Horticulture Principles and Practices

physiologically immature seeds must undergo certain enzymatic and biochemical
changes to attain maturity. These changes are collectively called after ripening.
Immature embryos cannot germinate. Of necessity, such fruits are stored for a period of
time to allow embryos to mature completely.
Seeds of ancient origins have been reported to germinate after the hard seed coat
was weakened. The sacred lotus (Nelumbo nucifera) is reported to have germinated after
2,000 years, and the arctic lupine (Lupinus articus) germinated in forty-eight hours after
being dormant for 10,000 years.
After Ripening
A period after harvesting
required by certain fruits to
complete embryo maturity.
9.10 IMPROVING GERMINATION CAPACITY OF SEEDS
Ideally, seeds should germinate within the reasonably expected period that is characteristic
of the species, provided the right conditions for germination are present. Germination
of seeds should be quite predictable by the grower, especially in commercial operations
where timing of harvest is essential to obtain premium prices for horticultural produce.
Furthermore, in commercial operations, uniformity of germination and maturity are critical,
especially where any aspect of production is mechanized. When machines are used
to harvest a crop, they are unable to distinguish between ripe and unripe fruits. Additional
labor costs are incurred to manually sort out the harvest to remove immature fruits before
marketing.
Some horticultural practices initiate germination processes in a seed to a safe stage
and then discontinue it (see primed seed in). These seeds germinate quickly. Because of
dormancy, seeds extracted from mature pods or fruits fail to germinate if planted immediately
without a rest period. Through breeding activities, dormancy has been bred out
of some cultivated species, since after a species becomes domesticated, the responsibility
of caring for and protecting plants is transferred from nature to humans. In some
plants, such as legumes, seeds germinate in the pod while still on the plant. At the other
extreme, some seeds need special treatment to germinate.
Structural dormancy is imposed by seed testa or other protective covering, which
prohibits the entry of moisture and air to start the physiological process of germination.
Species with hard, impervious seed coverings occur in trees such as pine and in some
legumes. To help such seeds germinate, certain methods (mechanical and chemical) may
be employed to loosen the seed covering or initiate physiological processes for germination.
These processes are described in the following sections. To address the situation of
double dormancy from multiple causes, one may combine two or more of these treatments.
9.10.1 PHYSICAL
Mechanical
Mechanical processes include a variety of methods adopted to scratch the surface of the
seed to loosen the covering, which is called scarification. Seeds may be mechanically
rubbed between sheets of abrasive tissue (such as sand paper). They may also be scratched
by tumbling them in a rotating drum lined with a coarse material (such as emery cloth).
Some people also mix abrasive material such as gravel with seeds in the drum. Mechanical
bruising of seed must be done carefully to avoid damaging the embryo.
Temperature
Scientists have observed that forests that have experienced a fire produce new seedling
growth soon after a rainfall, whereas forests that have not burned do not have such new
growth. The inference is that the heat (high temperature) weakens the seed covering
enough to enable it to imbibe moisture. In horticultural applications, some seeds may be
placed in boiling water (with heat turned off) for about twenty-four hours. Some seeds
require cold temperature treatment (stratification) to germinate.
9.10 Improving Germination Capacity of Seeds 297