Time Of Stratification In Weeks

Figure 2.9 Water and oxygen uptake by apple seeds during stratification in relation to germination. From Lewak and Rudnicki (1977); modified from Duczmal (1963).

The megagametophyte absorbed less water and the seed coat the least (Stanley, 1958). The rate and pattern of water uptake by embryos are influenced by dormancy-breaking treatments. For example, in embryos of sugar pine seeds, water uptake increased as stratification time under moist conditions was increased. The rate and amount of water imbibed by seeds also vary with the nature of the seed coat, size and chemical composition of the seed, and water temperature.

Respiration

Respiration of seeds involves oxidative breakdown of organic constituents, primarily sugars, starches, fatty acids, and triglycerides, to provide energy in the form of ATP, which drives anabolic aspects of germination. The ATP is required for synthesizing new cellular constituents in seedlings and for forming protein-synthesizing machinery in producing enzymes for degradation and conversion of storage compounds. Respiration also provides much reducing power in the form of NADH. Hence, respiratory products play an important role in seed germination (Kozlowski, 1992).

Oxygen uptake of dry seeds is extremely low, but it increases greatly as water is imbibed and varies appreciably during three or four phases (Figure

2.9). In phase I, the imbibition phase, a burst of respiration is associated with activation of mitochondrial enzymes. In phase II, the rate of respiration is stabilized as seed hydration has slowed and preexisting enzymes are activated. Phase III is characterized by a second respiratory burst that is associated with activity of newly synthesized mitochondria and respiratory enzymes. Oxygen availability to the embryo also may be increased by rupture of the seed coat. In phase IV, which occurs only in depleted storage tissues, the rate of respiration decreases as respiratory substrates are exhausted (Bewley and Black, 1982).

When dormant sugar pine seeds imbibed water at 5°C they showed a rapid increase in 02 uptake, ATP, and moisture content during the first 4 days (Murphy and Noland, 1982). A plateau phase followed until 60 days, after which a second marked increase occurred in all three features as dormancy was broken. The pattern was different when dormant seeds imbibed water and were maintained at 25°C. Water was absorbed much faster during the first 4 days, and a second increase in 02 uptake did not occur after 60 days because the seeds did not break dormancy.

A high energy charge (EC) is important for termination of seed dormancy because of the role of ATP in respiration (see Chapter 6 of Kozlowski and Pallardy, 1997). The energy charge is the sum of the mole fraction of ATP plus one-half the mole fraction of ADP divided by the sum of the mole fractions of ATP, ADP, and AMP:

The EC increased greatly during stratification of sugar maple seeds (Table 2.5), and, even at high absolute ATP levels, the embryonic axes elongated in response to gibberellic acid and kinetin only when the energy charge was near 0.8 or greater. The EC increased during the first 2 weeks of stratification of northern red oak acorns when germination was low (Hopper et al., 1985). Thereafter the EC decreased before increasing again at 8 weeks after stratification. A second rise in the EC coincided with increased seed germinability. During the 28-day germination period, fluctuations in EC were correlated with growth of roots and shoots. Hopper et al. (1985) suggested that the EC may be useful for indexing the degree of seed dormancy and seedling vigor.

The rate and duration of each phase of respiration may be expected to differ among seeds of different species and genotypes because of variations in seed morphology, amounts of carbohydrate reserves, seed coat permeability, rates of imbibition, and metabolic rates (Bewley and Black, 1985).

Table 2.5 Effect of Stratification at 5°C on ATP, ADP, AMP, and Energy Charge in Sugar Maple Seeds"

Adenylates (nmol/g dry weight)

Table 2.5 Effect of Stratification at 5°C on ATP, ADP, AMP, and Energy Charge in Sugar Maple Seeds"

Adenylates (nmol/g dry weight)

Stratification period (days)

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