INFLUENCE, RÉVERSIBLE PAR LA LUMIÈRE ROUGE‐CLAIR, DE COURTS ÉCLAIREMENTS ROUGE‐FONCÉ A LA FIN DE LA PHOTOPÉRIODE, SUR LA CROISSANCE ET LA TENEUR EN CHLOROPHYLLES DE MARCHANTIA POLYMORPHA L
Authors: Fredericq H. , Greef J. A. De 
Published: January 2008

Abstract— 1. Thalli of Marchantia polymorpha L. grow plagiotropically with 4, 8 or 16 hr of white fluorescent light per day (24‐hr cycles), and have round lobes; male thalli are not as uniformly flat as the female thalli and their lobes are not so broad.2. The growth of both kinds of thalli is strongly modified by 5 min far‐red (or even 1 min) at the end of short days. Already after the first cycle of 8 hr fluor. light + 5 min fr+16 hr darkness, the margins of the young parts are bent upwards; further growth is orthotropic and the newly formed lobes are much narrower and elongated and their green colour is different. After 2 weeks of treatment, the appearance is completely different.3. The strong action of the terminal far‐red irradiation is fully reversible by red light or by white fluorescent light. The efficiency of the rather short exposure times and the repeatable nature of the reversibility, the last irradiation determining the type of growth, strongly suggest that the orientation and the growth of the thalli is controlled by the same red, far‐red photo‐reaction as in the case of the growth of higher plants and on the basis of our physiological experiments we think that the same photoreversible pigment, the phytochrome, is present in the thalli of Marchantia.4. Our hypothesis is that the far‐red absorbing form, P730, is responsible for the horizontal growth and for the broad and round shape of the lobes.Even under short days, the percentage of P730 left during part of the long night must be sufficient for flat growth, since a 5 min far‐red irradiation at the middle of the 16‐hr night still causes orthotropism, and this is reversible by red light.Complete absence of P730 for 16 hr per day should then allow a stronger orthotropism and etiolation than the absence of P730 for only 8 hr per day; we in fact found that the effect of a far‐red irradiation is much stronger at the end of an 8‐hr photoperiod than at the end of a 16‐hr photoperiod.5. These and other similarities between our results on light‐grown, green Marchantia thalli and the results of Downs et uZ.(1) on light‐grown pinto beans, with white fluorescent light during the photoperiods in both cases, is stressed.6. We have obtained similar but weaker effects in two other species of liverworts.7. Under an 8‐hr photoperiod of red fluorescent light, Marchantia grows horizontally; a terminal 1‐min far‐red exposure causes orthotropism and etiolation, reversible by another short red exposure. However, the controls under 8 hr of red fluorescent light have narrower lobes which are not round, as compared to the controls under 8 hr of white fluorescent light. On the basis of other results, we suppose that this last mentioned difference is not due to differences in light intensities, but rather that, besides the participation of the P730 form as indicated by the experiments with terminal exposures after white light photoperiods, also the blue light included in the white fluorescent light plays a role in the formation of broad round lobes.The same blue light (fluor. lamps, filter and CuSO4. 5H2O) which has no effect as a 15‐min terminal exposure after 8 hr of white fluorescent light, causes orthotropism and etiolation when given as the only light source during the whole photoperiod, and this much more under 18‐hr photoperiods as under 8‐hr ones, and mostly in the middle of the field, where the intensity is higher (tinpublished results of Maton and Van Den Abeele(12)).8. Rather short terniinal far‐rcd irradiations, espccially after 4‐ and 8–hr photoperiods of white fluorescent light, result in markedly lower levels of chlorophyll content in the young growing parts, and this is reversed by a subsequent daily red irradiation.Even at the end of a 16‐hr photoperiod, a daily 10 min far‐red exposure results in a lowercd chlorophyll content.It is not necessary to treat for 2 weeks. Already after five cycles, each of 8 hr fluor. light + 5 min far‐red + 16 hr darkness, the chlorophyll content was lowered by 16 per cent on a fresh weight basis and by 35 per cent on a dry weight basis, as compared to the controls under the smie photoperiod.9. The content of the basal parts of the thalli was not measured. In two experiments, 5 min far‐red given at the end of each 1‐hr photoperiod caused a lower chlorophyll content in the young parts, and already after the fifth cycle the basal parts had turned to a whitish colour, whereas the controls remained completely green.10. Our results concerning the effect of terminal far‐red exposures and its reversal by red light, on the chlorophyll metabolism of light‐grown, green thalli of Marchantia are discussed in relation to the results obtained by other authors on etiolated seedlings of higher plants.11. Recently De Greef and Fredericq (unpublished results) found a strongly lowered chlorophyll content of the lobes of thalli in continuous darkness at ± 22°C; at 4°C this effect is much slower, and in this respect there are considerable differences between male and female thalli.