TOW: Dormancy, Seed and Bulb
Roy M. Sachs (Sun, 11 Jan 2004 17:33:14 PST)

Dormancy is one of the terms used to describe a state of seeds, bulbs
and buds when they do not grow even though environmental conditions
are otherwise favorable for growth. As a TOW the subject of dormancy
will be divided into two parts, a) dormancy in seed and b) in bulbs.

SEED. Most seed during, and for sometime after, formation in fruit do
not germinate and some require special treatments that will be
considered below.

After-ripening is the time between the formation of a mature seed and
its ability to germinate; in some species there is no after-ripening
period and seed germinate in the fruit (a highly undesirable feature
in most edible fruit where the germinating seed resemble large white
nematodes).

I have observed that seed of some freesia and alstroemeria taken
from green fruit rarely germinate, but they do fairly quickly when
taken from dried fruit. Thus the after-ripening period is satisfied
in the fruit as it dries. But there may be other cases where
germinability increases for some period during seed storage. Forum
members can help here.

Immature embryo. Seed of many orchids do not germinate unless
provided with a medium that nurtures embryo development, without at
the same time nurturing the development of fungi and bacteria. We
know that orchid seed propagation in sterile culture is a major
horticultural industry, but I don't know of other geophyte
species/genera where immature embryos are commonly produced (certain
hybrids aside, where embryos must be removed from an apparent
inhibiting influence of the endosperm).

Impermeable/Mechanically Resistant Seed Coats. Many legumes and some
grasses have very hard, mechanically resistant, or gas or water
impermeable, seed coats. How many geophytes have seed that fall in
this category? Is the problem solved by a brief treatment in strong
acid, by nicking the seed coats with a scalpel, rolling them in emery
cloth, etc? These are the measures that classroom teachers employ to
eliminate the seed coat as the barrier to germination.

The forum has had several reports from members who recommend not
giving up on seed that don't germinate quickly, even in a year or
two, and I suspect that they're waiting for the seed coats to be
digested or weakened by weathering or soil-borne bugs (microbial and
insect).

Physiological dormancy. There are embryos that will not grow for
internal, mostly unknown, reasonsŠ..this is often termed,
physiological dormancy.

Methods of breaking physiological dormancy:

Chilling. Germination in most alstroemeria is hastened by a 3 to 4
week chilling treatment following imbibition. This is not an
uncommon requirement for seed, but I have not been able to find
references for other species, particularly in the literature on
breeding of bulbs or seed producers where breaking seed dormancy
would be emphasized. Hopefully, forum members, particularly seed
suppliers, will be able to contribute specific information.

Leaching of seed, say for 24 hours, also works well to promote
germination in some seed that contain water soluble inhibitors. These
are mostly phenolic compounds that turn the leachate brown. I
suspect that seed-borne inhibitors might be quite common. Work with
lower Mojave desert annuals showed that germination, following
precise amounts of rain, is related to leaching of inhibitors. Some
of the recommendations for floating seed on water may promote
leaching of inhibitors.

Although I have not shown that leaching is necessary in alstroemeria
I do soak them for 24 hours to ensure imbibition and in the process I
remove a lot of phenolics as shown by the dark-brown leachate.

Growth Regulators. Gibberellic acid and relatives promote germination
of many seed, but I've seen nothing specific for geophytes, nor have
i seen literature on applications of cytokinins or ethylene that
would apply to us. We need help from PBS members who have been
successful with growth regulators.

I tried gibberellic acid treatments on alstroemeria seed since some
of the species of interest seemed slow to germinate or had low
germination percentage; although there was more rapid (but not
desirable) elongation of the emerging shoot; at least in the small
trials that I ran there was no improvement in germination percentage
so I discontinued the trials.

Light promotes the germination of some seed and slows the germination
of others, but once more I've seen nothing of interest regarding
geophytes. As a routine I bury all seed with approximately one inch
(2+ cm) of the perlite:///vermiculite/ mix that I use for germination and
have observed no difficulties.