Persistent drought severely inhibits plant growth and productivity, which negatively affects terrestrial primary productivity worldwide. Therefore, it is important to investigate the impacts of drought on plant leaf CO
2/H
2O exchange and water use efficiency. This study assessed the responses
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Persistent drought severely inhibits plant growth and productivity, which negatively affects terrestrial primary productivity worldwide. Therefore, it is important to investigate the impacts of drought on plant leaf CO
2/H
2O exchange and water use efficiency. This study assessed the responses of net photosynthesis (
Pn), stomatal conductance (
Gs), transpiration (
Tr), and instantaneous water use efficiency (WUE) to drought based on a worldwide meta-analysis of 112 published studies. The results demonstrated that drought decreased
Pn,
Tr, and
Gs significantly and differently among different moderators. C
4 plants had smaller
Pn reduction than C
3 plants, which gives C
4 plants an advantage in
Pn. But their WUE decreased under drought conditions, indicating a great flexibility in C
4 WUE. Annual herbs sacrificed WUE (−6.2%) to maintain efficient
Pn. Perennial herbs took a different strategy in response to drought with an increased WUE (25.1%). Deciduous tree species displayed a greater increase in WUE than conifers and evergreen species. Additionally,
Gs had a significant correlation with
Pn and
Tr, but an insignificant correlation with WUE, which could be because WUE is affected by other factors (e.g., air flow, CO
2 concentration, and relative humidity). These findings have significant implications for understanding the worldwide effects of drought on plant leaf CO
2/H
2O exchange and water use efficiency.
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