Plant growth and survival are ultimately constrained by the supply of water to leaves. Even under adequate moisture supply, photosynthesis is restricted by the stomatal pores in leaves as well as the efficiency of water movement through the plant. If the stomatal pores in leaves do not tightly coordinate water loss with changes in water supply, large negative pressures (tension) will develop in the plant's water conducting system (xylem), causing entry of air bubbles and ultimately catastrophic hydraulic failure and plant death. Air bubbles are catastrophic because a bubble will break the water column and cause that part of the xylem to be nonfunctional. Few studies have considered dynamic conditions under which water stored in plant tissues is released into the transpiration steam, buffering fluctuations in xylem tension. The overall objective of this research is to elucidate the relative roles of both dynamic and static properties of the plant hydraulic pathway from root to leaf in avoiding hydraulic failure. The researchers hypothesize that there is a continuum of relative reliance on different mechanisms conferring hydraulic safety: species and plant organs with low water storage capacity rely primarily on xylem structural features to avoid transport failure, whereas species and organs with higher water storage capacity avoid transport failure due to a transient release of stored water. A comprehensive understanding of how plants react to the dynamic stresses they experience on a daily basis is critical for identifying mechanisms allowing them to cope with variation in moisture supply under current and future climate regimes. Because water is typically one of the most important limiting factors to plant growth, the results will have broad implications for agriculture, forestry and management of ecosystems experiencing altered moisture regimes as a result of changes in land-use, climate change and other factors. The project involves training of two postdoctoral scholars, one masters student, and several undergraduate students. The project will involve both graduate and undergraduate students from Penn State University, AgroParisTech (France) and Panama.
Project Outcome Report: Our understanding of tree water transport (hydraulics) is critical for understanding forest responses to climate change, particularly changes in precipitation and/or temperature regimes. Prior to this project, the majority of research on hydraulics was based on small, easily obtainable branches, but very little work had been done on entire trees. For this project we have compared hydraulic conductance (permeability of wood or leaves to water flow), vulnerability to hydraulic dysfunction, and hydraulic capacitance (water storage and release) across the entire tree hydraulic continuum from root to leaf. We have also assessed how these parameters respond to decreasing water availability. Major findings: Declines in hydraulic function in leaves during drought are due to air bubble formation in the leaf veins and many conifers are more susceptible to bubble formation than broadleaved trees. Leaves in coniferous species act as hydraulic circuit breakers to protect the branches and trunk from experiencing hydraulic failure. Many broadleaf trees have greater control of water loss and can thus prevent air bubble formation in their leaf veins. We have also found that branches in conifer trees are highly resistant to air bubble formation. This contradicts a twenty year old hypothesis (hydraulic vulnerability segmentation hypothesis) that stipulates that plant parts should become more vulnerable to hydraulic dysfunction along the trajectory from trunk to leaves. Our data indicate that, at least for many conifer species, this is not true. Trees use water that is stored in their wood to buffer negative pressures that arise from transpiration (evaporation of water from the leaf). We have found that trees strongly regulate their stored water by closing stomata thereby reducing transpiration. The point at which stomata are closed is the negative pressure at which the tree essentially runs out of stored water in its branches. Stomatal closure has long been thought to regulate negative pressure in the leaf, as opposed to the branch, so this is an important finding. We have also found that some trees and woody vines are able to repair air bubbles more efficiently than others. Our data from this project show that different plants and different plant organs (root, stem, leaf) employ different strategies for preventing air bubble formation versus air bubble occurrence and subsequent repair. In woody tissues, broadleaf trees are more efficient at embolism repair than conifers. There are exceptions, but this is the general trend. Our data from this project cast doubt on several old, dogmatic assumptions about plant hydraulics: that vulnerability to air bubble formation should be lowest in the trunk and increase in each downstream organ, that plants use stomatal regulation only to control the negative pressure of leaves, and that air bubble formation is a rare event that only happens during extreme drought.
