Hydraulic conductance, resistance, and resilience: how leaves of a tropical epiphyte respond to drought

PREMISE: Because of its broad range in the neotropical rainforest and within tree canopies, the tank bromeliad Guzmania monostachia was investigated as a model of how varying leaf hydraulic conductance (K (leaf)) could help plants resist and recover from episodic drought. The two pathways of K (leaf), inside and outside the xylem, were also examined to determine the sites and causes of major hydraulic resistances within the leaf. METHODS: We measured leaf hydraulic conductance for plants in the field and laboratory under wet, dry, and rewetted conditions and applied physiological, anatomical, and gene expression analysis with modeling to investigate changes in K (leaf). RESULTS: After 7 d with no rain in the field or 14 days with no water in the glasshouse, K (leaf) decreased by 50% yet increased to hydrated values within 4 d of tank refilling. Staining to detect embolism combined with modeling indicated that changes outside the xylem were of greater importance to K (leaf) than were changes inside the xylem and were associated with changes in intercellular air spaces (aerenchyma), aquaporin expression and inhibition, and cuticular conductance. CONCLUSIONS: Low values for all conductances during drying, particularly in pathways outside the xylem, lead to hydraulic resilience for this species and may also contribute to its broad environmental tolerances.