Some seeds contain compounds that enforce dormancy until there is enough water in the soil to leach the inhibitors out of the seed. The concentration of such inhibitors diffusing out of seeds or roots may be great enough to prevent germination if seeds are sown too close together or too close to other plants.

But these same inhibitors, reduced to sufficiently low concentration, may stimulate germination. Many of these inhibitors are lactones; parasorbic acid and coumarin are two examples. They apparently prevent germination by inactivating certain enzymes necessary for elongation of the radicle.

LIGHT-SENSITIVE compounds are studied in the laboratory with a spectrophotometer.

Spectroscopic studies have the advantage that they non-destructively follow chemical changes occurring in intact seeds. cytochrome oxidase and cytochrome c, two light-absorbing enzymes associated with respiration, can be measured in seeds by this technique. These enzymes are oxidized as the seed imbibes water. The higher oxidation state is related to the increase in respiration that occurs during the uptake of water. Spectrometry has also shown that the essential yellow carotene pigments are formed in the cotyledons of certain legumes- before the radicle emerges.

The growth of many seeds and plants is affected by light in the red part of the spectrum. The pigment that mediates these effects was extracted from seeds and in seedlings in 1959 by a group of Department scientists Sterling B. Hendricks, H. W. Siegelman, K. H. Norris, and W. L. Butler.

It is a soluble protein present in cells in very low concentrations. This pigment acts as an enzyme in some reaction so basic to plant development that it controls germination of certain seeds and several other phenomena of the growth and development in plants.

The pigment exists in two forms. One absorbs red light with an absorption maximum at wavelength of 660 My (millimicron). The other absorbs far-red light at a maximum of 730 m .

When the pigment absorbs light in one form, it is converted to the other chemical form.

660 m P660<--------------

--------------->P730 730 m Many seeds are stimulated to germinate by light.

Red light at a wavelength of 66o m is most effective in promoting germination of lettuce seed. Far-red light, 730 m , inhibits the stimulating effects of red light. The pigment controls the germination of such light-sensitive seeds by its response to light. Red light puts the pigment in the far-red absorbing form (P730). This change permits germination to proceed. If the red light is followed by far-red light, the pigment is returned to its red-absorbing form (P660), and germination is inhibited. Work with young seedlings and seeds has shown that in the dark the pigment exists in the P660 form. For this reason, germination of those seeds does not occur without light.

The control of germination by the photoresponsive pigment is an example of a cellular mechanism that keeps the seed dormant until conditions for survival are favorable.

A light-requiring seed buried deeply in the soil will not germinate until it is uncovered enough to allow light to reach it. It need not be completely bare, however, because only very low light energy is required.

Knowledge of the role of this pigment in maintenance of the living seed explains many seemingly unrelated observations.

For example, seeds of birch will not germinate on the forest floor beneath a tree canopy, but they will germinate in an opening that receives direct sunlight. We now recognize a likely reason. Probably the red wavelengths are absorbed out of light filtered through green foliage while far-red, inhibiting light is transmitted.

WE HAVE SEEN that keeping seeds alive requires the consideration of many important physical and chemical factors.

Details of the changes of simple organic constituents and respiratory processes of quiescent seeds have long been known. Information about complex organic compounds and seed hormones is just unfolding.

Other information and an understanding of facts already known must be sought.

We know enough about the manipulation of storage environments to minimize undesirable changes in most seeds for one or several years. Yet many so-called short-lived seeds do not retain their viability even under the best known procedures.

Perhaps the new research techniques and hypotheses will provide better ways to lengthen the lifespan and increase the germinative ability of the living seed.

R. G. STANLEY is a biochemist at the Institute of Forest Genetics of the Pacific Southwest Forest and Range Experiment Station of the Forest Service, Berkeley, Calif. Dr. Stanley is a graduate of Michigan State University and the University of California.

W. L. BUTLER is a biophysicist associated with the Market Quality Research Division of the Agricultural Marketing Service at the Plant Industry Station, Beltsville, Md. He is a graduate of Reed College and the University of Chicago.