Ozone changes the linear relationship between photosynthesis and stomatal conductance and decreases water use efficiency in rice.

Ozone is an important air pollutant that affects growth, transpiration, and water use efficiency (WUE) in plants. Integrated models of photosynthesis (A) and stomatal conductance (G) (A-G) are useful tools to consistently assess the impacts of ozone on plant growth, transpiration, and WUE. However, there is no information on how to incorporate the influence of ozone into A-G integrated models for crops. We focused on the Ball-Woodrow-Berry (BWB) relationship, which is a key equation in A-G integrated models, and aimed to address the following questions: (i) how does ozone change the BWB relationship for crops?; (ii) are there any difference in the changes in the BWB relationship among cultivars?, and (iii) how do the changes in the BWB relationship increase or decrease WUE for crops? We grew four rice cultivars in a field under ambient or Free-Air Concentration Enrichment (FACE) of ozone in China and measured A and G using a portable photosynthesis analyzer. We simulated WUE in individual leaves during the ripening period under different BWB relationships. The results showed that ozone significantly changed the BWB relationship only for the most sensitive cultivar, which showed an increase in the intercept of the BWB relationship under FACE conditions. These results imply that changes in the BWB relationship are related to the ozone sensitivity of the cultivar. Simulations of an A-G integrated model showed that increases in the intercept of the BWB relationship from 0.01 to 0.1 mol(HO) m s indicated decreases in WUE by 22%. Since a reduction in WUE indicates increases in water demand per unit of crop growth, air pollution from ozone could be a critical issue in regions where agricultural water is limited, such as in rainfed paddy fields.