reactions in the plant other than CO2 assimilation, we can choose between three possible experimental approaches: 1. Using seedlings with large supplies of stored food, so that their energy requirements can be satisfied from this storage. 2. Using whole plants or parts of plants, growing them on complete nutrient medium containing carbohydrates.