Dormancy-breaking and germination requirements for seeds of Symphoricarpos orbiculatus (Caprifoliaceae) 1
Initial germination experiments
ABSTRACT
Fruits (drupes) of
Symphoricarpos orbiculatus ripen in autumn and are dispersed from autumn to spring. Seeds (true seed plus fibrous endocarp) are dormant at maturity, and they have a small, linear embryo that is underdeveloped. In contrast to previous reports, the endocarp and seed coat of
S. orbiculatus are permeable to water; thus, seeds do not have physical dormancy. No fresh seeds germinated during 2 wk of incubation over a 15°/6°–35°/20°C range of thermoperiods in light (14-h photoperiod); gibberellic acid and warm or cold stratification alone did not overcome dormancy. One hundred percent of the seeds incubated in a simulated summer → autumn → winter → spring sequence of temperature regimes germinated, whereas none of those subjected to a winter → spring sequence did so. That is, cold stratification is effective in breaking dormancy only after seeds first are exposed to a period of warm temperatures. Likewise, embryos grew at cold temperatures only after seeds were exposed to warm temperatures. Thus, the seeds of
S. orbiculatus have nondeep complex morphophysiological dormancy. As a result of dispersal phenology and dormancy-breaking requirements, in nature most seeds that germinate do so the second spring following maturity; a low to moderate percentage of the seeds may germinate the third spring. Seeds can germinate to high percentages under
Quercusleaf litter and while buried in soil; they have little or no potential to form a long-lived soil seed bank.
No freshly matured seeds germinated during 2 wk of incubation in light. Further, no seeds germinated during 2 wk of incubation at 15°/6°–35°/20°C following cold, warm, cold plus warm, or warm plus cold stratification.
Effect of GA3 on germination and embryo growth (Gibberellic Acid)
No seeds germinated in any of the three treatments or in the control during 12 wk of incubation at 25°/15°C in light (
Table 3). Gibberellic acid also did not have much effect on embryo growth, although GA3 concentration, incubation length, and their interaction were significant (
P ≤ 0.0021). Maximum embryo length attained was 0.69 ± 0.01 mm (mean ± 1 SE) after 12 wk of incubation in 1000 mg/L GA3 at 25°/15°C (
Table 3). Thus, embryo length increased only ∼21%.
Effect of simulated sequence of seasonal temperature regimes on embryo growth, dormancy break, and germination
The sequence of temperature regimes had significant effects (
P = 0.0001) on germination percentages and on embryo growth. Embryos in fresh seeds initially incubated at 25°/15°C did not grow until after seeds were transferred to the lower temperatures (
Fig. 3a). However, embryos of seeds initially incubated at 5°C did not grow until after seeds received a warm period and then a second cold period (
Fig. 3b). Embryos in the four controls grew very little: from 0.57 ± 0.02 (mean ± 1 SE) to 0.67 ± 0.06 mm at 5°C, to 0.63 ± 0.03 mm at 15°/6°C, to 0.61 ± 0.04 mm at 20°/10°C, and to 0.59 ± 0.04 mm at 25°/15°C (data not otherwise shown). Embryo length increased ∼100% between seed maturity and germination (
Fig. 3).
Seeds placed initially at 25°/15°C did not germinate until after they were transferred through the sequence of temperatures: 25°/15°C (12 wk) → 20°/10°C (4 wk) → 15°/6°C (4 wk) → 5°C (12 wk) → 15°/6°C (4 wk), and then 100% of the seeds germinated at 15°/6°C (
Fig. 3a). In contrast, seeds placed initially at 5°C did not germinate until after they were exposed to the sequence of temperatures: 5°C (12 wk) → 15°/6°C (4 wk) → 20°/10°C (4 wk) → 25°/15°C (12 wk) → 20°/10°C (4 wk) → 15°/6°C (4 wk) → 5°C (12 wk) → 15°/6°C (4 wk) → 20°/10°C (4 wk), and then they germinated to 96% at 20°/10°C (
Fig. 3b). No seeds germinated in any of the four controls.
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