On the critical low temperature for flower bud differentiation in fall blooming chrysanthemums

Abstract

Temperature is one of the most important environ-mental factor in chrysanthemum growing in cold seasons from late fall to early spring. When growing temperature is too cold, chrysanthemum cannot form flower buds even under short day conditions. In this study, minimum temperature ranges which allow the flower bud differentiation were tested on 49 varieties of fall flowering chrysanthemum. Plants grown under the long day condition prolong-ed by artificial illumination were transfered to natural day (short day) at ten day intervals from September 15 to November 5. Flower bud differen-tiation, bud formation, and flowering were inspected on those plants. In some varieties, plants, which had been trans-fered to the natural day as early as September 25, failed to differentiate flower bud, while in some other varieties, plants were able to differentiate flower bud even when they had been transfered as late as November 5. In most varieties, however, flower bud could not be observed on most plants which had been transfered after October 15 to 25. Minimum tem-peratures during October 15 to 25 ranged from 14.6 to 11.4°C. It seemed that this range was the mini-mum temperature which allow the flower bud dif-ferentiation in most varieties tested, though it must be widened from 16.5 to 7.6°C., if all varieties tested were included. It was found that there was no relation between the season of flowering (earliness) and their mini-mum temperature ranges for flower bud formation. Flower buds were susceptible to cold injury, and they became more sensitive as they developed larger.