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.

We review the photosynthetic responses to drought in field‐grown grapevines and other species. As in other plant species, the relationship between photosynthesis and leaf water potential and/or relative water content in field‐grown grapevines depends on conditions during plant growth and measurements. However, when light‐saturated stomatal conductance was used as the reference parameter to reflect drought intensity, a common response pattern was observed that was much less dependent on the species and conditions. Many photosynthetic parameters (e.g. electron transport rate, carboxylation efficiency, intrinsic water‐use efficiency, respiration rate in the light, etc.) were also more strongly correlated with stomatal conductance than with water status itself. Moreover, steady‐state chlorophyll fluorescence also showed a high dependency on stomatal conductance. This is discussed in terms of an integrated down‐regulation of the whole photosynthetic process by CO₂ availability in the mesophyll. A study with six Mediterranean shrubs revealed that, in spite of some marked interspecific differences, all followed the same pattern of dependence of photosynthetic processes on stomatal conductance, and this pattern was quite similar to that of grapevines. Further analysis of the available literature suggests that the above‐mentioned pattern is general for C₃ plants. Even though the patterns described do not necessarily imply a cause and effect relationship, they can help our understanding of the apparent contradictions concerning stomatal vs. non‐stomatal limitations to photosynthesis under drought. The significance of these findings for the improvement of water‐use efficiency of crops is discussed.

The effect of diffusional and photochemical limitations to photosynthesis was assessed in field-grown water-stressed grapevines ( Vitis vinifera L.) by combined measurements of gas exchange and chlorophyll fluorescence. Drought was slowly induced, and the progressive decline of photosynthesis was examined in different grapevine cultivars along a continuous gradient of maximum mid-morning values of stomatal conductance ( g ), which were used as an integrative indicator of the water-stress conditions endured by the leaves. Initial decreases of g were accompanied by decreases of substomatal CO2 concentration (Ci), the estimated chloroplastic CO2 concentration (Cc) and net photosynthesis ( A N), while electron transport rate (ETR) remained unaffected. With increasing drought, g , A N, Ci and Cc further decreased, accompanied by slight decreases of ETR and of the estimated mesophyll conductance ( g mes). Severe drought led to strong reductions of both g and g mes, as well as of ETR. The apparent carboxylation efficiency and the compensation point for CO2 remained unchanged under severe drought when analysed on a Cc, rather than a Ci, basis, suggesting that previously reported metabolic impairment was probably due to decreased g mes.

Plant water use efficiency (WUE) is becoming a key issue in semiarid areas, where crop production relies on the use of large volumes of water. Improving WUE is necessary for securing environmental sustainability of food production in these areas. Given that climate change predictions include increases in temperature and drought in semiarid regions, improving crop WUE is mandatory for global food production. WUE is commonly measured at the leaf level, because portable equipment for measuring leaf gas exchange rates facilitates the simultaneous measurement of photosynthesis and transpiration. However, when those measurements are compared with daily integrals or whole-plant estimates of WUE, the two sometimes do not agree. Scaling up from single-leaf to whole-plant WUE was tested in grapevines in different experiments by comparison of daily integrals of instantaneous water use efficiency [ratio between CO assimilation (A) and transpiration (E); A/E] with midday A/E measurements, showing a low correlation, being worse with increasing water stress. We sought to evaluate the importance of spatial and temporal variation in carbon and water balances at the leaf and plant levels. The leaf position (governing average light interception) in the canopy showed a marked effect on instantaneous and daily integrals of leaf WUE. Night transpiration and respiration rates were also evaluated, as well as respiration contributions to total carbon balance. Two main components were identified as filling the gap between leaf and whole plant WUE: the large effect of leaf position on daily carbon gain and water loss and the large flux of carbon losses by dark respiration. These results show that WUE evaluation among genotypes or treatments needs to be revised.This work was performed with financial support from the Spanish Ministry of Science and Technology (project AGL2011-30408-C04-01) and from Conselleria de Educación, Cultura y Universidades (Govern de les Illes Balears) and the European Social Fund through the ESF Operational Programme for the Balearic Islands 2013–2017 (project PD/027/2013).Peer Reviewe

• Whether decreases in Rubisco activity and the availability of ribulose-1,5-bisphosphate (RuBP) regeneration are responsible for drought-induced depression of photosynthesis is under debate. • Here, leaf water potential and relative water content, gas exchange, chlorophyll fluorescence, initial and total Rubisco activity and RuBP content were determined during the time course of drought development in five C₃ species: Rhamnus alaternus, Rhamnus ludovici-salvatoris, Nicotiana sylvestris, Phaseolus vulgaris and Vitis vinifera. Water was withheld until photosynthesis approached zero (between 6 and 12 d depending on the species). • Relative water content and water potential progressively dropped with drought in Rhamnus and Vitis, but not in the other two species. While RuBP content and Rubisco activity remained constant, declining eventually only in the more severe drought situations, light-saturated stomatal conductance (g_s) and photosynthesis (A_N) decreased progressively during drought in all species. This strongly suggests a dominant role of decreased g_s in photosynthesis downregulation during drought in these species, which is supported by increased electron transport to A_N ratio. • It is concluded that impairment of Rubisco activity and RuBP content do not limit photosynthesis until drought is very severe. Moreover, the relative water content at which these mechanisms are impaired is strongly species-dependent.