Abstract
Classical approaches to estimate mesophyll conductance ignore differences in resistance components for CO2 from intercellular air spaces (IAS) and CO2 from photorespiration (F) and respiration (Rd). Consequently, mesophyll conductance apparently becomes sensitive to (photo)respiration relative to net photosynthesis, (F + Rd)/A. This sensitivity depends on several hard-to-measure anatomical properties of mesophyll cells. We developed a method to estimate the parameter m (0 ≤ m ≤ 1) that lumps these anatomical properties, using gas exchange and chlorophyll fluorescence measurements where (F + Rd)/A ratios vary. This method was applied to tomato and rice leaves measured at five O2 levels. The estimated m was 0.3 for tomato but 0.0 for rice, suggesting that classical approaches implying m = 0 work well for rice. The mesophyll conductance taking the m factor into account still responded to irradiance, CO2, and O2 levels, similar to response patterns of stomatal conductance to these variables. Largely due to different m values, the fraction of (photo)respired CO2 being refixed within mesophyll cells was lower in tomato than in rice. But that was compensated for by the higher fraction via IAS, making the total re-fixation similar for both species. These results, agreeing with CO2 compensation point estimates, support our method of effectively analysing mesophyll resistance.
Highlights
Quantifying the C O2 diffusion inside leaves of C 3 plants is important in both physiological and ecological contexts
Physiologists assess leaf photosynthetic efficiency and capacity, and both of them depend on how C O2 from the atmosphere travel to the chloroplast stroma and how much CO2 released by respiration and photorespiration [“(photo) respired CO2” hereafter] can be refixed by Rubisco (Busch et al 2013; von Caemmerer 2013)
The model of Farquhar, von Caemmerer and Berry (1980; “the FvCB model” hereafter), which is widely used as a component for this projection, requires the C O2 level at carboxylation sites of Rubisco (Cc) as its input
Summary
Quantifying the C O2 diffusion inside leaves of C 3 plants is important in both physiological and ecological contexts. Physiologists assess leaf photosynthetic efficiency and capacity, and both of them depend on how C O2 from the atmosphere travel to the chloroplast stroma and how much CO2 released by respiration and photorespiration [“(photo) respired CO2” hereafter] can be refixed by Rubisco (Busch et al 2013; von Caemmerer 2013). The model of Farquhar, von Caemmerer and Berry (1980; “the FvCB model” hereafter), which is widely used as a component for this projection, requires the C O2 level at carboxylation sites of Rubisco (Cc) as its input. The drawdown of Cc, relative to the C O2 level in the ambient air (Ca), depends on stomatal conductance for CO2 transfer (gsc) and on mesophyll conductance (gm), such that (von Caemmerer and Evans 1991): Cc = Ci − A∕gm (1)
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