Variable embolism in roots and leaves: a dynamic hydraulic signal mediating stomatal control of transpiration

MEINZER F.C., DOMEC J.C., WOODRUFF D., BUCCI S.J., SCHOLZ F.G., MCCULLOH M., WARREN J.M., GOLDSTEIN G., BROOKS J.R., GARTNER B.

Snowbird, Utah, USA

Simposio; ASPB Conference: The Biology of Transpiration: From Guard Cells to Globe; 2006

A major fraction of total plant hydraulic resistance resides in the root system and leaves.  In addition, the xylem of roots and leaves is typically more vulnerable to embolism than that of stems.  These traits imply that roots and leaves are likely to represent hydraulic bottlenecks under a range of atmospheric and soil conditions.  Here we describe our ongoing work on the role of dynamic variations in root and leaf embolism in mediating stomatal control of transpiration in trees from three contrasting vegetation types: temperate coniferous forests, a Brazilian savanna, and a lowland tropical forest.  In semi-arid ponderosa pine and mesic Douglas-fir stands, daily maximum stomatal conductance decreased exponentially with increasing seasonal loss of conductivity in shallow roots.  Root embolism varied diurnally in four co-occurring Brazilian savanna species, and daily maximum stomatal conductance decreased linearly with increasing afternoon loss of root conductivity in a similar manner across species. In both the coniferous forest and Brazilian savanna ecosystems, the effects of soil drying on root embolism were mitigated by hydraulic redistribution of water from deep root in moist soil to shallow roots in drier soil, which partially uncoupled the water potential of shallow roots from that of the surrounding dry soil. Douglas-fir needles were highly vulnerable to embolism compared to the stems to which they were attached, causing the needles to embolize and refill on a daily basis.  The onset of diurnal stomatal closure was associated with a threshold value of leaf water potential that coincided with nearly complete loss of needle hydraulic conductance from embolism.  Vertical trends in leaf vulnerability to embolism and leaf water potential thresholds for the onset of stomatal closure in the crowns of tall Douglas-fir trees were tightly coordinated with vertical gradients in maximum xylem tension. These and other results to be discussed imply that rapidly reversible embolism in roots and leaves constitutes part of a hydraulic signal involved in normal daily stomatal regulation to prevent xylem tension from reaching values that could provoke embolism in stems where its reversal may not be as vigorous.