Insufficient water is currently one of the environmental factors most limiting to crop yields and is likely to remain so even with global climate change. Although rising atmospheric carbon dioxide concentrations may substantially reduce stomatal conductance (gS), we can expect this to result in only small reductions in evapotranspiration at the field scale (Bunce, 2004). Furthermore, despite projected increased global precipitation, the increased magnitude of rainfall events also leads to a prediction of increased frequency of drought (Groisman and Knight, 2008). Therefore, adapting crops to drought conditions remains a reasonable goal.
Dry soils reduce plant biomass production primarily by slowing leaf area development or by decreasing photosynthesis, although reproductive processes can be more sensitive and specific targets of stress in some species. In many crops, dry conditions occur after full canopy development so that slowing of leaf area development would have little impact on biomass production, but reduced photosynthesis would limit it. Thus, improvement of photosynthesis during dry conditions is a worthwhile objective. Strategies to improve photosynthesis during drought should be based on knowledge of what plant processes are most limiting to photosynthesis in stressed plants.
It is generally accepted that moderate water stress inhibits photosynthesis primarily by decreasing gS, which limits the carbon dioxide supply to the leaf interior (Flexas et al., 2004; Flexas and Medrano, 2002; Lawlor and Cornic, 2002). At higher atmospheric carbon dioxide concentrations, the stomatal limitation to photosynthesis decreases (Sage, 2004), and photosynthesis would be expected to be less inhibited by the same relative reduction in gS. However, assessments of stomatal and biochemical limitations to photosynthesis need to take into account new information about limitations to photosynthesis imposed by the movement of carbon dioxide from inside the stomata to the site of fixation inside the chloroplast, the “mesophyll conductance,” and how it may be affected by water stress and by carbon dioxide concentration as well as taking into account the well-known difficulties in leaf gas exchange analysis presented by “patchy” stomatal closure.
Mesophyll conductance has been measured primarily using chlorophyll-a fluorescence combined with leaf gas exchange (Harley et al., 1992) or online carbon isotope discrimination (Evans et al., 1986). These methods rely on uncertain assumptions and involve expensive instrumentation. The oxygen sensitivity of photosynthesis is determined by the carbon dioxide concentration at Rubisco and so provides a relatively inexpensive estimate of mesophyll conductance (Bunce, 2009) and its environmental dependence. The observation that mesophyll conductance can decrease with water stress (e.g., Warren, 2008) calls into question the use of small-scale fluorescence methods to indicate the presence of “patchy” stomatal behavior (Flexas et al., 2004). As an alternative method of assessing the importance of patchy stomatal behavior, we can exploit the fact that stomata can be reopened despite water stress by treatment with low carbon dioxide concentrations (Bunce, 2007; Centritto et al., 2003) and use this to separate stomatal and non-stomatal inhibition of photosynthesis by water stress.
I report the use of the oxygen sensitivity of photosynthesis and exposure to low carbon dioxide concentrations to reopen stomata of moderately stressed leaves of four C3 species to better understand the stomatal and non-stomatal limitations to photosynthesis at the current ambient and projected future carbon dioxide concentrations.
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