In Populus trichocarpa (black cottonwood), net photosynthesis (A ) varies with latitude and, in northern genotypes, is supported by higher stomatal conductance (g ). We report here a parallel cline in mesophyll conductance (g ) and link this variation to carbonic anhydrase (CA) activity. Using concurrent carbon isotope discrimination and chlorophyll fluorescence methods, we examined the effects of acetazolamide, an inhibitor of CA, on g in six representative genotypes (three from either end of the north-south cline). Acetazolamide reduced CA activity, g , g , chloroplast CO concentration (C ) and A at normal CO (400 μmol mol ), the latter being reversible at saturating CO . Absolute reductions in A , g and CA activity were greater in northern genotypes than in southern genotypes (P < 0.025) but percent reductions were similar. In contrast, northern genotypes showed lower percent reduction in C compared to southern genotypes (P < 0.025). The northern genotypes had greater CA activity relative to both leaf area (two-fold) and mass (1.8-fold) (P < 0.016). The relationship between CA activity and g was similar whether the variation was inherent or inhibitor induced. We suggest that greater CA activity contributes to higher g in northern P. trichocarpa genotypes, but other diffusion pathway components may also be involved.
Summary Carbonic anhydrase (CA) is an abundant protein in most photosynthesizing organisms and higher plants. This review paper considers the physiological importance of the more abundant CA isoforms in photosynthesis, through their effects on CO2 diffusion and other processes in photosynthetic organisms. In plants, CA has multiple isoforms in three different families (α, β and γ) and is mainly known to catalyze the CO2 false↔ HCO3- equilibrium. This reversible conversion has a clear role in photosynthesis, primarily through sustaining the CO2 concentration at the site of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco). Despite showing the same major reaction mechanism, the three main CA families are evolutionarily distinct. For different CA isoforms, cellular localization and total gene expression as a function of developmental stage are predicted to determine the role of each family in relation to the net assimilation rate. Reaction–diffusion modeling and observational evidence support a role for CA activity in reducing resistance to CO2 diffusion inside mesophyll cells by facilitating CO2 transfer in both gas and liquid phases. In addition, physical and/or biochemical interactions between CAs and other membrane‐bound compartments, for example aquaporins, are suggested to trigger a CO2‐sensing response by stomatal movement. In response to environmental stresses, changes in the expression level of CAs and/or stimulated deactivation of CAs may correspond with lower photosynthetic capacity. We suggest that further studies should focus on the dynamics of the relationship between the activity of CAs (with different subcellular localization, abundance and gene expression) and limitations due to CO2 diffusivity through the mesophyll and supply of CO2 to photosynthetic reactions.
Leaves balance CO 2 and radiative absorption while maintaining water transport to maximise photosynthesis. Related species with contrasting leaf anatomy can provide insights into inherent and stress-induced links between structure and function for commonly measured leaf traits for important crops. We used two walnut species with contrasting mesophyll anatomy to evaluate these integrated exchange processes under non-stressed and drought conditions using a combination of light microscopy, X-ray microCT, gas exchange, hydraulic conductance, and chlorophyll distribution profiles through leaves. Juglans regia had thicker palisade mesophyll, higher fluorescence in the palisade, and greater low-mesophyll porosity that were associated with greater gas-phase diffusion (g IAS ), stomatal and mesophyll (g m ) conductances and carboxylation capacity. More and highly-packed mesophyll cells and bundle sheath extensions (BSEs) in Juglans microcarpa led to higher fluorescence in the spongy and in proximity to the BSEs. Both species exhibited droughtinduced reductions in mesophyll cell volume, yet the associated increases in porosity and g IAS were obscured by declines in biochemical activity that decreased g m . Inherent differences in leaf anatomy between the species were linked to differences in gas exchange, light absorption and photosynthetic capacity, and drought-induced changes in leaf structure impacted performance via imposing species-specific limitations to light absorption, gas exchange and hydraulics.
Mesophyll conductance (gm) determines the diffusion of CO2 from the substomatal cavities to the site of carboxylation in the chloroplasts and represents a critical component of the diffusive limitation of photosynthesis. In this study, we evaluated the average effect sizes of different environmental constraints on gm in Populus spp., a forest tree model. We collected raw data of 815 A-Ci response curves from 26 datasets to estimate gm, using a single curve-fitting method to alleviate method-related bias. We performed a meta-analysis to assess the effects of different abiotic stresses on gm. We found a significant increase in gm from the bottom to the top of the canopy that was concomitant with the increase of maximum rate of carboxylation and light-saturated photosynthetic rate (Amax). gm was positively associated with increases in soil moisture and nutrient availability, but it was insensitive to increasing soil copper concentration, and it did not vary with atmospheric CO2 concentration. Our results showed that gm was strongly related to Amax and to a lesser extent to stomatal conductance (gs). Also, a negative exponential relationship was obtained between gm and specific leaf area, which may be used to scale-up gm within the canopy.
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