To sustain carbon capture via photosynthesis plants must maintain leaf hydraulic conductance and leaf cell turgor. If plants lose leaf hydraulic conductance, stomata must close to prevent desiccation and if turgor is lost, plant cells may undergo plasmolysis and no longer be functional. Although stomata are the primary regulator of leaf water status, the ability to adjust cell turgor and leaf hydraulic parameters could also ensure that leaves continue to photosynthesize under progressively dryer conditions. It has been proposed that species that experience more negative leaf water potentials (anisohydric) may be more able to adjust leaf hydraulic parameters than those that experience less negative leaf water potentials (isohydric). To test this, we measured leaf water potentials, pressure-volume parameters, hydraulic conductance and vulnerability to hydraulic dysfunction in leaves of four co-occurring species in central Texas: Diospyros texana, Juniperus asheii, Prosopis glandulosa and Quercus fusiformis. Quercus and Prosopis were previously identified as strongly isohydric, whereas Diospyros and Juniperus were identified as anisohydric.
Diospyros and Juniperus experienced more negative predawn and midday water potentials than Quercus or Prosopis. Turgor loss points (TLP) and osmotic potentials at full turgor became more negative in anisohydric Diospyros and Juniperus during the driest part of the season, with Diospyros having the greatest changes (TLP changed by -1.5 MPa). Leaves of Diospyros and Juniperus were more resistant to hydraulic dysfunction during the driest part of the season (as compared to wetter parts of the season) but this effect was much greater in Diospyros. In isohydric Quercus, resistance to leaf hydraulic dysfunction increased throughout the season but did not appear to be related to available soil moisture. All species except Quercus experienced midday water potentials that were more negative than their TLP, suggesting midday impairment in cellular processes in these species. Species that are more anisohydric may be more able, in general, to adjust leaf-level hydraulic parameters than more isohydric species. This would allow anisohydric species to continue to photosynthesize as leaf water potentials become more negative. This adjustability could be especially important for species growing in arid or semi-arid habitats where both near-zero and highly negative leaf water potentials are experienced during a single growing season.