Physiology of Seed Germination

The essential events in seed germination are resumption of growth by the embryo and its development into an independent seedling. Many changes are set in motion as germination begins, including the following: (1) seed hydration, (2) increased respiration, (3) enzyme turnover, (4) increase in adenosine phosphate, (5) increase in nucleic acids, (6) digestion of stored foods and transport of the soluble products to the embryo where cellular components are synthesized, (7) increase in cell division and enlargement, and (8) differentiation of cells into tissues and organs. The exact order of the early changes is not clear, and there is considerable overlap; however, with few exceptions, absorption of water is a necessary first step. Increase of hydration is associated with cell enlargement and cell division in the growing points as well as release of hormones that stimulate enzyme formation and activity. Although an increase in fresh weight of the seed accompanies imbibition, there is an early loss in dry weight due to oxidation of substrates and to some leakage of metabolites. After the root emerges and begins to absorb minerals and the cotyledons or leaves become photosynthetically active, the dry weight of the young seedling begins to increase until it regains and then surpasses the original seed weight.

Hydration

Water must be imbibed by seeds to increase protoplasmic hydration and set in motion a chain of metabolic events associated with germination. Imbibition of water softens hard seed coats, and the swelling of the imbibing embryo bursts the seed coat, permitting emergence of the radicle.

Movement of water from the soil into the seed depends on water relations of both the seed and the surrounding soil (Hadas, 1982; Bewley and Black, 1985). There is a large difference between the water potential of the dry, planted seed and moist soil, largely because of the low SP of the dry seed coat, cell walls, and storage components of the seed. As the seed imbibes water, this gradient decreases because the ^ of the seed increases (becomes less negative) and the vf' of the surrounding soil decreases. Hence, the rate of water uptake by the seed declines. Subsequent water uptake by the seed is appreciably influenced by the ^ of the soil surrounding the seed, hydraulic conductivity of the soil, soil bulk density as affected by compaction, and extent of contact of the seed with soil particles.

Uptake of water by dry seeds typically occurs in three phases (Figure 2.9). An initial imbibitional phase of rapid water uptake is followed in order by low or negligible uptake and a phase of rapid uptake that culminates with radicle emergence (Hadas, 1982). The imbibitional phase of water uptake occurs in dormant, nondormant, viable, and nonviable seeds and involves physical hydration of the cell matrix, including walls and cytoplasmic colloids. As the seed imbibes water, a moving wetting front forms so there is a marked difference in the water content of the wetted cells and the adjacent, unhydrated cells.

In the lag phase of water uptake, the matrix components are no longer important in determining seed water status, and the ^ of the seed is a function of its osmotic potential and the pressure potential that results from the pressure of the swollen contents on the cell wall. During this phase the seeds become metabolically very active.

The third phase of rapid water uptake occurs only in germinating seeds. At first this increase in water uptake is associated with volume changes in cells of the radicle as it elongates. Subsequently, water uptake from the surroundings is regulated by a decrease in the osmotic potential associated with osmotically active compounds formed during hydrolysis of stored food reserves (Bewley and Black, 1985).

The amount of water absorbed by various parts of the seed during germination varies greatly. In sugar pine, the embryo, which was the smallest seed component, absorbed the most water as a percentage of its dry weight.

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