Epicotyl Dormancy in Viburnum Acerifolium (Caprifoliaceae)
Hidayati, Siti N., Baskin, Jerry M., Baskin, Carol C., The American Midland Naturalist
Seeds (true seed plus endocarp) of Viburnum arerifolium have underdeveloped linear embryos and deep simple epicotyl morphophysiological dormancy. Seeds mature and are dispersed in autumn, and radicles and epicotyls emerge the following autumn and late autumn-winter, respectively. Embryos in seeds incubated at 25/15 C became fully elongated in 16 wk, whereas those incubated at 5 C hardly grew at all. The optimum continuous temperature regime for both rate and percentage of radicle emergence was 25/15 C. At this regime, however, >20 wk were required for radicles to emerge in a high percentage of the seeds. GA^sub 3^ had little or no effect on radicle emergence. In seeds subjected to a winter [arrow right] spring [arrow right] summer [arrow right] autumn temperature regime in controlled environments, radicles emerged in autumn in the 20/10 [arrow right] 15/6 C portions of the sequence. In those subjected only to the summer [arrow right] autumn portions of the sequence, radicles also emerged in autumn at 20/ 10 [arrow right] 15/6 C. Thus, in nature, the cold winter season has no effect on dormancy-break. Radicles of seeds planted in a nonheated greenhouse in November 1998 emerged in October 1999, when mean maximum and minimum temperatures were about 19 and 9 C, respectively. The optimum continuous temperature regime for both rate and percentage of epicotyl emergence was 15/6 C. For seeds planted in the nonheated greenhouse in November 1998, rate of epicotyl emergence peaked in early January 2000, when mean maximum and minimum temperatures were about 10 and 5 C, respectively.
Many plant species, including those of the Caprifoliaceae, have underdeveloped embryos (Martin, 1946), and embryo growth must occur prior to radicle emergence (Baskin and Baskin, 1998; Hidayati, 2000). Underdeveloped embryos are either rudimentary (globular to oval-oblong) or linear (several times longer than broad) (Martin, 1946), and they may be dormant or nondormant at the time of seed maturity. If embryo growth and radicle and shoot emergence are completed at suitable incubation temperatures on a moist substrate in about 30 d, without a dormancy-breaking pretreatment, seeds have morphological dormancy (MD) (Nikolaeva, 1977; Baskin and Baskin, 1998, 2004). However, if embryos do not grow when seeds are placed under temperature, light, moisture and oxygen conditions that generally are favorable for germination, they also have physiological dormancy (PD). PD is broken while seeds are subjected to a specific regime of environmental conditions which, in nature, simply may occur with the passage of time, and embryo growth occurs either during or after PD is broken (Nikolaeva, 1969; Baskin and Baskin, 1998). seeds with underdeveloped physiologically dormant embryos have morphophysiological dormancy (MPD). This kind of dormancy requires a dormancy-breaking pretreatment such as exposure to moist cold (0-10 C) and/or to moist warm (≥15 C) temperature regimes before the seeds can germinate (Nikolaeva, 1969, 1977).
Eight kinds of MPD have been distinguished based on: (1) temperatures required to break dormancy, (2) temperatures required for embryo growth and (3) whether GA^sub 3^ overcomes dormancy (Nikolaeva, 1977; Baskin and Baskin, 1998, 2004). These eight kinds of MPD can be divided into two categories: (1) relatively high temperatures (≥15 C) are required for embryo growth in seeds with simple MPD and (2) low temperatures (0-10 C) are required for growth in those with complex MPD. The kinds of MPD can be subdivided further depending on the level of physiological dormancy: nondeep, intermediate and deep (Nikolaeva, 1977; Baskin and Baskin, 1998, 2004). Deep simple epicotyl MPD, generally referred to as "epicotyl dormancy," is one of the eight kinds of MPD (Nikolaeva, 1977; Baskin and Baskin, 1998, 2004). seeds with epicotyl dormancy produce a root at warm temperatures, but shoots do not grow until after the seeds with emerged roots have been exposed to a period of cold (0-10 C) temperatures (Barton, 1933). …