Leaf factors affecting the relationship between chlorophyll fluorescence and the rate of photosynthetic electron transport as determined from CO ₂ uptake

CO₂ uptake and chlorophyll fluorescence were measured under non-photorespiratory conditions in leaves from 14 plant species. The rate of CO₂-dependent electron transport (JCO₂) was calculated as four times rate of gross photosynthesis. The quantum yield of electron transport in photosystem II was estimated from the ratio ΔF/Fm′, where ΔF is the difference between steady-state and maximal fluorescence in the light. As photon flux density (PFD) increased, JCO₂ increased linearly first, and then reached saturation. The product (ΔF/Fm′) PFD, which is a function of electron transport rate, showed a similar response. Therefore, the relationship between (ΔF/Fm′)PFD versus JCO ₂ was proportional. However, under high light, a linear correlation was not always maintained. Factors affecting the linear correlation were analyzed by measuring CO₂ uptake and chlorophyll fluorescence under illumination from either the upper (adaxial) or lower (abaxial) leaf surface, and by using plants with anatomically symmetric leaves having palisade tissues on both sides. Consequently, it was shown that the parameter ΔF/Fm′ is based on chlorophyll fluorescence emitted from chloroplasts present near the illuminated surface. Further, it was suggested that this restriction of the origin of fluorescence actually measured is significant in a leaf with high chlorophyll content, resulting in the deviation from linearity in the relationship between JCO₂ and (ΔF/Fm′)PFD.