Annals of Botany 20: 587-622, 1956
© 1956 Annals of Botany Company
RESEARCH-ARTICLE |
Effects of Natural and Artificial Light in Arctic Latitudes on Long- and Short-day Plants as Revealed by Growth Analysis
Research Institute of Plant Physiology, Imperial College of Science and Technology London
The effects on growth and flowering of two short-day and two long-day plants when grown under different conditions of illumination are described. The plants fully investigated were Kalanchoe blossfeldiana and Xanthium pennsylvanicum and the annual varieties of Hyoscyamus niger and Beta vulgaris. Wintex barley, Iberis umbellata, and tomato were also grown in some selected treatments. The conditions investigated comprised continuous full daylight (24 hours), full daylight for the whole of the daily photoperiod and full daylight for half the photoperiods, the other half consisting of either daylight reduced by shading or light from incandescent lamps or fluorescent tubes (daylight-matching type), all of the same low intensity. Two lengths of photoperiod were used for each species, one nearly optimal for flowering, the other closer to the critical day-length; and the order of the low and high light treatments was varied. These factors were combined factorially.
Data were collected (or derived) for the following characteristics, though not always for all the species grown: dry weights, leaf areas, heights, water contents, epidermal cell sizes, net assimilation rates, times to flowering, leaf-number increments until flowering, numbers of inflorescences, stomatal apertures, and leaf postures.
Among other effects, the data revealed that in all four species investigated the adverse effects on over-all growth to be expected from reduction of the daily photoperiod or of the intensity of illumination are in fact minimized. This compensation was effected mainly by large increases in leaf areas, even though in all cases half the daily photoperiod consisted of full daylight. There are indications that increased efficiencies (net assimilation rates) may also have been involved. The leaf-area increases appear to have been due to increased cell size rather than cell number and a close positive correlation with water content was found.
The most striking among the effects on flowering was the failure of sugar-beet to bolt when half of its photoperiod (totals of 20 and 14 hours) consisted of light from fluorescent lamps. The flowering of barley and Hyoscyamus was also delayed considerably under these conditions. The deficiency of red in the spectrum of the fluorescent light is believed to have been the cause. By contrast, the flowering of Iberis, a crucifer, was not affected.