One of the factors that influence what seeds benefit from indoor sowing is seed size. If seeds are tiny, like lettuce and herbs like basil, they can easily get lost if you direct sow them in the garden. The problem is once they germinate, it’s hard to separate the weeds from the desired plants.
Ensuring that seed trays are kept watered is also important at this stage.
When Direct Sowing Makes Sense
Know the date for the possible last frost for where you live. Many online resources or your county cooperative extension service can provide this information. Once you know the date, work backward from there.
Craig takes his densely planted seed starts and plants them into their cells once they are about 3-inches tall. (Photo: Stephen Garrett)
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where Ni is the number of seeds germinated, Ti is the number of days from the start of the experiment, N is the total number of germinated seeds at the end of the experiment (Naylor 1981).
If germination does not occur because of the impermeability of the seed coat or pericarp, the seed is classified as showing PY (Baskin et al. 2000). If the water uptake rate is more than 20% within 24 h, the seed is judged to have water permeability (Baskin and Baskin 2003). Therefore, we found that there was no PY in E. jolkinii seeds, which had increased water absorption within 6 h.
With the enactment of the Nagoya Protocol, biodiversity and biogenetic resources are highly valued, and efforts are being made to develop and industrialize native plants (Oh et al. 2018). E. jolkinii is a medicinal plant material, and leaves and stems have been used as medicinal materials in oriental medicine. To verify and exploit these effects, studies on the pharmacological effects of extracts including their antioxidant and antibacterial activities, as well as melanin synthesis in melanoma cells have been conducted (Park et al. 2005; Kim et al. 2006; Kim et al. 2017). In addition, the unique, large flower structure and herbivorous form of E. jolkinii are considered to be of high ornamental value. To this end, researchers have evaluated the environment and growth characteristics of native habitats (Shin et al. 2018a), the vase life of cut flowers (Oh et al. 2018; Song et al. 2019a), cuttings for propagation (Shin et al. 2017), and cultivation according to pot size and nutrient conditions (Shin et al. 2018b). However, the dormancy characteristics of seeds and the conditions and characteristics of seed germination have not yet been reported.
E. jolkinii seeds are surrounded by a hard pericarp with bumps (KBIS 2019). The seeds were dark brown and globose or spherical (Fig. 1A). The surface appeared smooth, but had a mesh structure (areolate) (KNA 2017). The collected seeds were 2.12−2.85 mm in length and 2.10−2.35 mm in width, with an average of 2.18 mm. The weight of 100 seeds was approximately 473 mg, and the percentage of sound seeds was approximately 66%. After cutting the seeds, no endosperm vigor was observed in some of the seeds (Fig. 1D).
The FGPs at 5, 15, 20, and 25 °C were 51.7%, 83.5%, 2.6%, and 0.0%, respectively (Fig. 3). No seeds germinated under the four temperatures for four weeks. Less than 5% of the seeds germinated at 20 °C and 25 °C. At 5 °C and 15 °C, germination started after 47.3 and 39.0 days, respectively (Table 1). The mean germination times at 5°C and 15 °C were 53.8 and 82.3 days, respectively. The FGP was higher at 15 °C, but the germination progression was faster at 5 °C. In addition, T50 values were 48.7 and 85.0 days at 5 and 15 °C, respectively.
Seed dormancy refers to the plant’s self-suppression of germination under adverse environmental conditions for survival and is classified as physiological dormancy (PD), morphological dormancy (MD), morphophysiological dormancy (MPD), physical dormancy (PY), and combinational dormancy (PY + PD) (Baskin and Baskin 1998; Baskin and Baskin 2004; Fenner and Thompson 2005). In PD, dormancy is affected by cold stratification, after-ripening, and gibberellic acid (GA) treatment. PD can be divided into three types: non-deep PD, intermediate PD, and deep PD, depending on the depth of dormancy (Nikolaeva 1977; Baskin and Baskin 2014). Non-deep PD is the most common form of seed dormancy. This type of dormancy can be broken by short periods of chilling stratification, after-ripening (dry storage), or gibberellin treatment. Intermediate PD usually requires at least 2 months of chilling or gibberellin application as a substitute for chilling, whereas deep PD requires a long period of at least 3 months of chilling stratification to relieve dormancy and generally does not respond to exogenous gibberellin application (Geneve 2003; Baskin and Baskin 2004). MD and MPD are ecophysiological characteristics that occur because seeds have immature embryos at the time they are separated from the parent plant (Adams et al. 2005a, 2005b). Immature embryos are defined not by the size of the embryo, but by how much the embryo must still grow before germination (or before the seed coat is split) (Baskin and Baskin 2005; Chen et al. 2015). Seeds in the genus Euphorbia have been reported to be mostly nondormant or to show PD (Best et al. 1980; Sen and Chatterji 1966; Gómez and Espadaler 1997; Narbona 2002). E. esula can break dormancy after a 3-month period of after-ripening treatment (Best et al. 1980), and E. caducifolia, E. characias, and E. nicaensis, which are not native to Korea, do not exhibit dormancy. However, none of these studies have clearly revealed the morphology and development of the embryo. Therefore, detailed research is needed to determine the characteristics of non-dormancy and PD.
Germination was defined as the point at which radicles emerged at least 1 mm beyond the seed coat. Radicle emergence was checked every 3 days. After the final germination observation, poor-quality seeds were excluded from calculation of the germination percentage. The final germination was measured after 16 weeks of seed incubation. The germination characteristics, final germination percentage (FGP), day of germination, mean germination time (MGT) and days to 50% germination (T50) in days were calculated, as follows: