If you have already purchased a fresh lot of Grand Rapids seeds, I would suggest that you test the germination of the seeds you already have at a higher temperature.
It is quite likely that you will be able to find a temperature where dark germination is low, but germination after R is still very high.
There are some seedlots of light sensitive lettuce that exhibit very low dark germination at temperatures ranging from about 10 - 30C,
but germinate well after an exposure to light. These seedlots are nice to work with, but not easy to find.
More commonly, Grand Rapids and other light sensitive varieties exhibit a high level of dark germination at low temperatures (eg 10-25C) but dark germination levels decline with increasing temperatures, sometimes very abruptly.
I have several seedlots that germinate at about 60-100% in the dark at 20-24C, but germinate less then 10% at 27 or 28C.
Of course , if temperatures are too high, above (eg >30C) germination in both light and darkness will decline to zero.
for a bit of a discussion see
Kristie, D.N. and A. Fielding. 1994. Influence of temperature on the Pfr level required for germination in lettuce cv. Grand Rapids. Seed Science Research 19-25.
Dave Kristie
Dept of Biology
Acadia University
Wolfville NS
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Subject: Plant-ed Digest, Vol 36, Issue 2
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Today's Topics:
1. RE: looking for red/far red sensitive lettuce seeds (Mark Spiro)
----------------------------------------------------------------------
Message: 1
Date: Fri, 4 Apr 2008 12:52:37 -0400 (EDT)
From: "Mark Spiro" <spiro from bucknell.edu>
Subject: RE: [Plant-education] looking for red/far red sensitive
lettuce seeds
To: "'Gini Berg'" <giniberg from cfu.net>,
<plant-ed from magpie.bio.indiana.edu>
Message-ID: <009701c89674$4978ded0$063b5286 from bucknell.edu>
Content-Type: text/plain; charset="us-ascii"
Wards sells Grand Rapids seeds that work well. However, you have to
measure germination within a couple of days of imbibition to see a
difference in the germination rate of light and dark grown seeds.
After a few days the seeds germinate under both light and dark conditions.
(Far-red still does prevent germination for several days).
It is best to run a test experiment ahead of time to find the optimal time
period.
Finally, you have to make sure that the seeds have not had any exposure to
the cold since this will eliminate the light requirement.
Mark D. Spiro
Associate Professor
Associate Chair
Department of Biology
Bucknell University
Lewisburg, PA 17837
spiro from bucknell.edu
phone: 570-577-3486
-----Original Message-----
From: plant-ed-bounces from oat.bio.indiana.edu
[mailto:plant-ed-bounces from oat.bio.indiana.edu] On Behalf Of Gini Berg
Sent: Thursday, April 03, 2008 5:18 PM
To: plant-ed from magpie.bio.indiana.edu
Subject: [Plant-education] looking for red/far red sensitive lettuce seeds
Does anybody have a source for (or a big pile of) red/far red sensitive
lettuce seeds that I could use for a classic phytochrome teaching
experiment? Mine are old, and now either all germinate (regardless of
light) or fail to germinate (ditto). I've tried again from the same
source, but they all germinate now.
Thanks.
--Gini Berg
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