Using a cost-benefit model, the leaf nitrogen concentration and root : shoot ratio that maximize whole-plant relative growth rate are determined as a function of the above-ground environment (integrated daily photon flux density and the concentration of carbon dioxide at the site of fixation within the leaf). The major advantage of this approach is that it determines the adaptive significance of leaf physiology by considering the functional integration of leaves and roots. The predicted response to increasing daily photon flux densities is an increase in optimal leaf N concentration (N opt ) and a concomitant increase in root: shoot ratio. Increased carbon dioxide concentrations, on the other hand, reduce N opt and only slightly change root: shoot ratio. The observed increase in leaf nitrogen concentration found in plants growing at high altitudes (low CO 2 partial pressure) is also predicted. Since these responses to light and CO 2 maximize the whole-plant relative growth rate, the observed adjustments that plants make to light and carbon dioxide concentration appear to be adaptive.We show that the relationship between photosynthesis and leaf nitrogen concentration is complex and depends on the light and CO 2 levels at which photosynthesis is measured. The shape of this function is important in determining N opt and the opposite response of leaf nitrogen to light and carbon dioxide is shown to be the result of the different effects of light and CO 2 on the photosynthesis-leaf nitrogen curve. © 1991 Annals of Botany Company.