Plant size directly correlates with water use efficiency in Arabidopsis. The researchers studied how different factors affect plant transpiration and water use efficiency in Arabidopsis thaliana. They found that bigger plants are more efficient in using water. Despite variations in stomatal density and ABA content, water use efficiency was directly linked to plant size. Mutations in certain genes affecting water use efficiency also led to smaller plant size. This suggests that plant size is an important trait for efficient water use in Arabidopsis.