Axenic cultures of Lemna minor
were grown on a medium containing sugars and amino acids. In continuous darkness the growth rate was one-tenth of the maximum in continuous light. In darkness early death revealed a thiamine deficiency; this deficiency was counteracted, without increasing the multiplication rate, by feeding the plants thiamine or its thiazole component. The rate of cell division, the rate of frond expansion, and the frond multiplication rate were increased by illumination; at low intensities this increase was in the nature of a photostimulus effect. The increase in growth rate was accompanied by a decrease in dry weight. The sugar uptake from the medium per frond per day was constant, independent of the light intensity and multiplication rate. At intensities of above 100 μW cm -2
continuous red light, the multiplication rate increased in proportion to the rate of photosynthesis.
Periodic brief illuminations yielded a stimulation of the frond multiplication rate of up to 4 times the dark value. Red illuminations of 2 minutes every 48 hours were sufficient to prevent thiamine deficiency; this effect of the red illuminations was reversible by far red. Stimulation of the multiplication rate was likewise reversible by far red; the effect of continuous low intensity red light was reduced by a simultaneous far red irradiation, while the effect of brief illuminations was reduced by far red following the red. From this it was concluded that phytochrome is a photoreceptor for these light effects. The results of experiments with different far red and red illumination schedules suggested that in plants with much phytochrome only a small part of it is active but that the Pfr level in this part decreases at a rate not very different from that of the bulk phytochrome as measured by the spectrophotometer.
Part of the light effect was not reversible by far red. This part increased by increasing the length of the illumination periods, the light intensity, and the number of illuminations per day. In this increase blue light was much more effective than it was in the far red reversible part of the light effect. This indicates a photoreaction different from phytochrome photoconversion.
Kinetin specially increased the far red non-reversible part of the photostimulation. Together with the intensity dependency of this part of the stimulation, this gave rise to the supposition that the non-reversible part of the light effect was limited by the supply of substrates in phytochrome controlled reactions during the light periods. Thus it would seem plausible to assume that the far red non-reversible part is built up during the red and the blue light periods, whereas the reversible part is developed during the intervening dark periods.