Interrelated Effects of Temperature and Photoperiod on the Growth and Flowering of Clematis

Photoperiodic responses in the growth and flowering of Clematis cultivars were studied in relation to temperature variation throughout the four seasons. The results were as follows:1. In the summer, Clematis plants which had been grown under long-day conditions developed flowers on terminal buds only. Lateral buds did not initiate flower buds under short- and long- day conditions at high temperature. In autumn, terminal buds ceased to grow under short-day conditions. This resulted in alternation of the flower-bud initiation position from terminal to lateral buds.2. On the basis of flower-bud initiation under different day-length and temperature conditions, Clematis cultivars may be classified into three groups as follows: 1) Group A: Flower buds are initiated under short-day conditions in autumn without winter chilling.2) Group B: Flower buds are initiated at the apex of new shoots, after being subjected to chilling, irrespective of day length.3) Group C: An intermediate group between A and B.3. When subjected to winter chilling for a short period, plants which had initiated flower buds in the previous autumn flowered earlier under long-day conditions. However, no influence of photoperiod was recognized when the chilling period became longer. When the plants were cultivated without winter chilling, growth started immediately after the photoperiod was changed from short-day to long-day. leading to almost normal flowering.These results revealed that identical cultivars of Clematis show different photoperiodic responses in the initiation and development of flower buds, depending on temperature conditions.