Inhibition of Premature Flowering by High Temperature Interruption during Winter Season in Phalaenopsis Hybrids

Abstract

High temperature above 28°C is necessary to inhibit flowering in Phalaenopsis. During winter season, however, growers maintain a greenhouse temperature conditions below 25°C because of greenhouse heating cost. This cultivation practice induces premature flowering. This study was conducted to develop new cultivation strategy by preventing premature flowering through energy-saving flowering inhibition using high temperature inhibition. The experiment in Chapter I was performed to determine the growth period of the juvenile stage and low temperature duration required for inducing premature flowering. Clones of Phalaenopsis Hwasu 355 grown in a greenhouse for 2, 4, and 8 months (2, 4, and 8-month-old), which have 1-2, 2-3, or 3-4 newly developed leaves, respectively, were used in the experiment for determining the juvenile period. These plants were grown under low temperature at 25/20°C with 7 different durations: 0, 1, 2, 3, 4, 5, and 10 weeks. After each treatment, plants were transferred to 28/28°C. In addition, to observe flower-stalk differentiation 8-month-old plants were treated with 7 weeks of low temperature. With the results of chapter I, the objective of the experiment in Chapter II was to determine the cultivation strategy for inhibition premature flowering by high temperature inhibition. Clones of 8-month-old Phalaenopsis Hwasu 355 and Doritaenopsis Mantefon plants were cultivated at four different temperature regimes for 16 weeks: continuous low temperature (L) (no interruption), 1 week of high temperature (H) every 1 week (1L + 1H), 1 week of high temperature every 2 weeks (2L + 1H), and 2 weeks of high temperature every 2 weeks (2L + 2H). During the experiment, the low temperature and high temperature were set at 25/20°C and 28/28°C, respectively. In Chapter I, the length of juvenile period of Phalaenopsis plants seemed to be between 4 and 6 months, with 3 fully developed leaves. Low temperature for 4 and 5 weeks could induce flower-stalk emergence in 8-month-old Phalaenopsis Hwasu 355, but the elongation of flower-stalks stopped. About 8 weeks of low temperature was required to induce visible inflorescences (flower-stalks longer than 0.5 cm). In chapter II, the inhibition of premature flowering was observed in plants treated with high temperature interruption treatments. However, the flowering was slightly inhibited in Doritaenopsis Mantefon, whereas that was completely inhibited in Phalaenopsis Hwasu 355. The inhibition effects increased with shortening the interval between high temperature interruption treatments. The number of days to visible inflorescence increased with lengthening the duration of total high temperature. These results indicate that high temperature interruption can prevent premature flowering in winter. Furthermore, high temperature interruption can be used to improve previous cultivation practice through raising the flower quality by preventing premature flowering. However, because the low temperature sensitivity is different among cultivars, detailed studies are needed for specific cultivars.