Articles
THE TIMING OF LOW TEMPERATURE TREATMENTS ON STEM ELONGATION AS AFFECTED BY LIGHTING STRATEGIES
A possible interaction between the timing of a period of low temperature and the timing of day-extension (using fluorescent lamps) was investigated in two similar experiments with Begonia x hiemalis, Impatiens New–Guinea, Fuchsia x hybrida and Salvia splendens. Daylength was extended to 16 hours by lighting either in the first period of the night (early evening) or in the last period of the night (before sunrise). Except in the case of Begonia x hiemalis, no significant interaction was found in either experiment between the timing of lighting and the timing of an 8-hour low temperature period (21°C down to 15°C) on elongation growth.
Diurnal patterns of temperature-induced elongation response were studied in four replicate experiments, each of 6 weeks.
Six, 4-hour, temperature drop treatments of 6°C (20°C down to 14°C) were given at times evenly spread throughout a 24-hour day, and combined with a 4-hour day extension given either in the first period of the night or before sunrise (using fluorescent lamps). For Fuchsia x hybrida and Impatiens New–Guinea, temperature-drop treatments during daylight reduced elongation growth, but low temperature given during the whole daylight period (DIF) was more effective than a 4-hour temperature drop treatment given during the same period.
For Begonia x hiemalis and Euphorbia pulcherrima temperature drop treatments were also effective outside the daylight period, and were nearly as good as DIF with respect to height reduction.
The lighting strategy did not alter the response to temperature drop, although the response pattern of Begonia x hiemalis appeared to be postponed by day-extension given in the first period of the night.
More practical experiments with several cultivars of Begonia x hiemalis also demonstrated an interaction between temperature-drop treatment given in the last period of the night and the timing of (supplementary) lighting.
Compared to a control treatment with constant day/night temperature, a temperature drop drop given 10 to 16 hours after sunset reduced plant height by 20% if lighting was given before sunrise (6 hours darkness after sunset), but no reduction was gained if lighting was given immediately after sunset (6 hours darkness before sunrise).