Seed placement
2. Dispense seeds into the palm of your hand or onto a seed tray and place seeds individually on the surface of the wet soil.
Seed tray: We typically use 4” x 6” note cards with all edges folded up ~3/4” to prevent the seeds from rolling away (figure 3).
Figure 3: Seed tray made from a note cards. Seeds may be planted by hand, with the use of fine tip forceps, or by using the jagged and wettened end of a broken large toothpick or bamboo skewer.
Place seeds: Both terms merit comment. To “place” the seeds means to deposit them gently on the soil surface. There is no need (and it may be disadvantageous) to press the seeds down into the soil. If your fingers are small and nimble, use them; alternatively, you can use forceps but be careful—depending on the style and width of the forcep tips seeds can sometimes “pop” loose and land (a) where you can’t find them and/or (b) precisely where you don’t want them. A third option is to use the jagged tip a broken toothpick or bamboo skewer that has been wetted with water or saliva. Capillary action holds the seed to the wooden edge during transit but releases the seed easily when it is placed onto wet soil.
Seeds: The quality of seeds that are planted is among the most important factors that determine the vigor of the resulting plants. Embryogenesis/seed development in FPsc and many other varieties of B. rapa is “messy”—some proportion of fertilized ovules abort development altogether at some point during embryogenesis; in others, development proceeds slowly and with apparent difficulty, resulting in undersized and sometimes misshapen embryos/seeds at maturity. The cause(s?) of spontaneous abortion or stunted embryo development are unknown; in any case, the proportion of “bad seeds” is higher in plants grown under sub-optimal conditions. “Good seeds” are full and round (potato-shaped, actually; see fig. 4), whereas the seed coats of “bad seeds” are typically shrunken and/or wrinkled, an outward manifestation of the embryo within. Many of the latter can germinate but the resulting plants are frequently and likewise misshapen, and overall vigor is typically reduced as compared to plants grown from “good” seeds. We have a mantra in the lab that goes something like “Crappy plants make crappy seeds and crappy seeds make crappy plants”. We try to package only the highest quality of seeds for distribution, but even after our efforts there may be variation in seed quality. Furthermore, if your experiments span multiple generations, you need to be aware of bad seeds. In any case, it is important to select the best seeds available when planting and to grow plants under the best possible conditions in order to maximize the proportion of high quality seeds.
[It’s worth noting that most types of B. rapa “parts” you may see in the veggie section of the supermarket are the product of selective breeding programs imposed by humans, likely conducted over many thousands of years and seed production has rarely been a driving force since the portions that are intended to be eaten are produced during the vegetative phase of plant development. Think of the turnips (B. rapa var. rapifera); the fleshy leaves of Bok choi (B. rapa var. pekinensis) and Pak Choi plants, and the spicy edge provided by the thin and serrate leaves of Chinese mustard greens (B. rapa var. Mizuna). It’s all about the leaf, and as long as seed –production is sufficient to provide for next season’s crop, maximizing seed yield and seed quality has not been as direct a target for selection by those breeders. An exception to this general rule are B. rapa oil seed crops grown in northern US and international latitudes (rapeseed) and in Asiatic countries such as India, China and Nepal, where the seeds are an important source of spices and condiments.
Figure 4: Plant the best seeds that you have available—size matters, and bigger is better. Try to avoid shrunken or otherwise misshapen seeds, as well as any whose embryos had bypassed dormancy and started to germinate (vivipary).