Bundle-sheath leakiness and intrinsic water use efficiency of a perennial C4 grass are increased at high vapour pressure deficit during growth

Bundle-sheath leakiness (ϕ) is a key parameter of the CO₂-concentrating mechanism of C₄ photosynthesis and is related to leaf-level intrinsic water use efficiency (WUE_i). This work studied short-term dynamic responses of ϕ to alterations of atmospheric CO₂ concentration in Cleistogenes squarrosa, a perennial grass, grown at high (1.6 kPa) or low (0.6 kPa) vapour pressure deficit (VPD) combined with high or low N supply in controlled environment experiments. ϕ was determined by concurrent measurements of photosynthetic gas exchange and on-line carbon isotope discrimination, using a new protocol. Growth at high VPD led to an increase of ϕ by 0.13 and a concurrent increase of WUE_i by 14%, with similar effects at both N levels. ϕ responded dynamically to intercellular CO₂ concentration (C _i), increasing with C _i Across treatments, ϕ was negatively correlated to the ratio of CO₂ saturated assimilation rate to carboxylation efficiency (a proxy of the relative activities of Rubisco and phosphoenolpyruvate carboxylase) indicating that the long-term environmental effect on ϕ was related to the balance between C₃ and C₄ cycles. Our study revealed considerable dynamic and long-term variation in ϕ of C. squarrosa, suggesting that ϕ should be determined when carbon isotope discrimination is used to assess WUE_i Also, the data indicate a trade-off between WUE_i and energetic efficiency in C. squarrosa.