The effects of averaging sub- and supra-optimal temperatures on the flowering of Chrysanthemum morifolium

Abstract

SummaryThe effects of short-duration averaging of sub- and supra-optimal temperatures were tested in chrysanthemum (Chrysanthemum morifolium Ramat.). Plants of three cultivars grown in short-days (SD) at a daily average of 19°C (12 h day at 24°C, 12 h night at 14°C) showed an average flowering delay of 8.5% compared to plants grown at a continuous 19°C. The reciprocal transfer of plants between these treatments after 14 SD showed that the delay caused by averaging was largely due to slower flower initiation, but that slower flower development also contributed in two of the cultivars. However, the delay due to daily averaging was around 47% less than that predicted by the arithmetic mean of continuous growth at supra-optimal (24°C) and sub-optimal (14°C) temperatures. Averaging over 2 d (alternate days at 24°C and 14°C) gave a greater delay than daily averaging, but the delay was still less than predicted (amelioration reduced from 47% to 24%). There was little further delay as a result of averaging over 14 d (alternate 7 d periods at 24°C and 14°C). Inertial delays in the attainment of thermal equilibria at shoot apices, when air temperature settings are changed, may give effective average tissue temperatures closer to the optimum, and may help explain the reduced delay in flowering due to temperature averaging.We suggest, however, that there is probably some other, more important, factor involved. Energy saving in chrysanthemum cultivation by using solar gain to give supra-optimal day temperatures, and compensation with sub-optimal night temperatures can be expected to give only minor flowering delays, but growing with day temperatures higher than night temperatures (‘positive DIF’) will increase the need for height regulation.