PARENT SESSION
Oral Session #63: Plant Ecology: Water Relations. Presiding: J. Cleverly.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Hall of Ideas H.
Water Relations of a Liana Infected by a Xylem-Limited Bacterium and Concurrently Drought Stressed.
McElrone, Andrew1, Forseth, Irwin 1, 1
ABSTRACT- Recently, the appreciation of the importance of pathogens in natural communities has increased. One pathogen of particular importance is Xylella fastidiosa, a xylem-limited bacterial pathogen with a diverse and extensive host range that causes bacterial leaf scorch in hosts. This pathogen provides a unique system to assess the effects of a parasite on water transport in plants. Our 1999 study showed that drought stress enhances symptom severity and progression along the stem. Our objective here was to investigate the mechanism behind the interaction of drought stress with infection by X. fastidiosa of the eastern deciduous forest vine, Parthenocissus quinquefolia. A 2 x 2 complete factorial design with two water treatments (high, HW and low, LW) and two infection treatments (control, C and infected, I) was used, and measurements included: leaf water potentials, transpiration, shoot hydraulic conductance, percent embolism, and xylem vessel dimensions. No significant differences in hydraulic architecture or conductance were found between C and I plants prior to drought. Drought treatment significantly reduced leaf water potentials and transpiration at all leaf positions throughout the day in late summer. Infection significantly reduced leaf water potentials and transpiration in the most basal leaf positions in late summer. Whole shoot hydraulic conductance was reduced by both LW and I treatments for plants harvested in mid September. However, percent embolised vessels and mean vessel diameter was affected by drought treatment only (no significant infection effects). These results imply that the major effect of infection by X. fastidiosa occurs due to reduced hydraulic conductance, and not increased cavitation and embolism of xylem elements.
KEY WORDS: xylem-limited plant pathogen, drought, embolism, water relations