Lateral Diffusion of CO2 in Leaves Is Not Sufficient to Support Photosynthesis

Abstract

Lateral diffusion of CO 2 was investigated in photosynthesizing leaves with different anatomy by gas exchange and chlorophyll a fluorescence imaging using grease to block stomata. When one-half of the leaf surface of the heterobaric species Helianthus annuus was covered by 4-mm-diameter patches of grease, the response of net CO 2 assimilation rate (A) to intercellular CO 2 concentration (C i ) indicated that higher ambient CO 2 concentrations (C a ) caused only limited lateral diffusion into the greased areas. When single 4-mm patches were applied to leaves of heterobaric Phaseolus vulgaris and homobaric Commelina communis, chlorophyll a fluorescence images showed dramatic declines in the quantum efficiency of photosystem II electron transport (measured as F q #/F m #) across the patch, demonstrating that lateral CO 2 diffusion could not support A. The F q #/F m # values were used to compute images of C i across patches, and their dependence on C a was assessed. At high C a , the patch effect was less in C. communis than P. vulgaris. A finite-volume porous-medium model for assimilation rate and lateral CO 2 diffusion was developed to analyze the patch images. The model estimated that the effective lateral CO 2 diffusion coefficients inside C. communis and P. vulgaris leaves were 22% and 12% of that for free air, respectively. We conclude that, in the light, lateral CO 2 diffusion cannot support appreciable photosynthesis over distances of more than approximately 0.3 mm in normal leaves, irrespective of the presence or absence of bundle sheath extensions, because of the CO 2 assimilation by cells along the diffusion pathway.

Stomatal Size, Speed, and Responsiveness Impact on Photosynthesis and Water Use Efficiency

“…Measurements of maximum and actual photosynthetic efficiency were identical to the wild type in areas with stomata; however, leaf areas without stomata showed lower maximum and actual photosynthetic efficiency, illustrating that stomatal patterning determined CO 2 concentration and photosynthesis across the leaf lamina and that lateral fluxes of gas could not compensate for reduced vertical diffusion as a result of reduced stomatal numbers (Büssis et al, 2006). This work agrees with reports that suggest that lateral fluxes can limit photosynthesis (Morison et al, 2005) but depend on species and leaf anatomy (Lawson and Morison, 2006).…”

Spatial Dependence for Hydrogen Peroxide-Directed Signaling in Light-Stressed Plants

“…However, it may be that bundle sheath cells cannot increase rates of CO 2 assimilation to dissipate EEE (Fryer et al, 2003). This notion comes from observations that CO 2 for photosynthesis in bundle sheath cells may be produced from malate transported from the roots (Hibberd and Quick, 2002) and not from the atmospheric CO 2 , which is unlikely to exhibit significant rates of diffusion from stomatal cavities to vascular tissues in photosynthesizing leaves (Morison et al, 2005). The combination of high rates of electron transport combined with a limited capacity for CO 2 fixation would favor the photoreduction of O 2 and increased production of H 2 O 2 by the water-water cycle (Fryer et al, 2003).…”

Stomatal Crypts Have Small Effects on Transpiration: A Numerical Model Analysis

“…Evidence supportive of this was presented by Hassiotou et al (2009a), who showed that in the genus Banksia, encryption increases with increasing leaf lamina thickness. Whether or not the diffusion resistance within the intercellular air spaces affects photosynthesis is dependent on the leaf anatomy and porosity, the cell shape and packing, and the pattern of stomatal openings (Morison et al, 2005). Facilitated diffusion by crypts may be particularly beneficial for thick leaves with densely packed mesophyll cells.…”