• Diffusive limitations to photosynthesis;
  • metabolic limitations to photosynthesis;
  • drought;
  • salinity;
  • photosynthetic metabolism;
  • stomatal-mesophyll conductance

Abstract: Drought and salinity are two widespread environmental conditions leading to low water availability for plants. Low water availability is considered the main environmental factor limiting photosynthesis and, consequently, plant growth and yield worldwide. There has been a long-standing controversy as to whether drought and salt stresses mainly limit photosynthesis through diffusive resistances or by metabolic impairment. Reviewing in vitro and in vivo measurements, it is concluded that salt and drought stress predominantly affect diffusion of CO2 in the leaves through a decrease of stomatal and mesophyll conductances, but not the biochemical capacity to assimilate CO2, at mild to rather severe stress levels. The general failure of metabolism observed at more severe stress suggests the occurrence of secondary oxidative stresses, particularly under high-light conditions. Estimates of photosynthetic limitations based on the photosynthetic response to intercellular CO2 may lead to artefactual conclusions, even if patchy stomatal closure and the relative increase of cuticular conductance are taken into account, as decreasing mesophyll conductance can cause the CO2 concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO2 concentration. Measurements based on the photosynthetic response to chloroplast CO2 often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt-stressed leaves.