Seedling Emergence

The term 'seedling' is simply another way of saying 'young plant' and implies no specific age or stage range. Trees are often referred to as seedlings into their second or third year simply because they are small compared to their adult form, whereas a fast-growing annual may be called a seedling only for a matter of days. The distinction between seed germination, seedling emergence and establishment is not always clear and the terminology can be confusing. Germination normally means that the seed is physiologically active and the embryo is undergoing mitosis to produce a shoot and/or root that emerges from the seed coat. Emergence usually refers to the appearance of a shoot above the soil or a root from the seed. Establishment is generally considered to occur once a seedling no longer depends on seed reserves (endosperm and cotyledons), i.e. it is photosynthetically independent. We discuss establishment in Chapter 7.

Factors affecting seedling emergence

The seedling stage often has the highest mortality rate of a plant's life cycle (Harper, 1977) because seedlings are vulnerable to environmental stress. As with germination, the timing of seedling emergence is important because it determines whether an individual will be able to compete with its neighbours, subject to herbivory or disease, and timing of other life history events (Forcella et al., 2000). The timing of seedling emergence is determined by the interaction of seed size, dormancy, germination, and the rate of stem and root elongation with abiotic factors (e.g. soil temperature, temperature fluctuations, soil moisture, depth of burial, light) (Allen and Meyer, 1998; Forcella et al., 2000; Roman et al., 2000). For example, Benvenuti et al. (2001) examined the effect of burial depth on 20 weed species. Time to emergence increased with depth of burial, and few seeds germinated below 10 cm. In addition, they found that the depth at which 50% of a species' seeds germinated was related to seed weight. For example, species with light seeds (<1 g) had at least 50% emergence only when buried less than 5.5 cm, whereas species with large seeds (>8.5 g) had more than 50% emergence at up to 7 cm burial. Therefore, larger seeds were able to emerge from a greater depth.

0 0

Post a comment