Physiol. Plant. 48; 385-388. 1980 FLUORESCENCE AND PHOTOSYNTHETIC CAPACITIES 385 Dichlorophenyl dimethylurea (DCMU) induced increase in chlorophyll a fluorescence intensity - An index of photosynthetic oxygen evolution in leaves, chloroplasts and algae By G. KULANDAIVELU and H. DANIELL School of Biological Sciences, Madurai Kamaraj University, Madurai 625 021, India (Received 8 June, 1979; revised 17 September, 1979) Abstract Dichlorophenyl dimethylurea (DCMU) treatment in photo- synthetic samples resulted in an increase in the level of steady state chlorophyll a fluorescence at room temperature which was directly proportional to the photosynthetic efficiency. The applicability of this method for the rapid determination of the efficiency of oxygen evolution in leaves, algae, mesophyll cells and chloroplasts has been investigated. Especially reliable values with less than 5% error were obtained if the fluorescence meas- urements were made under low excitation intensities with a sam- ple chlorophyll concentration below 1.0 [i,g/ml. Key words: Fluorimetry, photosynthetic capacity, in vitro and in vivo conditions. Introduction A portion of the light intercepted by the plant is absorbed by the photosynthetic pigments and creates a supply of singlet electronic excited states. Most of this energy, nearly 85% under optimal conditions, is used in photo- chemistry. Though Chi a has a high fluorescence in sol- ution, its yield is low in vivo (about 3%), since most of the energy is used in photosynthesis. In the early literature, the yield of Chi a fluorescence was taken to be an inverse measure of the degree of efficiency of the photochemical apparatus. The transient changes during the fluorescence induction (the "Kautsky effect"; Kautsky and Hirsch 1931) were approximately the inverse to changes in the rate of photosynthesis. Inhibition of photosynthesis by certain poisons (Kautsky and Zedlitz 1941) or increase in light intensity beyond the saturation level (Shiau and Franek 1947) results in an increase in fluorescence yield. But the relationship between fluorescence and photo- synthesis has proved to be more complicated than this simple inverse correlation. After the discovery of the two pigment systems, the relationship between fluorescence and photosynthesis has been investigated in greater detail (see Govindjee et al. 1967, Govindjee and Papageorgiou 1971, Goedheer 1972, Papageorgiou 1975). The transient changes during the phenomenon of fluorescence induction were studied mostly with respect to the rate of O2 evolution. The "Kautsky effect" consists of fast and slow changes (Gov- indjee and Papageorgiou 1971). The fast changes occur within a few seconds after the onset of illumination and reflect the redox states of the primary acceptor of PS II. During this phase the changes in the fluorescence are approximately inversely proportional to the changes in the rate of photosynthesis. The slow changes last for sev- eral minutes, and probably reflect variations in the physi- cal state of the chloroplast membranes and the kinetic balance of the two light reactions. Simply defined, in- crease and decrease in the fluorescence level during the early temporary steady state is, respectively, by the addi- tion of electrons from PS II and through draining by PS I. When the electron transport is blocked before plas- toquinone by DCMU, the fluorescence level increases; and this increase in level (fluorescence level in the pre- sence of DCMU minus fluorescence in the absence of DCMU, A F) reflects the relative photosynthetic effi- ciency (Samuelsson and Oquist 1977). The authors suggested that this method could be employed for study- ing the photosynthetic capacity in algae. Though most of their fluorescence measurements demonstrated more or less parallel changes with respect to photosynthetic ac- tivity, they indicate the actual photosynthetic capacities with errors as high as 30%. Hence, we have developed optimum conditions for fluorescence measurements, so that the technique can be applied to determine the O2 evolving capacity with less than 5% error. Results are also presented on the estimation in short periods of the photosynthetic efficiency in intact leaves. 0031-9317/80/030385-04 $03.00/0 © 1980 Physiologia Plantarum