How did Joseph Priestley’s observations of plants contribute to research on photosynthesis?

Joseph Priestley’s observations were foundational to the study of photosynthesis. In 1771, he demonstrated that plants possess the ability to restore "common air" that had been rendered incapable of supporting combustion or animal life through processes like burning or respiration.123

Priestley’s key experiment involved placing a sprig of mint into a closed container where a candle had previously burned until it could no longer support a flame. He discovered that after several days, the plant had produced a substance—later identified as oxygen—that allowed the air to again support combustion.13

These findings provided the essential evidence that nature possesses a mechanism for purifying the atmosphere, which directly informed the subsequent research of scientists such as Jan Ingenhousz and Jean Senebier. Ingenhousz later expanded on Priestley’s work by proving that this restoration of air only occurs when plants are exposed to light and that it is performed specifically by the plant's green tissues.2314