Studies on Greening of Etiolated Seedlings
Authors: DE GREEF J. A. , CAUBERGS R. 
Published: April 2006

Evidence is given that a selective light‐pretreatment of the embryonic axis exerts a deep influence on the greening in primary leaves of 8‐day‐old etiolated bean seedlings (Phaseolus vulgaris cv. Limburg). After a subsequent dark incubation of sufficient length and a final exposure of the entire plants to continuous illumination the lag phase of chlorophyll synthesis is completely removed. In particular the highly meristematic hook tissue seems to be responsible for this light effect. Lengthening of the dark period following pre‐irradiation increased the capability of chlorophyll production in the main white light period, reaching its maximum after about 12 hours of darkness. The period of dark incubation for elimination of the lag phase is considerably longer in plants with shielded leaves than the length of the lag phase in etiolated seedlings of the same age, exposed entirely to continuous light. This difference may be explained by the synergistic effect between leaves and embryonic axis. Evidence for this interorgan cooperation is given by experiments with a selective light‐pretreatment of leaves and embryonic axis. After a 5 min pre‐exposure to white light of whole plants the leaves of some of the plants were shielded and these plants received a further pre‐illumination of 2 hours on their embryonic axis. In all the pre‐irradiated, etiolated plants the lag phase of chlorophyll synthesis was eliminated during the main white light period, following a dark incubation of 2 hours. Additional and preferential light activation of the embryonic axis during the pretreatment had no significant effect on chlorophyll production during the white light illumination after a 2 hours dark incubation, but resulted in a lower yield of chlorophylls after 18 hours dark incubation compared to the white light controls, receiving no selective light‐pretreatment on the embryonic axis. From our results we can decisively conclude that a simultaneous light‐pretreatment of both, leaves and embryonic axis, is more effective and beneficial for building up a capacity of chlorophyll synthesis in the leaves than either a selective light‐pretreatment of the embryonic axis alone or a simultaneous pre‐illumination of leaves and embryonic axis, immediately followed by an additional preirradiation of the embryonic axis. Therefore, we think that several photoactive sites are involved in de‐etiolation processes of intact, etiolated seedings. Light activation of the embryonic axis stimulates the development of this organ and contributes to the greening processes in the leaf. At the same time, by irradiating the leaf, light activates the photo‐sensitive site in the leaf itself, which also develops a capacity for chlorophyll synthesis. Both photo‐acts are cooperative, explaining the enhanced chlorophyll production. Additional pre‐irradiation of the embryonic axis after a short illumination of whole plants favours its own development and reduces the synthetic capacity of the leaf. A prolonged far‐red pretreatment induces qualitatively the same response as white light. We assume that these effects on lag phase removal and chlorophyll production, induced in etiolated, primary bean leaves by selective irradiation of the embryonic axis, is a phytochrome‐mediated process. Our results indicate a transmission of light‐induced stimuli from one organ to another.