Lateral CO2 Diffusion inside Dicotyledonous Leaves Can Be Substantial: Quantification in Different Light Intensities1[W][OA]

Substantial lateral CO2 diffusion rates into leaf areas where stomata were blocked by grease patches were quantified by gas exchange and chlorophyll a fluorescence imaging in different species across the full range of photosynthetic photon flux densities (PPFD). The lateral CO2 flux rate over short distances was substantial and very similar in five dicotyledonous species with different vascular anatomies (two species with bundle sheath extensions, sunflower [Helianthus annuus] and dwarf bean [Phaseolus vulgaris]; and three species without bundle sheath extensions, faba bean [Vicia faba], petunia [Petunia hybrida], and tobacco [Nicotiana tabacum]). Only in the monocot maize (Zea mays) was there little or no evident lateral CO2 flux. Lateral diffusion rates were low when PPFD <300 μmol m−2 s−1 but approached saturation in moderate PPFD (300 μmol m−2 s−1) when lateral CO2 diffusion represented 15% to 24% of the normal CO2 assimilation rate. Smaller patches and higher ambient CO2 concentration increased lateral CO2 diffusion rates. Calculations with a two-dimensional diffusion model supported these observations that lateral CO2 diffusion over short distances inside dicotyledonous leaves can be important to photosynthesis. The results emphasize that supply of CO2 from nearby stomata usually dominates assimilation, but that lateral supply over distances up to approximately 1 mm can be important if stomata are blocked, particularly when assimilation rate is low.