Interrelated Effect between Thermo- and Photo-periodism on Growth and Development of Ornamental Woody Plants

Modification of growth resulting from photoperiodic responses to temperature treatment in Hydrangea was studied.1. Interrelated effects of winter chilling, temperature, and day-length during the growing period on growth were investigated. (1) Day-length did not affect the growth of any plots transferred to a greenhouse with a constant night temperature of 10°C. (2) When plants were transferred on December 15 to a greenhouse with a night temperature of 18°C, new shoot length, leaf length and inflorescence diameter were longer, and flowering time was earlier, under short-day rather than long-day conditions. However, in plants which were transferred on January 1, growth was promoted and flowering time was earlier under long day conditions. Day-length did not affect the growth of any plots except those with transfer dates of December 15 and January 1. (3) Under the 25°C treatment, growth was promoted under long-day conditions only when plants were transferred to the green house after January 15. Flowering time was earlier under long-day conditions independent of the transfer date.2 When cutting of Hydrangea was done in February, flower bud differentiation was induced in May and flowering occurred in August. Day-length had no effect on any of the flower development processes. However, when cutting was done in March, day length affected flower bud differentiation and flowering markedly, and there were more plants which initiated buds and flowered normally under short-day conditions than long-day conditions.