[Effects of CO₂ concentration and soil water content on short-term water-use efficiency at whole-plant level]

Uncovering the variations of short-term water-use efficiency (WUE_p) at whole-plant level in response to CO₂ concentration (C_a) and soil water content (SWC) can improve the understanding of plant survival strategies under climate change. In this study, Platycladus orientalis saplings were cultured in simulated climate chambers.There were totally 15 treatments, including C_a of 400 (C₄₀₀), 600 (C₆₀₀) and 800 (C₈₀₀) μmol·mol^-1 and SWC of 35%-45% field water holding capacity (FC), 50%-60%FC, 60%-70%FC, 70%-80%FC and 95%-100%FC. The WUE_p was measured by mini-lysimeters, weighting method, and static assimilation chamber. The results showed that both daytime (0.12-1.87 mol·h^-1) and nighttime transpiration rates (0.01-0.16 mol·h^-1) at whole-plant level reached the maximum at C₄₀₀×70%-80%FC, while the whole-plant daytime net photosynthetic rate (2.12-22.10 mmol·h^-1) reached the maximum at C₈₀₀×70%-80%FC. In contrast, nighttime respiration rate (0.84-4.41 mmol·h^-1) increased with increasing SWC, but decreased with increasing of C_a, reaching the maximum at C₄₀₀×95%-100%FC. For WUE_p (5.37-24.35 mmol·mol^-1), it reached the maximum at C₈₀₀×50%-60%FC, indicating that plants could use less water and fixed more carbon by adjusting adaptation strategies under high C_a and drought conditions. In addition, leaf instantaneous water-use efficiency was a good predictor of WUE_P when the canopy structure was similar.