Effects of Physical Environment on Photosynthesis, Respiration, and Transpiration

Abstract

Responses of photosynthetic, respiratory, and transpiration rates of plants to the levels of physical environmental factors are outlined. Water vapor movement from a leaf to the atmosphere in transpiration is quantitatively described using a gas diffusion model that incorporates the concentration gradient of water vapor and the conductance for water vapor. Changes in the levels of environmental factors affect transpiration rate directly through changes in the driving force of water vapor diffusion or indirectly through changes in stomatal aperture. In plants, there are two types of respiration, namely, dark respiration and photorespiration, although they are completely different metabolisms. Dark respiratory rate is sensitive to changes in temperature, gas concentrations, and light intensity, and the effects are summarized. Photosynthetic carbon dioxide (CO2) influx of a leaf is spatially divided into CO2 diffusion from the atmosphere to the chloroplasts and biochemical CO2 fixation within the chloroplasts. The former can be described with a model in a similar manner to that for water vapor diffusion. Net photosynthetic rate in C3 leaves shows a saturating-type increase in response to increases in CO2 concentration or photosynthetic photon flux density (PPFD), and the response curves are characterized by several parameters. Net photosynthetic rate is low at extremely low and high temperatures and shows an optimum level at intermediate temperatures.