Plant ecologists have identified empirically many quantitative trends, laws and rules related to the basis for variation in plant productivity including the law of constant yield in plant communities (Kira et al 1953), the self-thinning rule (Yoda et al 1963), the leaf economic spectrum (Wright et al 2004), and the rules for scaling biomass allocation on plant size (Enquist & Niklas 2002) among others. What is lacking is a unified mechanistic theory explaining such general patterns in plant productivity from individual plants to ecosystems. We start from a premise that in general the single most critical resource governing the ecology and evolution of variation in the processes involved in plant productivity is photosynthetically active radiation. We take this premise as an axiom, and review the mathematical derivation of a series of relationships organizing plant productivity at the leaf, plant and community levels.