Bundle-sheath leakiness and intrinsic water use efficiency of a perennial C(4) grass are increased at high vapour pressure deficit during growth

Bundle-sheath leakiness (ϕ) is a key parameter of the CO(2)-concentrating mechanism of C(4) 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 CO(2) 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 CO(2) concentration (C(i)), increasing with C(i). Across treatments, ϕ was negatively correlated to the ratio of CO(2) 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 C(3) and C(4) 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.