Understanding the coordination of hydraulic strategies with other traits in determining plant survival under drought

Drought negatively impacts plant growth and survival. The ability to maintain hydraulic functionality during water stress strongly influences whether plants will survive and recover from drought. Although our understanding of the mechanisms underlying drought-induced mortality have improved in recent decades, our understanding of the coordination between stomatal and hydraulic traits and their role in shaping drought resistance and enabling recovery remains poorly understood. In this thesis, I examined plant hydraulic traits across a range of contrasting species in order to better understand how hydraulics determines plant function under drought, governs gas exchange, and drives differences in drought resistance. By subjecting three contrasting Australian tree species to water limitation, we were able to determine the hydraulic vulnerability to embolism of leaves, stems and roots as well as the relationship between stomatal conductance and photosynthesis with decreasing water potential. We found that leaves and/or roots were more vulnerable than stems in Eucalyptus coolabah and Acacia aneura, however E. populnea did not show vulnerability segmentation. Additionally, in these species stomatal closure always occurred prior to significant hydraulic dysfunction. We confirmed this finding in three additional tree species, Arbutus unedo, Ligustrum japonicum and Prunus persica via direct imaging of leaf embolism formation by the Optical Visualisation (OV) method with simultaneous measurements of stomatal conductance on intact plants. Prior drought exposure resulted in higher rates of photosynthesis in E. coolabah and E. melliodora under subsequent water stress in comparison with plants that had not previously experienced drought, due to differences in stomatal regulation that enabled stomata to remain open for longer at lower potentials. Plants that had previously experienced drought also reduced leaf size and had lower overall biomass indicating that these species can acclimate to recurrent drought. In combination, our results strongly suggest that the ability to resist hydraulic dysfunction, rather than recover from xylem embolism underpins plant resilience to drought, with early stomatal regulation to prevent water loss and delay catastrophic xylem dysfunction crucial to plant survival and recovery