The effects of drought on Photosystem II (PSII) fluorescence and photosynthetic electron transport activities were analyzed in cotton. Water stress did not modify the amplitude of leaf variable fluorescence at room temperature in the presence of 343,4-dichlorophenyl)-1,1-dimethylurea (DCMU) nor at 77 K. It is therefore concluded that photon collection, their distribution between the two photosystems, and PSII photochemistry are unaffected by the stress. In droughted leaves at room temperature under low exciting light, the transitory maximum (F,) PFD, 300 ,mol quanta m-2s-' in case of cotton, 100 in case of Nerium oleander provided by a bank of metal halide lamps; in some experiments (Table IV) a higher PFD, 1200 ,umol quanta m-2s , was used for both species. Plants were watered twice daily and fertilized once weekly. They were submitted to water stress by withholding watering 6 weeks after germination in case of cotton and 4 months after propagation by cuttings in case of N. oleander. Drought was achieved in 8 d. Plants were rehydrated by rewatering the pots to field capacity. Measurements were performed on fully expanded leaves (the fourth to fifth leaves from the top).Leaf Water Potential Measurements. Leafwater potential was determined using Wescor C52 psychrometers (Wescor Inc., Logan, UT) by the dew point method or using a pressure chamber (PMS, Corvallis, OR).Fluorescence Measurements. Room temperature fluorescence kinetics at 685 nm were measured from attached and detached leaves using a bifurcated light pipe together with a quartz light mixing rod to excite and collect the fluorescent light from the upper leaf surface. The exciting blue light was produced by a DC stabilized projector tungsten lamp passing through a combination of filters (Schott BG18+BG38+KG3) via a photographic shutter (opening time: 2 ms). The fluorescence was detected through a 685 nm interference filter (Schott PIL 0.1) and a red colored filter (Coming 2-64) using a photomultiplier tube (RTC, XP 1017). The signal was simultaneously displayed on a storage oscilloscope for initial rapid phases of the induction and on a chart recorded for slower phases. Fluorescence measurements were made on attached leaves which have been dark adapted during all the night period. Leaf discs of 1.5 cm diameter, were used for experiments with DCMU. They were infiltrated in the dark with a 20 ,uM DCMU solution.