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 (phi) is a key parameter of the CO2-concentrating mechanism of C-4 photosynthesis and is related to leaf-level intrinsic water use efficiency (WUEi). This work studied short-term dynamic responses of phi 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. phi 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 phi by 0.13 and a concurrent increase of WUEi by 14%, with similar effects at both N levels. phi responded dynamically to intercellular CO2 concentration (C-i), increasing with C-i. Across treatments, phi 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 phi was related to the balance between C-3 and C-4 cycles. Our study revealed considerable dynamic and long-term variation in phi of C. squarrosa, suggesting that phi should be determined when carbon isotope discrimination is used to assess WUEi. Also, the data indicate a trade-off between WUEi and energetic efficiency in C. squarrosa.