title: Parenchyma-chlorenchyma water movement during drought for the hemiepiphytic cactus Hylocereus undatus.
abstract: OBJECTIVE
  Hylocereus undatus, a hemiepiphytic cactus cultivated in 20 countries for its fruit, has fleshy stems whose water storage is crucial for surviving drought. Inter-tissue water transfer during drought was therefore analysed based on cell volumes and water potential components.
  
  
  METHODS
  In addition to determining cell dimensions, osmotic pressures and water potentials, a novel but simple procedure leading to an external water potential of zero was devised by which cells in thin sections were perfused with distilled water. The resulting volume changes indicated that the parenchyma-chlorenchyma water movement was related to more flexible cell walls in the water-storage parenchyma with its lower internal turgor pressure (P) than in the chlorenchyma.
  
  
  RESULTS
  Under wet conditions, P was 0.45 MPa in the chlorenchyma but only 0.10 MPa in the water-storage parenchyma. During 6 weeks of drought, the stems lost one-third of their water content, becoming flaccid. About 95 % of the water lost came from cells in the water-storage parenchyma, which decreased by 44 % in length and volume, whereas cells in the adjacent chlorenchyma decreased by only 6 %; the osmotic pressure concomitantly increased by only 10 % in the chlorenchyma but by 75 % in the water-storage parenchyma.
  
  
  CONCLUSIONS
  The concentrating effect that occurred as cellular volume decreased indicated no change in cellular solute amounts during 6 weeks of drought. The ability to shift water from the parenchyma to the chlorenchyma allowed the latter tissue to maintain a positive net CO2 uptake rate during such a drought.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095-1606, USA. psnobel@biology.ucla.edu
authors.forename: Park S
authors.initials: PS
authors.lastname: Nobel
mesh_terms: D001692:Biological Transport; D029421:Cactaceae / Q000166:cytology* / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism; D004190:Disasters; D009997:Osmotic Pressure; D018547:Plant Stems / Q000166:cytology; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water; D002245:Carbon Dioxide