Cytokinin control of growth of Spirodela oligorrhiza in darkness.

Continuous heterotrophic growth of Spirodela oligorrhiza cultures following transfer to darkness requires cytokinins, or periodic brief treatment with red light. In the absence of cytokinins or red light growth ceases after 2-3 days. However, growth resumes spontaneously after 3-4 weeks in darkness to produce etiolated plants. The growth rate of these etiolated plants is not stimulated by kinetin.Although the kinetin concentration in treated plants reaches a plateau 30-60 min after adding kinetin to "dormant" plants in darkness new fronds do not appear for 24 h. Dormant colonies treated with kinetin in darkness for only 6-12 h subsequently grow in darkness at the same rate as plants treated with kinetin for 1, 2 or 3 days. Treatments which inhibit growth in the light, for example cold, chloramphenicol or actidione, eliminate the requirement for cytokinin and allow subsequent growth in darkness. The results suggest that a growth inhibitor may be present but ineffective in Spirodela growing in the light. The inhibitor is active in darkness but slowly decays. Kinetin appears to inactivate the inhibitor in darkness.