Front Range Seed Analysts
1993 Seed Forum Volume 7 Number 1
Mystery: Necrosis Noted on Milkvetch
by Annette Logan
Ever come across an abnormal seedling that is so peculiar, the mystery of its cause grips you and just won't let go? The following is a story of one analyst's search for truth in germination testing.
The story begins with a routine germination test of four samples of
Astragalus cicer (milkvetch) . Two of the BampleB germinated wonderfully
with a minimum of hard seeds left over at the end. About midway through
evaluating the third sample I took a closer look at the abnormals and began
to see a strange pattern of necrosis on the cotyledons. I recorded the
following observations:
1. slightly affected seedlings: green cotyledons with sunken reddish
veins and otherwise normal seedlings.
2. moderately affected seedlings: cotyledons more yellowed with the
reddish veins more prominent. Cotyledonary tissue texture is firm as in
a normal seedling. Seedling may take longer to shed the seed coat. Seedling
development may be delayed or permanantly impaired.
3. severe symptoms: swollen seeds never develop. orange-red cotyledons
with dark red, sunken veins are seen when the seed coat is removed.
Reactions around the lab went something like this: "Wow!, I've never
seen anything like this", "it's definately a fungus", "it's definately
not a fungus", "it's a nutrient deficiency", "it's genetic", "it's definately
environmental" .... Thus, my search for truth was sparked.
First, we ruled out a fungus because of the firm texture of the affected
cotyledons and because we thought the seedlings would have been overcome
by disease (they weren't) by the end of the two week test. The symptoms
did make me think of a nutrient deficiency like the hypocotyl collar rot
on beans which is caused by a calcium deficiency. So I tried a retest with
a .6% calcium nitrate solution.
Meanwhile, I decided to do some digging. If the problem was environmental,
how come two of the four Bamples were unaffected? After a conversation
with the donor, checking computer records, and some searching for misfiled
papers, I heard what I wanted to hear: the samples were from different
crop years. But the discussion turned up a new question. Allegedly, the
worst sample came from an unusually wet season. The donor hypothesized
that the damage occurred during development before the hard seed coat was
fully formed. I wondered, how can wet weather cause this type of necrosis?
Back in the germinator, my retest was showing no reaction to the calcium
nitrate. I thought of other kinds of necrosis. It wasn't caused by mechanical
damage because the pattern of sunken veins was mirrored on both cotyledons
and because there was not any evidence of bruising or broken radicles or
cotyledons. I thought of necrosis on lettuce seedlings. The color was the
same. But the pattern was following the veins instead of the spotted, spread
out pattern seen on lettuce. Still, one could say that the morphological
differences between lettuce cotyledons and the milkvetch cotyledons could
cause the pattern difference. Now I was getting warm.
Coworkers shared lettuce necrosis literature, and we came up with a
guess. If this milkvetch necrosis is like the necrosis seen in lettuce
then a possible cause could be aging of seeds predisposed (by growing conditions
and genetics) to this susceptibility. We guessed that the freshly harvested
seed of these samples would not show any necrosis and that these varieties
might be predisposed to development of necrosis after aging if they had
been stressed during the growing Beason. Unfortunately, the seed was 10-13
years old and there was no record of the germination at harvest time.
The donor had performed a germination test just a few months before
the seed was sent to the lab. His recorded germination percentages for
all the samples were in the 90's while mine were in the 60's and 70's.
What caused the difference between our results? The donor was doing a quick
radicle emergence test and never saw the cotyledons. When alerted to the
necrosis symptoms on the milkvetch samples, the donor had a theory that
the darker, honey colored seeds were the culprits. Upon cleaning and retesting,
he reported that most of the darker "weathered" seeds were necrotic and
most of the brighter yellow seeds were normal. My own retests are still
in progress.
I have a hunch that there is some truth in each of the guesses we put forth at the beginning of the study. Environment, genetics, aging, and deficiencies, all may play a part in the expression of this abnormality. If anyone out there has more information about this symptom in milkvetch, I would like to hear about it. We are documenting the symptom and the correlation with seed color in the imaging laboratory at the NSSL. I didn't find the whole truth, but the lesson here is that many heads are better than one when it comes to germination puzzles.
(for more information about lettuce necrosis and aging see: Bass, L. J. Amer. Soc. Hort. Sci. 95(5): 550-553 1970)