Relating Surface Energy Budgets to the Biochemistry of Photosynthesis: A Review for Non-Biologists

Transpiration from plant canopies is an important determinant of surface energy budgets at the vegetated land surface. A recent parameterization of plant transpiration and stomatal conductance (Sellers et al., 1992a) is based on enzyme kinetics in plant chloroplasts. This paper presents background concepts in plant physiology and biochemistry needed to understand and evaluate the parameterization. Photosynthesis is the process by which plants store solar energy in chemical bonds, thereby providing the source of all energy in the biosphere. Atmospheric carbon dioxide (CO2) is reduced to organic compounds in specialized plant organelles called chloroplasts. The process involves an energy generation mechanism (the “light reactions”) and a carbon fixation mechanism (the “dark reactions”). In the light-driven part, chlorophyll pigment is excited by visible radiation in two distinct wavelength intervals, producing an oxidation-reduction couple and resulting in a strong gradient of pH within leaf cells. The pH gradient is used to produce adenosine triphosphate (ATP), and a strong reductant known as NADPH2. These products are used to drive the “dark phase” of photosynthesis by coupling strongly exergonic reactions with slightly endergonic ones, resulting in the reduction of atmospherically derived CO2 and the formation of six carbon sugars such as fructose or glucose. The carbon fixation step is regulated by an enzyme known as “Rubisco,” and by the regeneration of key reagents in a series of reactions collectively known as the Calvin Cycle. The net carbon assimilation rate is parameterized in terms of three possible rate limits: Rubisco activity, reagent regeneration, and end-product inhibition. Net assimilation of carbon is then used to calculate the stomatal conductance, which is related to evapotranspiration from the plant canopy using an integration scheme based on nutrient and light economy.