Germination and growth

5. The seeds should germinate within a day or two and you will see the hypocotyl and cotyledons emerge from the layer of vermiculite. Keep the soil moist but not saturated during plant growth. Depending on the soil mix that you use, a supplemental feeding at about one or two weeks may be useful but don’t overdo it with the fertilizer. Plants should make 2-4 true leaves and then transition to flowering. Typically the first flowers are open at ~18 days after planting.

Keep the soil moist but not saturated: FPsc (and most other non-wetland plants) does not grow well in water-saturated soil. The roots need to breath and chlorosis or death will result if the plants are, in essence, drowning. There are two ways we know of to maintain optimal soil moisture levels: wicking and periodic flooding. Wicking is widely used in classrooms for propagation of the WFP variety of B. rapa and also works well for growing FPsc. Briefly, a segment of yarn, cotton string, or other absorbamt and water-conducting material is inserted in the pot before soil is added, threaded out the bottom and into a reservoir of water. Water wicks up the fibers and into the soil as transpiration and evaporation remove it during plant growth. Use of a wicking strategy imposes front-end costs since it takes time to wick each pot or cell but those costs may be recovered later in terms of vigorous growth and less need to constantly monitor soil moisture levels. The latter is especially useful in classroom settings since teachers (and students) can leave for the weekend and be confident that the plants will be well when classes resume (provided, of course, that the reservoir was topped-off with water before leaving). Periodic flooding is another useful approach to maintaining optimal moisture levels. The pots with soil and plants are immersed in a larger vessel filled with water to a level sufficient to saturate the soil. Briefly flooded, the flats/pots are removed and excess water is allowed to drain before moving plants back to lighted shelves. Again, here at the UW, we have the luxury of access to flood tables in which electronically-controlled water pumps actuate to flood and then drain the flats thrice weekly, but we use those facilities only when we are bulking seed stocks or otherwise growing exceptionally large numbers of plants. It would be rather painful if we used the approach for all of our experiments since it can be messy and it takes time. Instead, we settle for a middle ground approach, adding water to the white flat bottom (figure 1) in small doses several times per week, providing somewhat bigger “drinks” if we know that plants will be unattended for 2-3 days.

Please also be aware that water usage by your plants will vary as a function of developmental stage, reaching a maximum at or about the time of flowering and seed set when there is an abundance of photosynthetically active leaves that both consume water and serve as platforms for water loss through transpiration and evaporation.

Supplemental feeding: As noted in item 1, the extent to which supplemental fertilizer/nutrients are needed depends heavily on which soil mix is used. Also as noted, we have found that MetroMix 360 has sufficient resources to support good growth without any supplemental feeding; nonetheless, we typically feed them lightly with a balanced fertilizer containing trace elements/micronutrients just before the onset of flowering. We use Peters All Purpose plant food (20-20-20 with micronutrients) at ½ recommended strength and added instead of plain water when the flat bottoms/reservoirs are dry (or nearly so). As with many things, less is more. Paul Williams (father of the WFP program) once told me that his goal when growing B. rapa is to keep them adequately nourished but “just a little hungry”. Too much fertilizer can be detrimental in (at least) two ways. First, if added at high rates at planting, especially if added to a soil mix already endowed with fertilizer, salts in the soil may come out of solution and crystallize on plant surfaces. In extreme cases this leads to what we call ‘tip burn’, in which seedlings emerge, spread their cotyledons, and then growth stalls due to damage at the shoot apical meristem. Often plants suffering tip burn never resume growth and ultimately die; alternatively, cells of the apical meristem may reorganize to permit further growth but such meristems are apparently dysfunctional—leaves that emerge are often misshapen, we see fasciation of the stem, etc (figure 7). Those plants are clearly not well, and remember the maxim: crappy plants make crappy seeds, etc. Thus, if you suspect that your soil may be deficient in minerals and nutrients, wait about a week until the seedlings have emerged and the meristems are elevated from the soil surface to minimize contact with any crystallized salts. The second detrimental effect of excess fertilizer is the tendency of overfed plants to produce secondary branches and to continue flowering well past some optimal point (that optimum will be discussed in item 6, below). The net result is a population of plants that are intertwined due to branching and slow to reach maturity and senescence.

SeedlingsTip burnFlowers displaying immature florescence

Figure 7: Seeds will germinate and grow above the soil after ~3-4 days (left: seedlings of a family in which the albino mutant allele is segregating). Center: A stunted seedling with a condition known as “tip burn”, caused either by crappy seeds and/or excess fertilizer and salts that have leached from the soil and damaged tissues that comprise the shoot apical meristem. Immature floral inflorescences should be evident 2 weeks after planting (right) and ready for pollination in ~18-20 days.