Light saturation of growth and photosynthesis of the shade plant Marchantia polymorpha.

The growth rate of the shade plant Marchantia was at its maximum for a low illumination, 2-3 x io lx, and was inhibited by an excess of light. Photosynthesis by intact thalli and by isolated chloroplasts of Marchantia was saturated by a light intensity of 2-3 x io lx. These isolated chloroplasts were able to carry on satisfactory rates of photosynthesis, up to 35 fiM CO2/h/mg chlorophyll. The Hill reaction and photosystem II were also saturated by the same light intensities, demonstrating that the factor limiting the light saturation of photosynthesis is located in the chloroplast. The structure of chloroplasts was strongly modified by an excess of light, small grana and fret membranes being replaced by continuous grana.