The effects of light, 3-(3,4-dichlorophenyl)-1,1-dhmethylurea (DCMU), and am um ion on pool sizes of ATP were studied in Lemns pauckstats 6746 ( Modification of flowering responses by altering photosynthesis has been described for a number of species (4, 13). Kandeler reported that long day flowering of the duckweed Lemna gibba G3 was enhanced by DCMU or adenylates (10), and inhibited by ammonium ion (1 1). On the basis of such results, it was suggested that promotion of flowering by DCMU might be due to increased levels of ATP resulting from cyclic photophosphorylation, and inhibition of flowering by ammonium ion to an uncoupling of phosphorylation (9,13).Promotion of flowering by DCMU and adenylates has also been observed in Lemna paucicostata2 strain 6746-a photosynthetic mutant, strain 1073, flowered in the absence of DCMU (16-18). Under certain conditions, flowering of both strains was inhibited by ammonium ion (6,16,18).Studies on chloroplast suspensions (20) showed that the photosynthetic electron transport chain of the mutant was blocked between PQ3 and Cyt f, but was capable of PMS-catalyzed PSI cyclicphosphorylation, results consistent with the ATP hypothesis described above. Samples of fronds to be illuminated were blotted for 10 s with filter paper, weighed, and placed in 3 ml of medium in a 12-ml graduated conical centrifuge tube. The samples, each containing 15 to 25 mg of fronds, were incubated in the dark for 2 h to establish dark levels of ATP. Those cultures being maintained in nitrogen were flushed with water-saturated nitrogen at a rate of 2 liter/h during the dark pretreatment and the illumination periods.For illumination periods of 1 min or less, most of the medium was removed, leaving the fronds adhering to the side of the tube. For long term exposures, the medium was removed at the termination of illumination; replicate cultures were used for weight and ChM determinations. There were three or four replicates for each treatment.Illumination with white light for short term exposures was from a 300-w tungsten filament projector bulb filtered through 5 cm of water. For red and far red illumination, the light was filtered through Comning filters 4-97 + 2-73 or 7-59 + 2-73, respectively (3). For red, the peak wavelength was about 610 nm, the cut-off wavelengths were 560 and 680 nm, and the intensity at the surface of the plants was 93 ,uw cm-2. At this intensity of red light, the rate of 02 evolution in wild type fronds was 10.9 ,umol/mg Chl. h. The far red light had a low wavelength cut-off at 680 nm; the incident intensity between 680 and 730 nm was 118 yw cm-2. This intensity of far red light did not cause any detectable 02 evolution in wild type fronds.For long term illumination, the conditions were similar to those used in previously reported flowering experiments: cool-white fluorescent light (350 ft-c) at about 25 C. At the end of the period of illumination, cold ethanol (-70 C) was rapidly added to the samples which were then stored at -20 C for at least 16 h to maximize the AT...