Observations on the Flowering Photoperiodicity
Recueil des travaux botaniques néerlandais , Volume 40 - Issue 1 p. 392- 412
According to their reactions to different photoperiodic treatments plants can be divided into 4 groups; a definition of these groups is given on page 393. Anthemis tinctoria is a long day plant and belongs to group I. A long blue day, that means natural day light from 7.— a.m. till 3.— p.m. supplemented by blue light from sunrise till 7.— a.m. and from 3.— p.m. till sunset, suppresses the formation of reproductive organs in seedlings as well as in older plants. In seedlings, moreover, it checks the vegetative development whereas older plants grow normally under these conditions. Iberis amara is a long day plant and belongs to group IV. The variety investigated in 1941 reacted principally in the same way as the one of 1938 at Ghent, but the blossoming times in the various compartments were shifted closer together; the retardation in “red’* and “dark”, which was as much as 45 and 90 days at Ghent, was reduced to about 20 and 30 days; this is perhaps due to a greater production of florigen. Glycine hispida, G. hispida var. nigra and G. soja are short day plants, but a somewhat higher temperature during the night is a stronger stimulation for the flowering and especially for the ripening of the fruits than the 8 hour day. The three varieties belong very distinctly to group I. Hedera amurensis hort. and H. colchica Koch forma arborea are short day plants and very typical for group I. Sedum: all species investigated are extreme long day plants. 5. Kamtschaticum belongs to group I; 5. Aïzoon to group II; 5. Telephium is transitional between II and IV; 5. Selskianum belongs to group IV with perhaps a slight tendency towards II; S. spectabile and probably also 5. lividum are to be classed in group IV. Sedum offers the first examples of plants belonging to group IV which are not Cruciferae. Perilla ocymoides and P. ocymoides var. nankinensis: both varieties, but especially nankinensis are extreme types of short day plants belonging to group II. In natural day length they remain vegetative till the end of September. When the natural illumination is interrupted in May by a small number of short photoperiods they blossom in the middle of the summer and afterwards they show the reaction of reversibility, that is, the axes of the inflorescences resume their vegetative growth; towards the end of the season a second flowering stage follows. There seems to exist an equilibrium between the formation of florigen during short days and of auxins during long days. A large quantity of florigen, due to a great number of short photoperiods, checks the formation of auxins with the consequence that after the first flowering stage growth is not resumed and the life cycle comes to an end. Ocymoides is more sensitive in this respect than nankinensis; in the former 7 short days determine the premature flowering, reversal and second flowering; 14 short periods induce premature flowering, but prevent the reversal, the plants die; in the latter variety 7 short days have no influence, but 14 determine the onset of flowering, reversal and second reproductive stage. A case of reversibility in Epilobium angustifolium is reported; this is also most probably due to photoperiodic induction. It is suggested that closer observation in nature and more experiments can add valuable material to the physiology of flowering and to the experimental phototeratology. I want to express my heartfelt thanks to everyone in the Botanical Institute, the National Herbarium and the Botanical Garden at Leyden who helped me to carry out these researches.
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