Apex size as an index of juvenility in ornamental perennials

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Relationship between sensitivity to chilling for flower bud initiation in freesia and Dutch iris and their morphological characteristics, such as bulb and corm weight, leaf number and apex size, was investigated to identify the phasic transition from juvenile to adult in apical meristems.Apex of freesia corms enlarged gradually after releasing from dormancy and reached 0.19-0.20 mm in diameter, which is an reliable index of phasic transition from juvenile to adult in apical meristems, resulting in obtaining high sensitivity to chilling. Apex size of the upper buds on freesia corms was larger than that of basal ones and showed higher sensitivity to chilling. These data indicate that the phasic transition does not occur in individual plants but in individual axis.In Dutch iris, phasic change from juvenile to adult was not accompanied by any morphological change. While, application of ethylene to bulbs at the concentration of 50-100ppm promoted the phasic change immediately after the treatment and increase the sensitivity to chilling, though no morphological change was brought by the treatment with ethylene, indicating that physiological phasic change occurs without any morphological changes in Dutch iris.