title: Hydathode trichomes actively secreting water from leaves play a key role in the physiology and evolution of root-parasitic rhinanthoid Orobanchaceae.
abstract: OBJECTIVE
  Root hemiparasites from the rhinanthoid clade of Orobanchaceae possess metabolically active glandular trichomes that have been suggested to function as hydathode trichomes actively secreting water, a process that may facilitate resource acquisition from the host plant's root xylem. However, no direct evidence relating the trichomes to water secretion exists, and carbon budgets associated with this energy-demanding process have not been determined.
  
  
  METHODS
  Macro- and microscopic observations of the leaves of hemiparasitic Rhinanthus alectorolophus were conducted and night-time gas exchange was measured. Correlations were examined among the intensity of guttation, respiration and transpiration, and analysis of these correlations allowed the carbon budget of the trichome activity to be quantified. We examined the intensity of guttation, respiration and transpiration, correlations among which indicate active water secretion.
  
  
  RESULTS
  Guttation was observed on the leaves of 50 % of the young, non-flowering plants that were examined, and microscopic observations revealed water secretion from the glandular trichomes present on the abaxial leaf side. Night-time rates of respiration and transpiration and the presence of guttation drops were positively correlated, which is a clear indicator of hydathode trichome activity. Subsequent physiological measurements on older, flowering plants indicated neither intense guttation nor the presence of correlations, which suggests that the peak activity of hydathodes is in the juvenile stage.
  
  
  CONCLUSIONS
  This study provides the first unequivocal evidence for the physiological role of the hydathode trichomes in active water secretion in the rhinanthoid Orobanchaceae. Depending on the concentration of organic elements calculated to be in the host xylem sap, the direct effect of water secretion on carbon balance ranges from close to neutral to positive. However, it is likely to be positive in the xylem-only feeding holoparasites of the genus Lathraea, which is closely related to Rhinanthus. Thus, water secretion by the hydathodes might be viewed as a physiological pre-adaptation in the evolution of holoparasitism in the rhinanthoid lineage of Orobanchaceae.
authors.affiliation: Faculty of Science, University of South Bohemia, Branišovská 1760, České Budějovice 370 05, Czech Republic and Institute of Botany, Academy of Sciences of the Czech Republic, Dukelská 135, Třeboň 379 82, Czech Republic.
authors.forename: Petra
authors.initials: P
authors.lastname: Světlíková
authors.affiliation: Faculty of Science, University of South Bohemia, Branišovská 1760, České Budějovice 370 05, Czech Republic and Institute of Botany, Academy of Sciences of the Czech Republic, Dukelská 135, Třeboň 379 82, Czech Republic Faculty of Science, University of South Bohemia, Branišovská 1760, České Budějovice 370 05, Czech Republic and Institute of Botany, Academy of Sciences of the Czech Republic, Dukelská 135, Třeboň 379 82, Czech Republic.
authors.forename: Tomáš
authors.initials: T
authors.lastname: Hájek
authors.affiliation: Faculty of Science, University of South Bohemia, Branišovská 1760, České Budějovice 370 05, Czech Republic and Institute of Botany, Academy of Sciences of the Czech Republic, Dukelská 135, Třeboň 379 82, Czech Republic jakub.tesitel@centrum.cz.
authors.forename: Jakub
authors.initials: J
authors.lastname: Těšitel
mesh_terms: D005075:Biological Evolution; D019069:Cell Respiration; D005740:Gases / Q000378:metabolism; D006790:Host-Parasite Interactions / Q000502:physiology*; D016014:Linear Models; D031670:Orobanchaceae / Q000502:physiology*; D018515:Plant Leaves / Q000378:metabolism*; D018517:Plant Roots / Q000469:parasitology*; D018526:Plant Transpiration; D064201:Trichomes / Q000502:physiology*; D014867:Water / Q000378:metabolism*
chemical_list: D005740:Gases; D014867:Water
keywords: Ecophysiology; Lathraea; Rhinanthus alectorolophus; Triticum aestivum; holoparasite; hydathode trichome; orobanche; parasitic plant; respiration; rhinanthoid Orobanchaceae; root hemiparasite; transpiration; water regime; water secretion; xylem