Temperature effects on flower initiation in sweet orange (Citrus sinensis)

The effects of temperature on flower induction in sweet orange were studied over a range of shoot and root temperatures representing the conditions found in the field in commercial growing regions. Rooted cuttings of the orange cultivar 'Late Valencia' were used in the Canberra phytotron, with an extended photoperiod of 16 hours and a photo-temperature of 8 hours' duration. Following low temperature inductive conditions (5 weeks at 15/10°C), profuse flowering occurred at 24/19°; flowering was only slightly less at this air temperature with a root temperature of 30°, and at an air temperature of 30/25°. At 36/31° few flowers were formed and these abscissed before opening. However, when the root temperature was maintained at 22° with this shoot temperature a few flowers were formed. In another experiment a root temperature of 30° and shoot temperature of 24/19° reduced the number of flowers that formed (86 cf. 137). Only a small part of the effect of high temperature in inhibiting flower initiation was due to root temperature. Two attempts were made to induce flowering under non-inductive temperatures (27/22°) with low root temperatures (15 and 11°). Flowering could not be induced under these conditions, and the lack of flowers apparently was not due to inhibition of growth by low root temperature. Although few flowers were formed at 27/22° without any previous inductive treatment (15 flowers per plant), profuse flowering was induced by 27/13° (141 flowers/plant), which indicated that a low night temperature will induce flowering. A high root temperature (29°) during inductive conditions (15/10°) had little effect on the number of flowers formed. It appeared that the site of flower stimulation by temperature was in the shoot only, and that subsequent high root temperature effected floral development. It was concluded that flowering could be induced in the field during the summer by low night temperature, and root temperatures would probably have little influence on floral development.