What is one potential reason for why a plant may experience diminished photosynthetic rates?

When the temperature of the environment increases, stomata close and carbon dioxide is not delivered to mesophyll cells.

Explanation:

Reasons of diminished photosynthesis rate:

In the absence of enough light, a plant cannot photosynthesis very quickly, even if there is plenty of water and carbon dioxide and a suitable temperature. Increasing the light intensity increases the rate of photosynthesis , until some other factor - a limiting factor - becomes in short supply.

At the same time, light-dependent reactions of photosynthesis proceed and oxygen is produced. Thus, the ratio between oxygen and carbon dioxide increases. As a result, glucose production does not take place and the rate of photosynthesis decreases.

Some studies shows that aquatic plants notice that flooding often leads to a decrease in the rate of photosynthesis among aquatic plants. Flooding causes more sediment to be suspended in the water. The increased sediment concentration decreases the amount of light that reaches the plants.

sorry, but you do not have posted the diagram.

i think that is new experiments yield untrustworthy data and should be carefully considered, because you don't know what will happen with the experiments.