Document: PET-3-32-19

Xylem cavitation thresholds and embolism repair in Hibiscus rosa assessed with independent techniques.

MELCHER, P.J* ^1,3, F.CMEINZER ² and G.GOLDSTEIN ³

Harvard University, Cambridge, MA 02138, USA ¹
Hawaii Agriculture Research Center, Aiea, HI, 96701, USA ²
University of Hawaii, Honolulu, HI 96122, USA ³

Abstract:
Xylem cavitation thresholds and diurnal variation in the frequency of embolised vessels were evaluated in field- and greenhouse-grown plants of Hibiscus rosa using independent techniques. Xylem tension was induced by air-drying, by centrifugation of excised stems, and by allowing intact stems to undergo normal daily cycles of partial dehydration and rehydration in situ. Cavitation and embolism were estimated indirectly from variation in the hydraulic conductivity (k_h) of stem segments, and directly by counting percentages of liquid- and gas-filled vessels in images of stems previously frozen in liquid N₂ for viewing by cryo-scanning electron microscopy (cryo-SEM). Apparent cavitation thresholds varied according to the techniques employed to induce tension and evaluate cavitation. At 1 MPa estimated tension, excised air-dried stems had lost 25% of their original k_h, whereas k_h of centrifuged stem segments was 7% greater than its original value. Centrifuged stems did not show measurable loss of k_h below an abrupt tension threshold of 1.5 MPa but did show steadily decreasing weight due to increasing loss of water beginning at the smallest tensions imposed. It was hypothesized that water entering the xylem from surrounding tissues during centrifugation and subsequently lost from the segment ends may have transiently enhanced k_h through partial removal of incipient emboli. Cryo-SEM images of stems subjected to centrifuge-generated tensions showed a continuous, gradual increase in the percentage of embolised vessels from c. 2% at 0 MPa, to 7% at 1 MPa to c. 22% at 4.5 MPa. Intact stems allowed to partially dehydrate in situ appeared more vulnerable to cavitation than excised stems because loss of k_h was 50%, and 16% of the vessels were embolised in cryo-SEM images, when tension was 1 MPa. The percentage of embolised vessels assessed by cryo-SEM varied diurnally in intact plants with a maximum value of 23% at midday and minimum values of 5 to 7% at 0600 and 1800 h. Concurrent measurements of k_h and nontranspiring leaf balance pressures indicated that vessel refilling took place when considerable tension was present in neighboring vessels.

Keywords: cryo-scanning electron microscopy; hydraulic conductivity; xylem tension; xylem vulnerability; embolism repair

This abstract is being presented at: 10:30 AM in session:
Oral Session #45: Water Relations in Shrubs and Annuals.