The study looked at how a type of grass, Stipa bungeana Trin., reacts to drought stress by measuring its photosynthesis and water use efficiency throughout the day. They found that the grass had two peaks of photosynthesis, with the highest rates in the morning and lower rates in the afternoon. Transpiration rates followed a similar pattern, with higher rates when the soil was moister. Stomatal conductance, which controls gas exchange, was influenced by soil moisture and light/heat stress. The grass was most water-efficient in the early morning but became less efficient as soil moisture increased.