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Keywords:

  • bordered pits;
  • embolism;
  • hydraulic resistance;
  • pit membrane;
  • xylem

Contents

 Summary1
I.Introduction2
II.The basic structures2
III.Safety of transport - pits as safety valves5
IV.Hydraulic resistance of pits and pit membranes - the cost of safety9
V.Impacts of pit function on whole-plant hydraulics14
VI.Future directions15
 Acknowledgements15
 References15

Summary

Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids) that are essential components in the water-transport system of higher plants. The pit membrane, which lies in the center of each pit, allows water to pass between xylem conduits but limits the spread of embolism and vascular pathogens in the xylem. Averaged across a wide range of species, pits account for > 50% of total xylem hydraulic resistance, indicating that they are an important factor in the overall hydraulic efficiency of plants. The structure of pits varies dramatically across species, with large differences evident in the porosity and thickness of pit membranes. Because greater porosity reduces hydraulic resistance but increases vulnerability to embolism, differences in pit structure are expected to correlate with trade-offs between efficiency and safety of water transport. However, trade-offs in hydraulic function are influenced both by pit-level differences in structure (e.g. average porosity of pit membranes) and by tissue-level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connects vessels. In this review we address the impact of variation in pit structure on water transport in plants from the level of individual pits to the whole plant.