PARENT SESSION
Poster Session # 15: Plant Ecology.
Wednesday, August 6 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.
Cavitation repair under tension: Is it consistent with the cohesion-tension theory?
Black, R. Alan*,1, Pratt, R. Brandon1, 2, 1 School of Biological Sciences, Pullman, Washington, USA2 Natural Science Division, Malibu, California, USA
ABSTRACT- We examined diurnal patterns of cavitation in two genotypes of black cottonwood (Populus trichocarpa) displaying divergent stomatal behavior under water stress: one genotype incompletely closing stomata (ISC genotype) while the second closes its stomata (SC genotype). To test for cavitation we made diurnal measurements of xylem specific hydraulic conductivity (Ks), and ultrasonic acoustic emissions (UAE) under well-watered conditions in a greenhouse. The Ks of stems for the ISC genotype grown for one year were 47% lower at 13:00 hours compared to predawn. Predawn xylem water potentials (
x) were -0.20+0.02 MPa with no evidence of positive root pressure overnight. After two years of growth Ks displayed the same diurnal pattern but only when predawn x's were > -0.20+0.02 MPa. In contrast, the SC genotype displayed no diurnal difference in Ks and x at predawn was <-0.20 MPa. Ultrasonic acoustic emissions measured in situ were not different between the genotypes. For field grown plants, Ks's were not different between predawn, midday, and dusk measurements and predawn x's were < -0.30 MPa for both genotypes. Experimentally raising x of excised stems increased Ks by about 15% for both genotypes. Vulnerability to cavitation of lateral stems was not different between the genotypes (x at 50% loss of hydraulic conductivity (50) was -0.85+0.1 and -0.95+0.1 MPa for the ISC and SC genotypes respectively; p>0.05). We conclude that the decline in Ks diurnally displayed by the ISC genotype cannot be explained by differential vulnerability to cavitation, hydrogel behavior of pit membranes, or positive root pressure. Instead, the ISC genotype refilled embolized vessels when overnight x was < -0.20 MPa while the SC genotype did not refill vessels because its x was < -0.20. MPa. Our data are consistent with the Yang and Tyree (1992) model of cavitation repair and the cohesion-tension theory of water transport.
Key words: water relations, cavitation, hydraulic conductivity, Populus trichocarpa