At 6:03 AM 10/18/95 -0700, Kathleen Archer wrote:
>Regarding lettuce seed germination in fruit juice:
>>Lettuce seeds are not borne in juicy, fleshy fruits; they form in rather
>dryish fruit structures which split open when ripe releasing the seeds which
>are wind dispersed. Since they don't encounter a warm, juicy environment
>when they are maturing, would they be adapted to repond to germination
>inhibitors in fruit juices? Could some other germination inhibition process
Kathleen,
Good question, but here is what I find...
In our little project, the students put lettuce seeds
in six filter-paper-lined 10 cm Petri dishes. Four
dishes get 5 mL of distilled water. One gets 5 mL
of canned tomato juice (I just get those tiny cans
with a foil seal in a six-pack). One gets 5 mL
of 10-4 M abscisic acid. The dishes with juice, ABA,
and one dish with water go in white light. The
other three dishes go in red, far-red, and darkness,
respectively.
The seeds sprout well (80%+) in white and red light. We get
about 20-30% in dark. We get 0% in tomato juice, 0%
in abscisic acid, 0% in FR light. When the seeds in
FR light are put into white light, germination is
restored up to about 70%. The tomato juice and ABA
treated seeds do not sprout. The tomato juice gets
over-run with fungi after a few days (too much good
substrate) but the ABA treated seeds, if kept for
maybe a week or more, may begin to sprout up to maybe
5%. I interpret this to mean the ABA is being metabolized
by bacteria/fungi (not obvious but likely present)
and/or ABA is being photo-oxidized.
While the lettuce seeds are, indeed, as you say, shed
from a dry collection of cypselas in the involucre,
the maturing seeds are in more moist and warm conditions
in this place before the cypselas dry. I think that
investiture of ABA into seeds during the maturing phases
is probably universal, and that accumulations of very
large amounts of ABA are likely limited to fleshy-fruited
species. The fact that lettuce seeds respond to ABA
in this way makes sense to me.
Whether ABA is in the tomato juice in sufficient concentration,
whether ABA is the causative agent for no lettuce germination,
these are, indeed, open questions as I have not done the chem
analysis needed. It is completly possible that the lost response
is due to the acidity of the tomato juice, or perhaps even the
osmolarity of the juice. This would make an interesting project
for an independent study. Serial dilutions of tomato juice and
buffered tomato juice might be tried. Fungicides/antibiotics
might be added to the mix. One might use axenic culture...
even though this is tough with cypselas!
In any case it is a fun exercise that demonstrates the ABA
effect very nicely along with the phytochrome response.
BTW I use Agway 'Salad Bowl' lettuce and have found all
seed lots work well for me. I used to use 'Grand Rapids'
but got a lot of variability in the responses from seed lot
to seed lot.
ross
________________________________________________
( )______________________________________________)
\ Ross Koning \
\ Biology Department \
\ Eastern CT State University \
\ Willimantic, CT 06226 USA \
\ Koning at ecsuc.ctstateu.edu \
\ http://koning.ecsu.ctstateu.edu/default.html \
\ Phone: 860-465-5327 \
) Fax: 860-465-5213 )
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