When Robert Devlin addressed a group of Massachusetts cranberry growers a couple of years ago, he carried along two plump, crimson crab apples. Like a comedian with good timing, the plant physiologist held the apples aloft and proclaimed he had just found a new chemical that would produce jumbo-sized cranberries. ''Oh, they were excited,'' he recalls of the incident. ''They thought I was growing cranberries as big as crab apples.''
Dr. Devlin, who works out of the University of Massachusetts cranberry experiment station here, hasn't yet found the right brew to produce the Brobding-nagian berries. But he is closing in on finding chemicals that, among other things, will yield more fruit per vine (less than 25 percent of cranberry flowers now produce fruit).
Dr. Devlin is one of a growing number of scientists working in an emerging frontier of agriculture that could have a big impact on crop yields, and perhaps world food production, by the end of the century. Their goal: to find new chemicals that will ''stimulate'' the growth of a wide range of crops, from wheat to snap beans.
At the same time, however, the rise of the new chemistry is bound to stir debate in farming and environmental circles. These ''plant growth regulators'' are generally considered more benign than pesticides, but the danger of physiological effects from the chemicals exists nonetheless. Mounting concern about chemicals in the environment in general will ensure close monitoring of any developments.
Likely to be debated, too, is whether the emerging field will result in a greater dependence by farmers on the use of chemicals - and whether that will put the new technology beyond the financial grasp of third-world countries and even some farmers here.
Growth regulators are different than the familiar insecticides and herbicides now used. With pesticides, fields are sprayed to wipe out the weeds and bugs around the plants - in effect, boosting crop yields by destroying the competition. But chemical regulators work internally in the plant to control growth.
Regulators have long been used in some specialty crops. Orange trees, for instance, are routinely sprayed with regulators to loosen fruit and make it easier for mechanical harvesting. Chemical inhibitors are used to increase cotton yields by suppressing growth of excess leaves, producing more fluffy balls on the plants.
But now scientists are trying to tap their knowledge of chemistry to boost production in major food crops. Most of the world's big agri-chemical companies, as well as US government and university researchers, are involved in the chemical hunt. ''Most growth regulators in the past were used to affect plant shape, size, and 'fruit drop,' '' says Brian Young, an agricultural specialist at Battelle Columbus Laboratories. ''Now they're finding regulators that have a definite effect on yields. That's a whole new ball game.''
Monsanto Company is trying to come up with brews that will increase the yields of corn, wheat, and soybeans by 10 to 15 percent - a breakthrough that has eluded everyone so far. It is also working on new sugar-cane ripeners. The company says that some of its ripeners used in Hawaii are increasing sugar yields 10 percent.
The big St. Louis firm isn't the only one in the derby, though. Union Carbide , among others, is racing to find chemicals that will enhance the development of corn, barley, soybeans, and potatoes. By coming up with growth-stunting sprays, Carbide researchers also hope to produce shorter, sturdier stalks of wheat. Such chemicals have long been used in Europe.
At Texas A&M University, scientists recently were able to double the production of rubber from guayule, a desert plant, in the laboratory by dousing it with a chemical stimulant. Other areas where a new generation of chemicals may be put to work:
* Making plants more disease and freeze resistant.
* Producing salt-tolerant crops for use in briny coastal regions.
* Developing plants for desert agriculture.
* Coming up with more effective ''chemical lawn mowers'' - solutions that stunt the growth of grass.
* Increasing a plant's uptake of nutrients from the soil so fewer costly fertilizers have to be used.
Yet one point seems clear: None of this new alchemy will lead to any quick changes on the farm. For one thing, the technology is tough to crack. Indeed, Dr. John Marvel, manager of research for Monsanto's agricultural products division, contends that the search for new regulators is ''10 to 100 times'' more difficult than the development of other new chemicals, such as disease-control agents.
For the companies, the payoffs could be big - but so are the risks. Bringing a new chemical from the idea shop to the marketplace costs anywhere from $40 million to $100 million. On the other hand, the market for growth regulators, now about $255 million a year worldwide, should top $500 million by 1990, according to Monsanto. By the mid-1990s, Battelle Labs estimates it will be a $2 billion-a-year business.
The cash registers, in other words, won't be ringing for some time to come. In the meantime, advances in two other rapidly developing agricultural technologies - gene splicing and traditional plant breeding - may steal some of the new chemistry's thunder.
Nevertheless, asserts Dr. Devlin, ''plant growth regulators are going to be an explosive area in the future.''