Background and Objectives

Abstract

Photosynthesis is the fundamental process whereby plants capture and process sunlight and CO₂ as primary ecological resources supporting the growth and reproduction of an individual, and, ultimately, the survival of a species. Moreover, adaptations in resource acquisition and utilization are recognized as strong candidates for evolutionary selection in all species (Ackerly and Monson 2003a, Gutschick and BassiriRad 2003). Across the hierarchy of structural organization and spatial scale, extending from the chloroplast to the canopy and landscape, the efficiency of photosynthetic carbon assimilation declines substantially when assimilation is expressed per unit of plant biomass invested. For example, photosynthesis per unit mass is greatest for a single chloroplast, followed by a single cell, different cell layers, individual leaves, leaves arranged on a stem, and then leaves within crowns and canopies due, primarily, to increasing architectural constraints that require greater supportive biomass and generate increased mutual shading. Increasing size and accompanying structural complexity also place contraints on diffusional processes that require replacement by bulk transfer mechanisms. Thus, photosynthesis per unit biomass and ground area across any vegetative landscape is dramatically less than a hypothetical monolayer of all of the chloroplasts present, with each chloroplast photosynthesizing at its maximum capacity. Apparently, the adaptive advantages of size and greater structural/spatial complexity are most often related to a competition for sunlight that, apparently, outweighs any loss in photosynthetic efficiency expressed on an invested biomass basis.