Rapid declines in leaf-level physiological functions occur when foliar water potentials fall below the point at which turgor pressures become zero (turgor loss point) or when relative water contents fall below about 75%. In massive and tall trees with complex architecture, such as giant sequoia (Sequoiadendron giganteum), the mechanisms by which foliage maintains sufficient turgor pressures and water contents against height-related constraints and during drought remain poorly understood. Yet, while the recent 2012-2015 drought in California killed well over 100 million trees in the Sierra Nevada Mountains, the vast majority of Sequoiadendron survived even though many of their smaller neighbors perished. Our objective was to provide mechanistic insights into the components of water potential, including how turgor pressures and water contents change with height and over time. We generated pressure-volume curves on leafy shoots collected crown-wide from 12 large Sequoiadendron trees.
Results/Conclusions
The turgor loss point decreased with height at a rate indistinguishable from the gravitational potential gradient and was controlled primarily by tissue osmotica. Relative water content at the turgor loss point remained above 75% and was controlled primarily by the proportional distribution of water in apoplastic versus symplastic compartments. Hydraulic capacitance decreased only slightly with height, but importantly this parameter was nearly double in value to that reported for other tree species. From summer to fall measurement periods we did not observe osmotic adjustment that is known to depress the turgor loss point in other species. Instead we observed a proportional shift of foliar water into apoplastic compartments leading to a reduction in hydraulic capacitance while also maintaining high relative water content at the turgor loss point. Combined with tight stomatal control, this suite of foliar traits allows Sequoiadendron to routinely, but safely, operate close to its turgor loss point, and may permit survival through short-term drought by avoiding hydraulic failure. In addition, our results suggest that gravity plays an important role in the foliar water relations of the Earth’s largest tree species.