Agree with Dale and JD that exposing them to apples or other fruits that give off greater amounts of ethylene.
Found this list on line:
Ethylene producing foods
apples, apricots, avocados, bananas (ripe), blueberries, cantaloupe, cherimoyas, cranberries, figs, green onions, guavas, grapes, honeydew, kiwifruit, mangoes, mangosteen, nectarines, papayas, passion fruit, peaches, pears, persimmons, plums, potatoes, prunes, quince, tomatoes
Ethylene sensitive foods
asparagus, bananas (unripe), blackberries, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, chard, cucumbers, eggplant, endive, garlic, green beans, kale, leafy greens, leeks, lettuce, okra, onions, parsley, peas, peppers, raspberries, spinach, squash, strawberries, sweet potatoes, watercress, watermelon
Tomatoes do produce ethylene, but apples and the like definitely produce more. A funny experience of mine was storing and orchards worth of apples with about 200 pounds of potatoes in the root cellar - they set each other off in a big way. Went down about a month later and half of the apples were gone over and the same for the potatoes. That was a mess to clean up.