FOR SOME YEARS now, geneticists and agricultural planners have worried about the worldwide loss of genetic diversity among plant crops. With inflation, recession and an energy crisis upon us, that doesn't exactly sound like a heart stopper. But in the long range, it holds the potential for true biological disaster and, on a considerably shorter time scale, could seriously inhibit efforts of the developing nations to break out of the cycle of poverty.

As a consequence of the Green Revolution and of modern methods of high-yield agriculture, farmers around the world are planting fewer and fewer varieties of crops. The phenomenon is furthest along in the developed countries. In the United States, for instance, just six varieties account for 71 percent of the acreage planted for corn, seeds and silage; two for 42 percent of sugar beets; and three for all the acreage in millet. The same thing is going on in the Third World, with much more dangerous consequences because these countries include the regions of greatest genetric diversity from which plant breeders must draw the resources with which they work.

Maintaining genetic diversity is vital for two reasons. The first is the vulnerability of single varieties to pests, disease and climate change. When huge acreages are planted with a single variety or a few closely related varieties, entire harvests can be wiped out by one disease or by the temporary resurgence of an insect pest. Similarly, the harvest becomes terribly sensitive to any change in climate. At a time when the massive use of insecticides in constantly upsetting predator-prey relationships and changing the mix of insect populations, and when increased fossil-fuel consumption is thought to be leading to worldwide climate change, the planting of fewer and fewer varieties is particularly ominous.

Second, there is the need to preserve the genetic resources from which plant breeders can create new strains tailored to changing environments. Only from the large genetic pool that has evolved over eons can new plant characteristics -- of disease and pest resistance, length of growing season, water and fertilizer requirements, soil and sun needs, protein content and so on -- be drawn. As one U.N. official put it, "When farmers clear a field of primitive grain varieties, they throw away the key to our future."

What makes this trend even more worrisome is the growing concentration of the world's seed resources in the private holdings of multinational corporations. It has been reported, for example, that a single company, United Brands, holds two-thirds of the world's banana seeds. Access to and information about the contents of such corporate seed banks is generally limited to employees of the particular company. Plant breeders in Third World nations are thereby excluded from access to vital resources that came from their own countries to begin with.

Corporate control over plant resources is a growing domestic concern also. Seed companies owned by conglomerates like ITT and Union Carbide are lobbying Congress to extend patent protection to many types of vegetables. The companies argue that such protection is needed to provide the money for research on new breeds. But scientists fear that patent protection will inevitably lead to the disapperance of many, if not most, existing varieties.

A recent international study recommends a U.N. emergency fund to collect and maintain genetic reserves in a number of regional banks (a single bank is too vulnerable to biologic accident). There are suggestions that no laws allowing exclusive rights to plants be allowed. Others are trying to think up ways to preserve diversity in farmers' plantings. Whatever the right answers may turn out to be, at least one conclusion is clear -- current agricultural practices bear the seeds of future trouble.