Estimation of Mesophyll Conductance to CO₂ Flux by Three Different Methods

Abstract

The resistance to diffusion of CO2 from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, t3C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO2 diffusion determined in unstressed plants at high light. The relatively low CO2 partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced 02 sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO2 partial pressure upon switching to low 02. We found no correlation between mesophyll conductance and the ratio of intemal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO2 and 02 partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO2-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Slmmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO2 level.Leaves have a finite conductance for CO2 diffusion in the mesophyll (5,13

Winners always win: growth of a wide range of plant species from low to future high CO2

“…Forbs and woody species greatly increased SLA at low CO 2 , possibly to reduce mesophyll resistance in the leaf (Loreto et al. 1992; Medlyn et al. 2011) or to produce more carbon acquiring leaf area at a lower biomass expense.…”

“…At low CO 2 , a high SLA might be advantageous because of reduced diffusive resistance in the leaf (Loreto et al. 1992; Medlyn et al. 2011) and serve to increase the area available for photosynthesis at a lower carbon cost to biomass.…”

Leaf anatomy enables more equal access to light and CO2 between chloroplasts

“…The data have been separated into two groups : mesophytic leaves which are short-lived, herbaceous or deciduous species ; and sclerophytic leaves which either are evergreen or have a low specific leaf area. As the fluorescence method becomes unreliable when internal conductances exceed 0.3 mol m −# s −" , only data collected using the isotopic method were used Lloyd et al, 1992 ;Loreto et al, 1992 ;Evans et al, 1994 ;Epron et al, 1995 ;Syvertsen et al, 1995 ;Evans & Vellen, 1996 ;Roupsard et al, 1996 ;Lauteri et al, 1997. Solid lines from the origin indicate mean drawdown in CO # mole fraction (µmol mol −" ) across stomata (C a -C i ), or from substomatal cavities to the sites of carboxylation (C i -C c ).…”

Responses to water stress in an ABA‐unresponsive hybrid poplar (Populus koreana×trichocarpa cv. Peace) III. Consequences for photosynthetic carbon assimilation

“…These techniques, together with on-line measurements of carhon isotope discrimination in gas exchange systems (Evans et al, 1986;Lloyd et al, 1992) also underlined that CO.â vailabihty in the chloroplasts of well watered plants was much lower than that computed for the substomata! spaces, and therefore that substantial internal resistance to CO2 influx acted in addition to the stomatal ones (Loreto et al, 1992).…”