Photosynthesis is the primary process in crop production. It supplies reduced carbon for the construction of biomass and as the source of energy in metabolism. Leaves are the functional units of crop photosynthesis; their efficiency in capture and utilization of solar energy determines productivity. The transport of CO2 from the atmosphere to sites of fixation in leaves is limited by slower moving air within canopies, boundary layers of still air surrounding leaves, stomatal pores in leaf epidermis, and by the interior structure of leaves.

The area (LAI) and arrangement of foliage, i.e., canopy architecture, determine the interception of solar radiation by individual leaves of a crop. Leaf area and arrangement change during crop growth and, by leaf movement, during each day. Maximum crop production requires complete capture of solar radiation and 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.

This chapter begins with a discussion of photosynthesis and photosynthetic responses of leaves progressing to analyses and explanations of spatial and temporal variation of photosynthesis of crop canopies.

Photosynthetic systems

The central processes of photosynthesis are common to all plants but variants have evolved ancillary chemical, morphological, and physiological mechanisms that result in three photosynthetic systems with important ecological adaptations.