Abstract
The partial pressure of CO2 at the sites of carboxylation within chloroplasts depends on the conductance to CO2 diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (gm). We investigated how gm varies with leaf age and through a tobacco (Nicotiana tabacum) canopy by combining gas exchange and carbon isotope measurements using tunable diode laser spectroscopy. We combined these measurements with the anatomical characterization of leaves. CO2 assimilation rate, A, and gm decreased as leaves aged and moved lower in the canopy and were linearly correlated. This was accompanied by large anatomical changes including an increase in leaf thickness. Chloroplast surface area exposed to the intercellular airspace per unit leaf area (Sc) also decreased lower in the canopy. Older leaves had thicker mesophyll cell walls and gm was inversely proportional to cell wall thickness. We conclude that reduced gm of older leaves lower in the canopy was associated with a reduction in Sc and a thickening of mesophyll cell walls.
Highlights
In plants with the C3 photosynthetic pathway, mesophyll conductance, gm, quantifies the ease with which CO2 diffuses from intercellular airspace within a leaf to the sites of Rubisco carboxylation within chloroplasts [1,2]
The partial pressure of CO2 at the sites of carboxylation within chloroplasts depends on the conductance to CO2 diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance
We investigated how gm varies with leaf age and through a tobacco (Nicotiana tabacum) canopy by combining gas exchange and carbon isotope measurements using tunable diode laser spectroscopy
Summary
In plants with the C3 photosynthetic pathway, mesophyll conductance, gm, quantifies the ease with which CO2 diffuses from intercellular airspace within a leaf to the sites of Rubisco carboxylation within chloroplasts [1,2]. It is one of the three main physiological processes limiting CO2 uptake and fixation, the others being CO2 diffusion from the atmosphere to the sub-stomatal cavity (stomatal conductance, gs) and the biochemical activity of Rubisco and RuBP regeneration. The need to understand and maximize gm is part of the research efforts being made to enhance photosynthesis to improve crop yield. Crop models are incorporating leaf and canopy level parameters to better predict where photosynthesis improvements can be made (e.g. [10]), and an understanding of mesophyll conductance variation across leaf positions is crucial for this
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