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Capsicum plants (peppers) are essentially always "on". The capsaicin is in them, right there and available. Whether you eat them or not, they're already spicy.

The heat in radishes, horseradish, and wasabi, though, is a different story. Even the most potent wasabi is mild as a parsnip when it's sitting in the ground, minding its own business. They only become spicy when threatened.

They're mild until someone, or something takes a bite, or cuts them with a knife. Or, god forbid, grates them against shark skin.

The chemicals that burn you in them don't exist until their cells are ruptured, triggering an enzyme reaction that creates them. That's when the radishes go to war.