The gas used to ripen green picked fruit is Ethylene. Bananas come off the boat bright green and are warehoused where this gas is induced as needed to speed up the ripening process. I have seen it used to ripen lemons also.
True. Same gas used for Carbide lamps.
You can buy it at most farmer Coop stores in pellet form. Just add water.
Of course that is a lot of effort and expense for a few plums.
That said, it is the gas given off by ripe fruit so which is simpler to do?
One can reverse my advice and remember to never put ripe bananas or apples and tomatoes together unless you want the all those 'maters red at once.
Here's another tip, I grow pineapples in my personal green house. To get one to bloom, I use ripe apples and bananas to induce a plant to bloom. The gas will also do it. But since I eat a lot of fruit, the source is always available.