Front Range Seed Analysts
1993 Seed Forum Volume 7 Number 2
Life After Death: Resurrection of Aged Seeds
by Sheila Blackman
Research Associate, National Seed Storage Laboratory
Ever wonder why some seeds germinate while others can't seem to muster what it takes and dwindle in the soil? Germination requires an incredibly complicated series of events at all levels molecules, cells, tissues and organs. Each must occur at a precise time and be completed in correct sequence. Low quality seeds cannot complete this sequence appropriately. But this doesn't mean that the seeds are dead only that they can't survive under the conditions in which they were planted. We don't pronounce someone with diabetes "dead" simply because they have a metabolic problem. If given appropriate supplements, that person not only functions but thrives. If we could figure out the metabolic problem in low quality seeds, supplements might enable them to prosper, as well.
This is not a question born purely of scientific curiosity. Some lots
of corn seed received by the National Seed Storage Laboratory from South
America have completely lost their ability to germinate under field conditions.
This is because they were not stored under the best of conditions. These
are old and valuable stocks
it behooves us to do all we can to save them.
I came to the National Seed Storage Laboratory as a Postdoctoral Research Associate in late January. My contract is to see what can be done to coax some life out of seeds that have been brought to the lab in poor condition. Prior to January, I was a Research Associate at the Boyce Thompson Institute in Ithaca, NY and worked on aspects of soybean seed development. I am a biochemist/cell biologist by training. since arriving at the National Seed Storage Laboratory I've been really impressed by the wisdom of seed analysts who have come by their knowledge through years of careful observation. I would be most appreciative of any suggestions relating to this challenging and important project.
I've decided to approach this first by asking why some seeds won't germinate. What is missing or damaged? one clue might be how much energy the aged seeds can produce. This is because of two things: first, energy is a primary ingredient for germination and second, previous studies have shown that energy production in aged seeds of other species is lower than it is in f resh seeds. So I am beginning my search by comparing the ability of aged and fresh (high vigour) seeds to produce energy.
Autotrophic organisms obtain energy by oxidizing reduced carbon (carbohydrates
or sugars) to carbon dioxide.
The stage when a seed is germinating is the only one in the normal
life cycle of a green plant when it is autotrophic (ie. requires an external
supply of reduced carbon). once the seedling turns green and becomes photosynthetic,
it can obtain the energy it needs from sunlight. Corn seeds have a store
of reduced carbon (starch) in the endosperm that was supplied by the mother
plant during development. The embryo converts this reserve to carbon dioxide
to make energy until it becomes photosynthetic.
Measuring the amount of carbon dioxide that a seed gives off is one
way of monitoring the amount of energy it is making. Some
time ago, I f ound that aged corn seeds give of f less carbon
dioxide than fresh corn seeds do. So I guessed that they must be making
less energy than fresh seed.
There are two common "pathways" that organisms use to make energy. one
is aerobic (requires oxygen) and the other is anaerobic (does not require
oxygen) . Along each pathway, there are different enzymes (these are proteins,
each acting like an assembly line worker to make a product
in this case energy). I was surprised to find that embryos from
fresh seeds can germinate quickly even when they are deprived of oxygen
for the first 8 or so hours of germination. This seems to go against the
current thought that aeration (ie. a good supply of oxygen) is crucial
to successful germination. However, I have not yet tested later stages
of germination and, of course, the response of different species to oxygen
deprivation might also differ. I think that the energy needs of germinating
corn at least during these very early hours of germination
can be met by anaerobic pathways of energy generation. This is one more
clue as to what might have gone awry in aged seeds.
Another clue comes from the fact that the activity of many enzymes is low in aged seeds. In particular, a class of enzymes called "dehydrogenases" is unusually sensitive to aging. This is the basis of the tetrazolium test for seed vigour. It actually measures dehydrogenase activity. It turns out that this class of enzyme is extremely important in pathways of energy generation. So my guess is that the activities of some enzymes in pathways of anaerobic energy generation are lower than they are in fresh seeds.
Once the enzyme(s) that are both vital for germination and incapacitated
by aging are found, it (hopefully) will be a relatively easy matter to
give the seed an appropriate supplement. Ultimately, I will consider this
project a success only when I can "artificially resuscitate" aged seeds
and enable them to regenerate plants.