Effects of water deficit on photosynthetic capacity

Under drought, CO2 assimilation rates decrease already at small leaf water deficits. At least part of the inhibition is attributed to non‑stomata1 effects at the chloroplast level, with electron transport and phosphorylation being main targets of inhibition. These findings are questioned by direct measurements of photosynthetic capacity with systems that are not Limited by stomata, e.g. leaf slices in solution or leaves at ex‑ternal CO2 concentrations exceeding 5%. Here, photosynthesis was rather insensitive to dehydration down to 50–70% relative water content, and different plant species re‑sponded in a very similar way. More severe dehydration affected not only pboto‑synthesis, but also dark CO2 fixation and presumably also photorespiration. Rever‑sible and unspecific inhibition is thought to be mediated mainly by increased concen‑trations of solutes in dehydrated cells. Inhibition of photorespiration might favour photoinhibition when long‑term water stress is coupled with full sunlight. Photo‑inhibition, together with general senescence phenomena might be involved in long‑term effects of water stress under natural drought conditions. This offers an explanation for the conflicting results of short‑term water stress experiments and studies carried out under field conditions.