Low temperatures can harm watermelon seedlings' ability to photosynthesize. The colder it gets, the worse the damage. Stomata, tiny openings on leaves, play a big role in this. In moderate cold, stomata close up, reducing photosynthesis. But in severe cold, other factors cause the drop in photosynthesis. Watermelon plants try to protect themselves by changing their chlorophyll levels. Under severe cold, their protective mechanisms fail, causing damage to their photosynthesis.