Plant acclimation to the light gradient in canopies; functional significance and regulation by cytokinin

Abstract

Plants growing in dense leaf canopies are exposed to a vertical light gradient due to mutual shading of the leaves. Theoretical models have predicted an optimal leaf area index (LAI) and leaf nitrogen distribution at which whole-plant daily carbon gain is maximized. In this thesis, these model predictions have been experimentally tested using genetically transformed plants that have an altered LAI and leaf N distribution. It was shown that such plants, which retain leaves in the lower shaded part of the canopy achieve a lower daily carbon gain because of higher costs of respiration. Evidence is presented showing that plants increase their carbon gain and competitiveness in a dense canopy by shedding their lower leaves and utilizing remobilized N from these leaves to form additional leaf area in the upper, well-lit part of the canopy. Secondly, the mechanism has been investigated by which plants perceive and acclimate to the light gradient in canopies. The plant hormone cytokinin is delivered to leaves according to their respiration rates, which depend on irradiance. Cytokinin was shown to regulate many aspects of photosynthetic acclimation to light, also at the level of gene expression.