Poplar seedlings were studied to see how they respond to different levels of drought stress. The researchers looked at things like water content, photosynthesis, and antioxidant enzyme activity in the leaves. They found that as the drought stress increased, the seedlings had lower water content and photosynthesis rates. The seedlings also showed different responses depending on the severity of the drought stress, with some stress levels affecting the stomata and others affecting the photosynthetic system. Overall, the findings suggest that under light drought stress, the decrease in photosynthesis is mainly due to stomatal limitations, while under moderate and severe stress, it is more related to damage to the photosynthetic system and antioxidant enzymes.