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The effects of sonication of synchronized cultures of Scenedesmus obtusiusculus Chod. were examined. Two frequencies, 4 kHz and 19 kHz were applied during various phases of the diurnal mitotic cycles. As a result of 4-kHz sonication the average cell division decreased. During exposure to 19-kHz sonication the average rate of cell division initially increased slightly, but mitotic division was subsequently inhibited when the cells were exposed to prolonged sonication. A "relaxation" period of two generations was required before the cells regained their normal rate of productivity (cell number increase). The cells were more sensitive to sonic shock during the first quarter of their 24-h life cycle and apparently insensitive during the last quarter. No chromosomal effect was observed. It is suggested that sonication alters specifically timed events that occur at the earlier stages of the life cycle.

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Developing synchronous cells of Scenedesmus obtusiusculus Chod. were investigated during the 15-h light period of the 24-h life cycle. Measurements were made of photosynthesis and of the chlorophyll, carotene, nucleic acid, and protein content, including intracellular phosphorus.Photosynthetic oxygen evolution (μmol/mg chlorophyll) increased during the early growing stage and declined gradually in the older cells. Chlorophyll, carotene, protein, ribonucleic acid, and ribonucleic acid phosphate of these cells increased during the growth phase (dry weight basis) and declined in the sporulating phase, whereas the deoxyribonucleic acid and deoxyribonucleic phosphate decreased during the growth phase. Deoxyribonucleic acid increased slightly in the sporulating phase, but deoxyribonucleic acid phosphate remained relatively constant during this period. When calculated on a per cell basis, ribonucleic acid and ribonucleic acid phosphate increased steadily throughout the period of observation, whereas deoxyribonucleic acid and deoxyribonucleic acid phosphate remained constant during the first 5 h and then increased gradually. Analysis of the intracellular distribution of phosphorus(dry weight basis) revealed that methanol-soluble (lipid) phosphorus decreased steadily with cell growth, whereas the content of acid-soluble phosphorus and ethanol–ether-soluble (lipid) phosphorus fluctuated markedly. The pH of the culture medium increased concomitantly with cellular development during the light period.

Effects of gibberellin A3 (GA) on growth, pigments, protein, and nucleic acid content of the cotyledons of lettuce seedlings (Lactuca sativa L. cv. Grand Rapids) are reported. GA treatment increased both the total shoot and cotyledon fresh weights, and decreased chlorophyll content per cotyledon and on a dry weight basis. GA also decreased the protein content of the cotyledon, but increased the level of RNA and DNA measured on a dry weight basis. The hormone had no effect on the pheophytin: chlorophyll ratio, but increased the ratios of chlorophyll a: chlorophyll b, phytol: chlorophyll, and carotene: chlorophyll. GA also increased the sugar content of the cotyledons. Treatment of excised cotyledons with glucose had an effect similar to that of GA on chlorophyll and protein content. The data are discussed in relation to the mechanism of GA-induced chlorosis.