Control of Flowering of Orchardgrass (Dactylis glomerata L.) : IV. Aquirement of responsiveness to floral-induction treatment in seedling

The existence of juvenile stage is well known in orchardgrass ; that is, the imbibed seed and the seedling immediately after germination are not responsive to short day-length and/or low temperature which later promote flowering. However, inconsistent results have been reported about the duration of juvenile stage. The present studies were carried out to clarify the influence of planting density, fertility-level, and seedling-age on the responsiveness to floral-induction treatment. Seedlings were grown under continuous light in a warm (25℃ in day/15℃ at night) glasshouse for several weeks after sowing. Thereafter, they were subjected to floral-induction treatment of natural short-day-lengths and low temperatures for 2 to 6 weeks in spring and autumn. After the treatments, their heading behaviour was observed under continuous light in a warm glasshouse (Fig.1 and Table 2). The results are summarized as follows: In Experiment I, 7-weeks-old seedlings of "Aonami" and "Latar" grown at different four planting densities and two fertility-levels were subjected to floral-induction treatment for 6 weeks. The heading behaviour showed that the responsiveness to floral-induction treatment substantially increased when seedlings were grown at low planting density and at high fertility-level (Table 3). In Experiment II, seedlings of "Aonami" and "Nakei EV-No.1" were subjected to floral-induction treatment from 3-, 4-, 5-, 6- and 7-weeks-old to 9- or 10-weeks-old, respectively. From the heading behaviour, it was certified in both materials that the 5-weeks-old seedlings grown under continuous light in a warm glasshouse were responsive to floral-induction treatment. However, the effects of floral-induction treatment started with the 3- or 4-weesk-old seedlings were lower than those started with the older seedlings, although the seedling-age and plant stage expressed by leaf number of main stem increased equivalently until the end of treatment. When the treatments were started at younger seedling-ages, on the other hand, the plant height of the seedlings remained lower until the end of treatment. From these results, it was suggested that the responsiveness of seedlings to floral-induction treatment was influenced not only by the seedling-age and the plant stage in leaf number but also by the size of the seedlings (Table 4). In Experiment III, 3-, 4-, 5-, 6- and 7-weeks-old seedlings of "Aonami", "Latar", "Aberystwyth S143", "O.S.G. -7", "Nakei EV-No.1" and "Dactylis glomerata ssp. judaica" were subjected to floral-induction treatment for 6 weeks. In all materials tested, the effects of floral-induction treatment increased linearly with the seedling-age up to the 6-weeks-old, but the effects on the 7-weeks-old seedlings were almost the same or a little higher than those on the 6-weeks-old seedlings (Table 6). Therefore, it was concluded that the 6-weeks-old seedlings grown under continuous light in a warm glasshouse were considerably responsive to floral-induction treatment irrespectively of materials.