PARENT SESSION
Oral Session # 45: Plant Ecology III: Plant - Water Relations I.
Presiding: R Giuliani
Wednesday, August 6. 8:00 AM to 11:30 AM, SITCC Meeting Room 200.

Sap flux responses to drought and humidity for cottonwoods under current and increased CO₂concentrations.

Bobich, Edward^*,1, Grieve, Katie¹, Barron-Gafford, Greg¹, Murthy, Ramesh¹, ¹ Biosphere 2 Center, Oracle, AZ, USA

ABSTRACT- Sap flux in stems of woody plants depends on available soil water, xylem conductivity, and leaf water status, which in turn depends on stomatal control of water loss. Predicted increases in atmospheric CO₂concentrations ([CO₂]) and changes in rainfall patterns associated with global climate change can influence sap flux and hence the physiological success of certain species. In the present study, four-year-old eastern cottonwoods (Populus deltoides Bartr.) growing in a sealable glasshouse structure under current (400 ppm) and doubled (800 ppm) ambient [CO₂] were investigated with respect to their sap flux responses (measured using Granier-type probes) to drought combined with periodic changes in vapor pressure deficit (VPD). Total daily sap flux decreased an average of 80% after 5 weeks of drought for plants under both elevated and ambient [CO₂]; the decreases are likely due to increased cavitation of vessels as the drought progressed. Decreases in total daily sap flux occurred primarily during periods of high VPD (3.1 kPa); sap flux was relatively unchanged during periods of low VPD (1.2 kPa). Decreases in sap flux were responsible for over 90% of the decreases in hydraulic conductivity (calculated using sap flux, midday leaf water potentials, and soil water potentials) at midday for plants under both [CO₂]. For the first 4 weeks, total daily sap flux was greater for plants in doubled versus current [CO₂], reflecting the greater total leaf area and stomatal frequency for plants growing under the doubled [CO₂]. In addition, wood of plants under the doubled [CO₂] was 13% less dense than wood of plants under the current [CO₂], possibly indicating a greater conducting area per unit wood area. The results suggest that P. deltoides will take up and transport more water, allowing for greater photosynthesis, during short periods of drought in future atmospheric [CO₂].

Key words: sap flux, Populus deltoides, Biosphere 2 Center, drought