There is a long-standing controversy as to whether drought limits photosynthetic CO2 assimilation through stomatal closure or by metabolic impairment in C3 plants. Comparing results from different studies is difficult due to interspecific differences in the response of photosynthesis to leaf water potential and/or relative water content (RWC), the most commonly used parameters to assess the severity of drought. Therefore, we have used stomatal conductance (g) as a basis for comparison of metabolic processes in different studies. The logic is that, as there is a strong link between g and photosynthesis (perhaps co-regulation between them), so different relationships between RWC or water potential and photosynthetic rate and changes in metabolism in different species and studies may be 'normalized' by relating them to g. Re-analysing data from the literature using light-saturated g as a parameter indicative of water deficits in plants shows that there is good correspondence between the onset of drought-induced inhibition of different photosynthetic sub-processes and g. Contents of ribulose bisphosphate (RuBP) and adenosine triphosphate (ATP) decrease early in drought development, at still relatively high g (higher than 150 mmol H20 m(-2) s(-1)). This suggests that RuBP regeneration and ATP synthesis are impaired. Decreased photochemistry and Rubisco activity typically occur at lower g (<100 mmol H20 m(-2) s(-1)), whereas permanent photoinhibition is only occasional, occurring at very low g (<50 mmol H20 m(-2) s(-1)). Sub-stomatal CO2 concentration decreases as g becomes smaller, but increases again at small g. The analysis suggests that stomatal closure is the earliest response to drought and the dominant limitation to photosynthesis at mild to moderate drought. However, in parallel, progressive down-regulation or inhibition of metabolic processes leads to decreased RuBP content, which becomes the dominant limitation at severe drought, and thereby inhibits photosynthetic CO2 assimilation.

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The importance of both stomatal closure and reduced carboxylation efficiency on the photo- synthesis decline in response to long term water stress was previously measured in field-grown grapevines. Here we address the question of whether water stress affects the photochemical capacity of leaves, measuring gas-exchange rates and chlorophyll fluorescence under drought and moderate irrigat- ion at intervals through the summer season during three consecutive years. We conclude that usually water stress does not induce photoinhibition in field-grown grapevines, even when stomatal conductance and photosynthesis are reduced to very low values. Moreover, down-regulat- ion of photochemical reactions is low, leading to a general pattern of photosynthetic response to drought consistent in large reductions of stomatal conductance (g), followed by a consistent decrease of CO2 assimilation (A) but with a much lower effect on electron transport rate (ETR). In consequence, the intrinsic water-use efficiency (A/g) increased, as well as the ratio ETR/A. It is suggested that increased electron transport to alternative pathways, such as photorespiration, prevented further down-regulation of ETR under drought conditions. These results are in agreement with our previous reports for potted plants. However it is clear that, under field conditions with a much more slowly developed water stress, ETR reductions are more attenuated than in potted plants, reducing their incidence in carbon assimilation, which seems to be mainly regulated by stomatal closure.

A comparison of photosynthetic characteristics of 20 cultivars of grapevine (Vitis vinifera L.) from Mallorca (Balearic Islands, Spain) and two widespread cultivars, Cabernet Sauvignon and Chardonnay, was made under irrigation as well as in response to drought. Although these cultivars share a common origin, a high variability was found for several photosynthetic characters under irrigation. Interestingly, these variations were significant for gas-exchange parameters (net CO 2 assimilation, stomatal conductance and intrinsic water use efficiency) but not for chlorophyll fluorescence parameters (maximum photochemical efficiency, electron transport rate and non-photochemical quenching). Since water stress is the most limiting factor for plant production under the Mediterranean climate, it is presumable that these findings reflect specific selection pressures over physiological characteristics related to a balance between net carbon gain and water use. Some cultivars presented high carbon assimilation at the expense of a high water loss, whereas others were water savers, accompanied by low CO 2 assimilation even under irrigation. Escursach was found to be an interesting cultivar, presenting low water consumption at the same time as reasonably high carbon assimilation. These cultivars also showed different responses to drought, which allowed their classification in two main groups: alarmist cultivars, which showed strong reductions of stomatal conductance in response to relatively low decreases of leaf water potential, and luxurious cultivars, showing low reductions of stomatal conductance under water stress.

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