A Brief Consideration of Seedlings
The winter rains have started and seedlings are peeking out of the soil in our local plant communities. What gets this process going? The three especially important environmental factors affecting germination are water, oxygen, and temperature. Water is the most critical.
The seeds of flowering plants consist of an embryo, a seed coat, and stored food (endosperm). Most mature seeds contain only five to twenty percent of their total weight as water. Germination is not possible until the seed imbibes enough water for metabolic activities to begin. The amount of water imbibed is considerable and the seed swells. Pressure develops within the seed, causing the seed coat to rupture. The first structure to emerge from most seeds is the radicle, or embryonic root. This order of events reflects the primary needs of germinating seeds for water and for anchor in the soil.
During the early stages of germination, respiration may be entirely anaerobic, but as soon as the seed coat is ruptured, the seed switches to aerobic respiration requiring oxygen. If the soil is waterlogged the amount of oxygen available to the seed may be inadequate for aerobic respiration and the seed will fail to germinate. As the soil dries, the swollen seeds are able to produce the energy for growth.
Although many seeds will germinate over a wide range of temperatures, they usually will not germinate either below or above a certain temperature specific for each species. The optimum temperature varies with the metabolic requirements of the enzymes used in cell building and growth. The minimum temperature for most species is 0-5 degrees C; the upper, 45-48 degrees C; with the optimum of 25-30 degrees C. At either temperature extreme, the percentage of germination will be low.
The most crucial phase in the life history of the plant is the period from germination to the time the seedling no longer depends on its endosperm. At that time the plant is most susceptible to injury by a wide range of insect pests and parasitic fungi, and water stress to the tiny seedling can very rapidly be fatal.
What to Observe During a Winter Trip to San Diego County
If you have and opportunity to spend some time in southern California this winter, look at some seedlings and think about all the biological processes going on within the little plants to get themselves established. And remember that, all over, in the duff of the chaparral, coastal sage scrub, oak woodlands, and other plant communities, are thousands, millions, maybe even trillions of seeds waiting to germinate in some future season. One way that plant populations have adapted to protect themselves against extinction is that the seeds they produce germinate over many years, never all in one year.
In the coastal mesas and foothills of San Diego County, winter with its rains is the season of rebirth. We don’t have to wait for the snow to clear. Tidy-tips (Layia platyglossa), poppies, and mesa saxifrage are already germinating by December and will soon (if they aren’t already) be blooming. Some white-flowered Ceanothus species and many manzanitas (Arctostaphylos spp.) are showy in January along the coast, in canyons, in chaparral, and at higher elevations further inland. The tiny mesa saxifrage Jepsonia parryi can still be found in open areas of moist shaded chaparral and coastal sage scrub but its flowering period is almost over by January. Enjoy our bountiful winter!