The response of leaf respiration to water stress in Nothofagus species

Nothofagus solandri is regarded as more tolerant to drought than Nothofagus menziesii in the field. However, the physiology of responses to water limitation in these species is not well understood. In this study, the thermal sensitivity of leaf respiration and its underlying metabolism in response to drought were investigated in mature trees and saplings. Respiration (R-d) and photosynthesis (A(max)) were measured during drying and re-wetting cycles. In addition, respiratory pathway changes were evaluated by oxygen isotope fractionation and protein analyses. Under drought treatment in the glasshouse, both species showed similar photosynthetic performance, but under mild water stress N. solandri was able to increase A(max). Under moderate water deficit (around 2 MPa), N. solandri increased respiration at a base temperature of 10 degrees C (R-10) but then decreased it to initial values after re-watering. In N. menziesii, R-10 did not respond significantly to water-stress treatment. The temperature sensitivity of R-d (Q(10) and E-o) was unchanged for both species during the gradual deficit water treatment in the glasshouse. Although respiratory electron flow was mainly via the cytochrome pathway under all conditions, an increase in alternative oxidase/cytochrome oxidase protein content suggests that the alternative pathway is involved in modulating respiratory metabolism during the recovery after drought.