Sugar (sucrose) pre-treatment of seed to decrease contamination
Orchid seed is often contaminated with large numbers of bacteria or fungi, some of which are highly resistant to chemical sterilization. Since we can't assure seed is free of such
potential contaminants, we recommend the use of a sugar pre-soak to diminish the chances of contamination. Its use is particularly recommended with particularly valuable seed.
Sucrose pre-treatments are very simple; the concept behind it involves soaking the seed in a solution of sugar, which encourages encysted organisms to "come alive," making them
more susceptible to traditional chemical sterilization.
The sugar solution need not be very concentrated, although some use a solution that is as strong as a 50/50 saturated sugar plus water solution (i.e., half strength of a saturated solution
of sugar). I have used a weaker solution, in the following manner.
A small test tube (I use 7 ml "Vacutainer" tubes that have never been used for holding blood products as they were intended- I purchase them new, and wash and sterilize tubes and
their stoppers) is used to hold the seed; I dump in whatever quantity of seed I am going to use (from 1 to 5 cubic mm or more), and then an equivalent amount of sugar. Then I add distilled water-
usually 1-3 ml, but never so much that the seed is lost or diluted too much. I then swipe the bottom of the stopper across a bar of soap to add a *tiny* amount of wetting agent, and then stopper
the tube and gently agitate it.
Agitation is performed several times (at least every 8 hours) for 16-24 hours. During this period of time, if the seed did not sink before, it almost certainly will now. The tube may be
incubated at room temperature, or at 37 degrees C if this is convenient. After pre-treatment, I open the tube and use a piece of Whatman filter paper to remove the sugar solution by absorption. The
seed is then ready for chemical sterilization and sowing into flask.