Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2007
Publication Date: 11/1/2007
Citation: Mcelrone, A.J., Bichler, J., Pockman, W.T., Addington, R.N., Linder, C., Jackson, R.B. 2007. Aquaporin-mediated changes in hydraulic conductivity of deep tree roots accessed via caves. Plant Cell and Environment.
Interpretive Summary: Deep roots can contribute significantly to whole tree water use but have long been overlooked due to the difficulty of accessing these plant organs without causing significant damage during excavation. We overcame this limitation by utilizing caves in the Edward's Plateau region of central TX to access intact deep roots for two of the dominant tree species found in this ecosystem. Over several seasons, we documented deep root physiological responses that facilitate uptake of consistent water supplies from a perennial underground stream located at ~20 m depth. Deep roots reduced the resistance of their living cells, where water is initially absorbed, during periods of high water use by the trees aboveground (i.e. summer, full sunlight and midday). Additional measurements confirmed that water channels located in the living cells were responsible for the dynamic adjustments in root resistance to water uptake. Such physiological flexibility aids the use of reliable water resources from depth and may contribute to the success of these species in this dry environment.
Technical Abstract: Although deep roots can contribute substantially to whole-tree water use, little is known about deep root functioning because of limited access for in situ measurements. We used a cave system on the Edwards Plateau of central Texas to investigate the physiology of water transport in roots at 18-20 m depth for two common tree species, Quercus fusiformis and Bumelia lanuginosa. Using sap flow and water potential measurements on deep roots, we found that calculated root hydraulic conductivity (RHC) fluctuated diurnally for both species and decreased under shading for B. lanuginosa. To assess whether these dynamic changes in RHC were regulated during initial water absorption by fine roots, we used an ultra-low flowmeter and hydroxyl radical inhibition to measure in situ fine root hydraulic conductivity (FRHC) and aquaporin contribution to FRHC (AQPC), respectively. During summer, FRHC and AQPC were found to cycle diurnally in both species, with peaks corresponding to the period of highest transpirational demand at midday. During whole-tree shade treatments, B. lanuginosa FHRC ceased diurnal cycling and decreased by 75 and 35% at midday and midnight, respectively, while AQPC decreased by 41 and 30% during both time periods. A controlled growth-chamber study using hydroponically grown saplings confirmed daily cycling and shade-induced reductions in FRHC and AQPC. Winter measurements showed that the evergreen Q. fusiformis maintained high FRHC and AQPC throughout the year, while the deciduous B. lanuginosa ceased diurnal cycling and exhibited its lowest annual values for both parameters in winter. Adjustments in FRHC and AQPC to changing canopy water demands may help the trees maintain the use of reliable water resources from depth and contribute to the success of these species in this semi-arid environment.