My research interests are organismic physiology and ecophysiology of plants. I have been particularly interested in xylem structure and long-distance water transport. For every molecule of CO2 a leaf takes up, it loses several hundred molecules of water. Since water supply to leaves is often limiting for plant growth and gas exchange, xylem function has important ecological and evolutionary implications. 

Hydraulic architecture and limits to gas exchange

Hydraulic limits arise from the fact that both soil and xylem conductivity may drop during a drought. In the xylem, this is due to cavitation. Xylem cavitation or "air-seeding" results in embolized conduits that are unavailable for water transport. Air-seeding occurs in roots, stems, and leaves. We found that small roots are often particularly susceptible to cavitation (Hacke et al. 2000, Sperry & Hacke 2002). Conduits may be refilled under certain conditions, but any active repair mechanism is likely to be costly for the plant (Stiller et al. 2005).

Leaf water supply can also be limited by the soil, especially under drought conditions. Coarse soils with large pore spaces and high saturated conductances tend to show a much more abrupt decline in conductance with decreasing water potential than finer-textured soils. This is analogous to plant xylem: xylem with large vessels is very efficient as long as conditions remain favorable, but there may be a steep loss of conductivity as xylem pressure becomes more negative. For instance, this means that Kudzu shows dramatically higher conductivities in its narrow pressure range than xeric species, but Kudzu's vulnerable xylem prevents it from growing in drier conditions.

There is a link between the water transport capacity of a plant and stomatal function. Stomata must control xylem pressure to preserve hydraulic continuity of the soil-leaf continuum. While stomates of some riparian trees like birch maintain leaf water status within a narrow range, stomata of many xeric plants allow a much broader 'water use envelope' (Sperry et al. 2002).

How trade-offs influence the structure, function, and evolution of xylem

Xylem has evolved as a long-distance transport system for water and nutrients. However, it also provides structural support and storage. There is huge variation in xylem and conduit structure among plants (e.g., dicots vs monocots, angiosperms vs gymnosperms, stems vs roots). This is partly due to the vastly different negative pressure regimes. The necessity for reinforcing conduits against implosion and the economy of avoiding excessive safety factors appears to be responsible for a relationship between the reinforcement of conduit walls and air-seed pressure. On the tissue level, this translates into a correlation between wood density and air-seed pressure. Xylem that tolerates very negative xylem pressure without cavitation requires greater reinforcement, and is thus more costly (Hacke et al. 2001, 2004).

Besides this mechanical cost, there is a hydraulic cost of developing safe xylem. The structural basis of this relationship was unknown - until recently. In our data set (Wheeler et al. 2005, Hacke et al. 2006), the total pit area per vessel scaled with air-seed pressure. Vulnerable vessels had much more pit area than resistant ones. This is consistent with the idea that plants have no perfect control over the pore size in their pit membranes. The more pits there are in a vessel, the greater is the chance that a vessel has that rare large pore that will trigger cavitation. According to our data, there is not necessarily a correlation between the average and the maximum pore size in pit membranes.

In addition to having greater pit area, vulnerable vessels were also bigger than resistant ones. This resulted in a safety versus efficiency trade-off on the conduit level (not on the level of individual pits) in angiosperms.

The constraint on pit area and the resulting constraint on vessel size explain why pit resistance is so high. Pit resistance represented c. 55% of the total xylem resistance in our study species. A constraint on vessel size resulting from the need to provide adequate protection from cavitation also explains why vessels are not longer to overcome this high resistance (Sperry et al. 2005).

A recent comparison of angiosperm and gymnosperm xylem revealed that torus-margo pits are highly efficient. These sophisticated valves compensate to a large degree for the shorter conduit length of gymnosperms (Pittermann et al. 2005).