Plants must move on the order of 200-300 kilograms of water through their xylem for every kilogram of dry mass they assimilate from atmospheric CO2. Xylem transport has to be economic or plants could not turn a profit with such a poor carbon-for-water exchange rate. The driving force is free: capillary suction at cell wall pores coupled to a negative pressure gradient in xylem conduits. But negative xylem pressure exacts a toll on conducting efficiency, because the conduit network must be safe from dysfunction from cavitation and from collapse by implosion. The evolution of efficiency is limited by safety concerns. The proposed research will quantify the efficiency vs. safety trade-off across major xylem types, testing hypotheses for the functional significance of major xylem traits. The first objective is to quantify the flow resistance through inter-conduit pits and its importance for limiting total xylem resistance. Pits are crucial structures that protect against cavitation by air-seeding but constrict water flow and weaken walls. The hypothesis is that a high air-seeding pressure comes at the expense of a high pit resistance. The second objective is to determine how air-seeding of cavitation occurs in different pit types, and how the strength of the membrane constrains pit dimensions. The hypothesis is that pit dimensions are maximized to the point where membrane strength is limiting and air-seeding occurs at the verge of membrane rupture. The third objective is to determine the extent of mechanical stress in wood cell walls from negative xylem pressure. The prediction is that hydraulic stresses are significant and require compensating reinforcement that reduces conducting efficiency. The objectives will be accomplished by a combination of experimentation and biomechanical modeling across major xylem types and phylogenetic groups including ferns, gymnosperms, and angiosperms. The research tests many long-standing hypotheses of wood structure and function. The results are necessary for interpreting xylem evolution, evaluating the cost of water transport and exposing mechanisms that enhance competitive ability, and fulfilling the promise of genomics for linking genes to wood function. The research will create research opportunities for students at all levels.
