10 - Photosynthesis

Summary

OVERVIEW

Photosynthesis, the primary process in crop production, supplies the reduced carbon that serves in construction of biomass and as a source of energy in metabolism.

Leaves are the functional units of crop photosynthesis; their efficiency of capture and utilization of solar energy determines productivity. CO₂ diffuses from the atmosphere to the sites of fixation in leaves down the concentration gradient that photosynthesis establishes by biochemical fixation in the chloroplast. Diffusion along the pathway is limited by the boundary layer of air surrounding the leaf, by stomatal pores in the leaf epidermis, and by the interior structure of the leaves.

The area (LAI) and arrangement of the foliage, i.e. the canopy architecture, determine the interception of solar radiation by a crop and the distribution of irradiance among individual leaves. The community geometry, established by plant spacing and morphology of the plants, determines canopy structure. Leaf area and arrangement change during the life of a crop and, by leaf movement, even during the course of a single day. Maximum crop production requires complete capture of incident solar radiation and can only be achieved with supporting levels of water and nutrients. When water or nutrients are in short supply, productivity is reduced by incomplete capture of radiation and/or less efficient utilization of it.

Crop photosynthesis varies spatially and temporally in response to environmental factors and from day to day in response to the accumulated effects of past environments on canopy size and its physiological status.