UCLA

There has been increasing worldwide recognition of the importance of hydraulic physiology--the transport of water through the plant--in explaining plant growth and drought tolerance. By combining physiology and anatomy within an evolutionary framework, we can discover the mechanisms underlying species differences in hydraulic function, especially those of the leaf, the central organ in plant metabolism. I refined and developed new methods to investigate leaf water transport and its decline during drought, focusing on a critical measure of the capacity for water movement (leaf hydraulic conductance, K_leaf). I found that species most tolerant of K_leaf decline had small leaves with dense major veins, providing pathways for the water to bypass embolized conduits during drought giving a new, direct explanation to the fact that species of dry areas have small leaves. I also developed a new method to investigate the role of leaf shrinkage on water movement. As leaves shrink with dehydration, mesophyll cells lose connectivity, physically impacting water movement outside the xylem. I found that species most sensitive to K_leaf decline were those with strongest shrinkage in thickness. I then developed a new method to measure xylem hydraulic decline in leaves to test for a possible artifact of cutting leaf petioles under tension while under water. Such artifact has been recently found to occur in stems, and has put into question measurements of K_leaf. Across four diverse species, I found no sign of such an artifact in leaves, likely due to the lesser mechanical stress imposed when cutting a petiole vs. stem. Finally, I took an evolutionary perspective. I quantified the anatomical and physiological plasticity in leaves of six species of endemic Hawaiian lobeliads grown under different light regimes and found a high degree of plasticity in K_leaf with light, relating to leaf anatomical changes. Across 30 species of Viburnum I have identified the evolutionary shifts of leaf anatomy, water transport and drought tolerance. This work provides new techniques, clarity and applications toward understanding leaf water transport and its role in plant performance and drought tolerance, with applications for ecology, paleobiology and the conservation of species and ecosystems.