Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants.

Rates of CO(2) assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO(2) in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25 degrees C. Electron transport activity for CO(2) assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O(2), and between fluorescence and rates of CO(2) assimilation at 19 millibars but not 200 millibars O(2). The values of fluorescence measured at about 220 microbars intercellular CO(2) were not greatly affected by increasing O(2) from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO(2) partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO(2) partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO(2) partial pressures which reversed to a positive correlation at higher values of CO(2) partial pressures. Similar results were observed in the C(3) species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C(4) species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO(2) assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q(E)) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.