AT THIS time of the year, itinerant stores open almost everywhere. The fruit and vegetable trucks, those versatile stores on wheels, bring the culture of the country onto the tables of the city. And the city swarms to greet them. Metrobuses come to a screeching halt as drivers dash out for a basket of tomatoes. Horns honk and people bark the inevitable question, "Got any berries?"

As no two tomatoes are the same, no two trucks have identical merchandise. Some sell only what they grow, others supplement their output with wholesale purchases, and some truckers grow nothing themselves but buy strictly from the markets. Although price may be more erratic than the supply, market research surveys indicate that customers cone first for quality, then atmosphere and finally price.

Atmosphere is seldom more than a truck with some shelves, but the people who staff the stands are the real attention-getters, often carrying on nonstop conversations with each customer. Buyers, in turn, ask questions about vegetable preparation or fruit storage, and many who have settled on a favorite truck find it difficult to determine if they return because of the produce or the person.

For many of the small growers, it is less time consuming to sell at a farmers' market. To meet the demand, such group efforts are cropping up everywhere; any day but Sunday and Monday you can find fresh pickings, and by next year even those two exceptions may change.

Most of the programs are similar to Silver Spring's, which allows only approved, licensed sellers. Bill Polen, a number of the Montgomery County Agricultural Committee's executive board, explained that all produce must be locally grown and freshly picked; he has rejected watermelon when it was not yet available locally, and truckfuls of sublevel produce. At the end of the day, remainders must be thrown out or donated to charity.

"Prices have stayed the same for the last 10 years," Polen stated proudly. To enable customers to buy a single onion or two tomatoes and three potatoes, most of the stands sell items at 3 pounds for a dollar so customers can mix and match. Even so, farmers have reported selling as many as 1,500 ears of corn by 11 a.m.

Recognized as a stalwart of city stands is the Saturday market at 18th Street and Columbia Road in Adams Morgan. Four and five vendors arrive weekly, and two are major city suppliers.

For six years, Norm Hunter has been traveling from his Sleepy Creek Farm in Berkeley Springs, W. Va. He grows only organic vegetables and supplements his stand with products from neighbors and a small percentage from the city market. His helpers are young people who have moved onto the farm for a summer of living and helping.

The stand is beautifully arranged with an eye for the esthetic; little paper bag signs declare price and whether the item is organically grown. At the end of the day, remainders are sold to restaurants, wholesalers or co-ops.

Jim and Millie Crawford lead a similarly busy country-to-city existence. For nine years, they have been selling at Columbia Road the organic vegetables from their New Mornng Farm in Houstontown, Pa. They grow only vegetables, buying their fruit from neigbors. Like the other vendors, they have scales so that they can sell by the pound.

In the District, vendors spread their wares under umbrellas on the sidewalk, but in Montgomery County, vendors work out of the truck because they are not allowed to put anything on the sidewalk. Most have erected signs signaling their lead items, and display them from specially constructed shelves.

Whether selling from stands or trucks, produce vendors agree: This is no glamor business. Even when you can hide under the shade of the tree, there are not enough hours for sleep during the season.

The Zilvettis have found a nice shady spot inside the District at Massachusetts Avenue before Spring Valley. From late May until the middle of September they pull out their white tablecloths and display a garden variety of everything seasonal.

Some of their produce is grown on the family farm, other items come from neighbors, and the wholesaler fills in the gaps. Being sensitive to the customers' needs is an important aspect of the stands. Customers are delighted by the possiblities: One woman wanted soft tomatoes for sauce. Another woman came to pick up the green tomatoes she had ordered. This is a vegetable corner: Customers want zucchini, squash and beans.

Not every site is so clearly defined. People stop at Tony's (MacArthur Boulevard and Wilson Lane) for a little of everything. The truck's shelves hold the Italian bread, "Good bakers. Nice kids. Help them out." Large rounds of Amish cheeses are in a cooler. Preserves and cider sit out. There is a little bit of this and a lot of vegetables. "No, buy the yellow today. It's sweeter," advises the proprietor. Then, as an afterthought, he reminds the customers to use just a little water to cook corn (1/2 cup of water for six ears), cover the pot and cook 4 minutes.

Grandma's driveway and lawn is the scene of a business that is half in the District and half in Maryland, at 48th and Western Avenue. Bill Polen Jr., 24, put out a few tomatoes one day when he was 10, and they were literally in a salad before his father could inquire if any customers had stopped. Now, all that has changed.

The parents buy the seed and the fertilizer for the family farm in Gaithersburg, and the three children do the rest. Every September they pay the parents back, then divide up the rest. Last year each child received $600 for his tuition. "This is a nice change from the library," Bill, a medical student, commented.

Fewere stands line the streets in Prince George's County, but there was a lot of evidence that many farmers are just waiting until the popular Silver Queen corn is in season.

Meanwhil, in Virgnia there is the greatest diversity: roadside stands that look more permanent than seasonal, unmanned tables that have honor boxes. One of the largest seasonal markets in Northern Virginia is the Cox Farm stand. The family farm i in Centreville, and the Vienna store is a sign of progress. Looking at the professional operation, it is hard to remember that up to five years ago it did even have a cash register. Now it has computerized scale/registers.

Most of the Coxes' products are from the farm and from nearby local farmers. Eastern Shore growers supply the remainder of the stock. There is no shortage of sweet corn, with 60 acres reserved for its growth. It is picked daily, and the unsold items are $2/bushel the next day.

The parking lot is a major problem, so customers are given a 10 percent reduction if they walk instead of drive to the stand. Senior citizens also are given a similar reduction, with walking seniors receiving a significant 20 percent savings.

Hall Kern is not a grower, but a supporter, being the owner and manager of the Reston Farm Market, which sells items that are home-grown or homemade, local, unique or what they consider the best.

If you think your recipe for sweet watermelon rind pickles is superior to what is in stock, then bring it in for the taste contest. The winners -- and the stock of the market -- range from Kansas City's Gates barbecue sauce to Bob's Famous Homemade Ice Cream. Farmers walk in with produce that is still hot from the sun, products so fresh you want to buy the whole bushel.

Whatever fruit is in season Kern turns into soda (crush the fruit, cook it, strain the pulp, add honey, well water and carbonate) at $1 a bottle. Of course, boxes of ripe berries and tables of peaches are available for the traditionalists.

The Market is open from April 1 until Dec. 31, and to beat the winter chills it is furnished with three Franklin stoves.

Most stands operate just during the short growing season. Moutoux Orchards since 1968 has sold 25 different varieties of peaches, all Hale derivatives that are hand-picked when firm-ripe. Too fragile for shipping, the peaches are about a day or two from being soft and sweet. In addition to the peaches, it has a complete supply of vegatables and, in the fall, apples and pears.

Also operating during the short growing season is the Hunter Mill Farm. The Cockrill family's children began with a roadside stand, developed it into an honor box system and now have a large sheltered barn that is open from mid-June the end of October.

They pick daily to stock the stand, and corn is added hourly as it comes from the field. Produce that is not sold by day's end is fed to the cattle and sheep or used for canning. Already this year, Mrs. Cockrill has put up 120 quarts of green beans (blanch 3 minutes, cool, bag, freeze, warm 1 minute in boiling water). One of the farm's most popular items is broccoli. By mid-July more than 200 bushels had been sold.