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The activity of Photosystem I has been investigated during growth in the dark of light-grown cells (degreening) and illumination of dark-grown cells (greening) of Chlamydomonas reinhardi y-1. During degreening the decrease in chlorophyll content precedes that of P700 and plastocyanin resulting in a temporary enrichment of the photosynthetic membranes in electron transport components relative to chlorophyll. The absorption maximum around 685 nm present in light-grown cells is gradually shifted toward shorter wavelengths (671 nm). The photoreduction of methyl viologen (micromoles per mg of chlorophyll per hour) in light-grown cells is low and saturated by low light intensity (3 x 104 ergs x cm-2 s-1). During degreening the light intensity required for maximal activity increases and saturation can not be obtained even at high light intensity (5 x 105 ergs x cm-2 s-1). The photoreduction of methyl viologen decreases continually when measured at low light intensity, however, it shows a 3-fold increase during the first 3 days of degreening when measured at high light intensity. During the greening a mirror image of the above results is obtained. The P700 content and photoreduction of methyl viologen increase faster than chlorophyll accumulation. The photoreduction at low light intensity increases gradually and shows a tendency toward saturation; at high light intensity the reaction rate is higher and shows a maximum after about 3 hours of greening. The absorption spectrum shows a fast change of the maximum from around 671 nm toward longer wavelengths (680 to 685 nm) which is completed after 3 hours of illumination. However measurements of the efficiency of different Photosystem I activities with light of different wave lengths show two peaks of activity at 670 and 700 nm and a trough at 680 nm. Addition of purified plastocyanin to the reaction mixtures enhances the photoreduction of methyl viologen only in preparations from cells grown in the light, during the 1st day of degreening, and at the end of the greening. It is concluded that changes observed in the activity of Photosystem I at different stages of photosynthetic membrane development are due to changes in the relative content of electron transport components and chlorophyll as well as changes in membrane organization related to an efficient utilization of light at low intensities.

Gibberellin-like activity was detected in extracts from Enteromorpha prolifera (Chlorophyta) and Ecklonia radiata (Phaeophyta). Material eluted from chromatograms of extracts and active in dwarf maize also brought about growth responses in the species from which the extract had been made. The responses were similar to those brought about by gibberellic acid. It is concluded that gibberellins are normally involved in regulating growth in green and brown algae.