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 CO(2) in the leaves through a decrease of stomatal and mesophyll conductances, but not the biochemical capacity to assimilate CO(2), 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 CO(2) 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 CO(2) concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO(2) concentration. Measurements based on the photosynthetic response to chloroplast CO(2) often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt-stressed leaves.

Drought and salinity (i.e. soil water stress) are the main environmental factors limiting photosynthesis and respiration and, consequently, plant growth. This review summarizes the current status of knowledge on photosynthesis and respiration under water stress. It is shown that diffusion limitations to photosynthesis under most water stress conditions are predominant, involving decreased mesophyll conductance to CO 2 , an important but often neglected process. A general failure of photochemistry and biochemistry, by contrast, can occur only when daily maximum stomatal conductance (g s ) drops below 0.05-0.10 mol H 2 O m À2 s À1 . Because these changes are preceded by increased leaf antioxidant activities (g s below 0.15-0.20 mol H 2 O m À2 s À1 ), it is suggested that metabolic responses to severe drought occur indirectly as a consequence of oxidative stress, rather than as a direct response to water shortage. As for respiration, it is remarkable that the electron partitioning towards the alternative respiration pathway sharply increases at the same g s threshold, although total respiration rates are less affected. Despite the considerable improvement in the understanding of plant responses to drought, several gaps of knowledge are highlighted which should become research priorities for the near future. These include how respiration and photosynthesis interact at severe stress, what are the boundaries and mechanisms of photosynthetic acclimation to water stress and what are the factors leading to different rates of recovery after a stress period. Photosynthesis and respiration responses to drought and salinityBoth drought and salinity stresses reduce the capacity of plants to take up water from the soil (Munns 2002). At present, low water availability is the main environmental factor limiting plant growth and yield worldwide, and global change will likely make water scarcity an even greater limitation to plant productivity across an increasing amount of land (Chaves et al. 2003, Hamdy et al. 2003.The limitation of plant growth imposed by low water availability is mainly due to reductions in plant carbon balance, which is dependent on the balance between photosynthesis and respiration. Both processes are intimately linked. For instance, it has been shown that transgenic plants with modified respiration also alter their photosynthetic behaviour (Dutilleul et al. 2003, Nunes-Nesi et al. 2005, and an increased respiration rate seems necessary for photosynthesis recovery after a period of water stress (Kirschbaum 1988). Of the total Abbreviations -ABA, abscisic acid; Altox, alternative oxidase; A N , light-saturated net photosynthesis; APX, ascorbate perox-

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 (AN), while electron transport rate (ETR) remained unaffected. With increasing drought, g, AN, Ci and Cc further decreased, accompanied by slight decreases of ETR and of the estimated mesophyll conductance (gmes). Severe drought led to strong reductions of both g and gmes, 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 gmes.

Summary• 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 C 3 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. Key words: water stress, gas exchange, chlorophyll fluorescence, RuBP (ribulose-1, 5-bisphosphate) content, Rubisco activity. AbbreviationsA N , net CO 2 assimilation rate; A SAT , photosynthesis under light-and CO 2 -saturated conditions; C i , substomatal CO 2 concentration; ε , apparent carboxylation efficiency (initial slope of A N -C i curve); ETR, electron transport rate; F v / F m , efficiency of excitation capture by open PSII in dark-adapted leaves; g s , maximum light-saturated stomatal conductance; RuBP, D -ribulose-1,5-bisphosphate; RWC PD , predawn leaf relative water content; RWC MM , mid-morning leaf relative water content; Ψ , predawn leaf water potential.

Summary• Rubisco activity decreases under water stress, for reasons as yet unclear. Here, the covariation of stomatal conductance ( g s ) and relative water content (RWC), often observed during water stress, was impaired to assess the separate effects of these factors on Rubisco activity.• Three different treatments were applied to soybean ( Glycine max ) and tobacco ( Nicotiana tabacum ): leaf desiccation (LD), in which stomatal closure was accompanied by large decreases of RWC; water stress (WS), in which minor decreases of RWC were observed along with stomatal closure; and exogenous application of abscisic acid (ABA), which triggered stomatal closure without changing RWC.• Decreased RWC did not induce decreased initial Rubisco activity, which was impaired only in soybean by 40% when the g s dropped below 50 mmol m − 2 s − 1 , regardless of the treatment. The mechanism for decreased activity differed among treatments, owing to decreased activation in LD and to total activity and protein content in WS and ABA.• Despite the occurrence of Rubisco regulation, CO 2 availability in the chloroplast, not impairment of Rubisco activity, limits photosynthesis during WS.