Thomas J. Givnish explores how plants adapt to different light levels by considering whole-plant energy capture, not just leaf-level photosynthesis. He reviews traditional methods for identifying adaptations to light levels, focusing on three key energetic tradeoffs: gas exchange, support, and biotic interactions. Recent models are evaluated to understand trends in leaf nitrogen, stomatal conductance, phyllotaxis, and defensive traits in sun and shade plants. A re-evaluation of the classic study on Atriplex shows that adaptation to sun and shade depends on whole-plant energy capture, not just leaf-level responses. Calculations for Liriodendron reveal that the traditional light compensation point has little meaning for net carbon gain, and the effective compensation point is influenced by leaf respiration, construction costs, and support tissue. Support tissue costs significantly raise the effective compensation point, especially in tall trees. New models for canopy width/height ratio and mottled leaves in understory herbs are outlined. The paper emphasizes that whole-plant growth and competitive ability depend on canopy geometry, energy allocation, and the costs of non-photosynthetic organs. Three key tradeoffs—gas exchange, support, and biotic interactions—shape adaptations to light levels. These include the economics of gas exchange, where increased CO₂ uptake must be balanced against water loss; the economics of support, involving mechanical costs of leaf structure; and the economics of biotic interactions, balancing photosynthetic benefits against herbivore attraction. The paper argues that leaf-level traits may not fully explain adaptations to light levels when considered in isolation. Instead, whole-plant energy capture and the integration of traits into the canopy are crucial. The analysis of Björkman et al. (1972b) on Atriplex shows that when photosynthesis is measured per unit leaf mass or protein, leaves acclimated to specific light levels perform better, highlighting the importance of incorporating leaf construction costs. The study concludes that photosynthesis should be expressed per unit leaf mass or protein to better assess adaptation to light levels, as this accounts for the energetic costs of leaf construction and improves the accuracy of whole-plant energy capture assessments.Thomas J. Givnish explores how plants adapt to different light levels by considering whole-plant energy capture, not just leaf-level photosynthesis. He reviews traditional methods for identifying adaptations to light levels, focusing on three key energetic tradeoffs: gas exchange, support, and biotic interactions. Recent models are evaluated to understand trends in leaf nitrogen, stomatal conductance, phyllotaxis, and defensive traits in sun and shade plants. A re-evaluation of the classic study on Atriplex shows that adaptation to sun and shade depends on whole-plant energy capture, not just leaf-level responses. Calculations for Liriodendron reveal that the traditional light compensation point has little meaning for net carbon gain, and the effective compensation point is influenced by leaf respiration, construction costs, and support tissue. Support tissue costs significantly raise the effective compensation point, especially in tall trees. New models for canopy width/height ratio and mottled leaves in understory herbs are outlined. The paper emphasizes that whole-plant growth and competitive ability depend on canopy geometry, energy allocation, and the costs of non-photosynthetic organs. Three key tradeoffs—gas exchange, support, and biotic interactions—shape adaptations to light levels. These include the economics of gas exchange, where increased CO₂ uptake must be balanced against water loss; the economics of support, involving mechanical costs of leaf structure; and the economics of biotic interactions, balancing photosynthetic benefits against herbivore attraction. The paper argues that leaf-level traits may not fully explain adaptations to light levels when considered in isolation. Instead, whole-plant energy capture and the integration of traits into the canopy are crucial. The analysis of Björkman et al. (1972b) on Atriplex shows that when photosynthesis is measured per unit leaf mass or protein, leaves acclimated to specific light levels perform better, highlighting the importance of incorporating leaf construction costs. The study concludes that photosynthesis should be expressed per unit leaf mass or protein to better assess adaptation to light levels, as this accounts for the energetic costs of leaf construction and improves the accuracy of whole-plant energy capture assessments.