The study looked at how the amount of water in the soil affects the way tree peony plants do photosynthesis. They used potted tree peony plants and found that as the soil got drier, the plants' photosynthesis slowed down. This happened because the plants couldn't take in as much carbon dioxide or use as much light. When the soil was at 55% to 40% water, the main reason for the slowdown was because the plant's stomata were closing up. But when the soil was at 20% water, the slowdown was because of other factors. The best amount of water in the soil for the plants to do photosynthesis well was 70%.