Seed Germination

Seeds that are not dormant may not germinate if they have not encountered favourable environmental conditions: these are termed 'quiescent' (Foley, 2001). They are 'seeds in waiting'. Quiescent seeds are able to germinate immediately once they encounter favourable environmental conditions, but may revert to secondary dormancy (Fig. 6.6).

The critical factor for seeds is to be able to germinate at an appropriate time, a daunting task since environmental conditions vary on small spatial scales and also are rarely constant from day to day and year to year. For example, in temperate regions, seeds that germinate late often experience intense competition from other individuals and have a shorter growing season to complete their life cycle. Conversely, seeds that germinate early may experience high mortality from unfavourable environmental conditions (e.g. frost). However, the risk may be worth it as the ones that survive are better competitors for light and other resources and have higher fitness (Ross and Harper, 1972; Marks and Prince, 1981; Gross, 1984).

Most weeds are able to germinate throughout the growing season and this is an important reason why they are successful, though each individual will experience differential success for the reasons we just mentioned. For example, redroot pigweed and lambsquarters can germinate from April to October. In addition to the early season weather risks, any individuals in agricultural fields likely will be subject to weed man agement. Some may escape management and survive to set seed but the chances of success are generally low. However, this may be worth the risk because later germinating weeds (e.g. in mid- to late summer) can be far worse at reproducing successfully. Later germinating weeds may not be subject to management because it is not economically viable for a farmer to do so. These weeds may acquire enough light, water and nutrients to survive under the crop canopy but they may have insufficient time or resources to set seed. While the parent may survive, their offspring may not (or may not even exist) and their fitness is reduced. In fact, it is likely that most of the weed seeds returned to the seedbank will be from those individuals that germinated early, survived weather and management conditions, and produced seeds (Swanton et al, 1999).

The timing of seed germination is triggered by environmental cues. The most common cues are light quality and quantity, temperature, moisture and gases (O2 and CO2). These generally vary on large scales (latitude) but they also vary locally. On a local scale, seeds of different species are sensitive to different aspects of their physical microenvironment. For example, Sheldon (1974) found that annual sow-thistle and dandelion seeds that had the attachment end (end that was formerly attached to the seed head) in closest contact with a moist substrate had the highest percentage germination (Fig. 6.10).

Requirements differ among species but, generally, species with small seeds tend to require light for germination more than large seeded species (Milberg et al., 2000). Alternating temperatures are required for the seed germination of many weeds, including common lambsquarters, large crabgrass (Digitaria sanguinalis), field bindweed, orchardgrass (Dactylisglomerata glomerata) and some species of dock (Rumex spp.). The effect of temperature fluctuations depends on the:

Fig. 6.10. Effect of seed position (in relation to the soil surface) on the germination of dandelion and annual sow thistle. The dark end of the seed is the attachment end (adapted from Sheldon, 1974).

Fig. 6.11. The effect of temperature and moisture (water potential) on the germination of common lambsquarters. The highest percentage germination was achieved at moderate temperatures (17.7-27.5°C) and low water potential (high water availability) (based on data from Roman, 1998).

(-) Water potential

Fig. 6.11. The effect of temperature and moisture (water potential) on the germination of common lambsquarters. The highest percentage germination was achieved at moderate temperatures (17.7-27.5°C) and low water potential (high water availability) (based on data from Roman, 1998).

• amplitude (difference between maximum and minimum temperatures);

• mean temperature;

• thermoperiod (time above mean temperature each day) (Probert, 1992).

In some species, germination requirements are well understood and we can be predict what proportion of them will germinate based on moisture and temperature (Fig. 6.11) (Roman, 1998). We also understand that for some species, germination cues can be influenced by the maternal environmental conditions. For example, the temperatures during seed after-ripening affect germination requirements of redroot pigweed and common lambsquarters (Baskin and Baskin, 1987). When seeds develop at higher temperatures, more seeds germinate at higher temperatures.

0 0

Post a comment