Night-time stomatal conductance associated with xylem embolism repair in the desert shrub Encelia farinosa

Nighttime stomatal conductance has been reported for a diverse range of genera and plant life-forms.  Its proposed functions include increased daytime carbon gain, a reduction of hydraulic redistribution, increased nutrient acquisition, and recovery of hydraulic conductance from xylem embolisms that form during the day.  Here we report substantial nighttime stomatal conductance in the desert shrub Encelia farinosa associated with simultaneous xylem embolism repair while the xylem-water was under tension.  Measurements of stomatal conductance, stem and leaf water potential, stem hydraulic conductivity, and percent loss of stem conductance, a measure of the degree of embolisms, were conducted over 24 hour periods for plants grown in a botanical garden and in the field.  When soil water was available, the degree of embolisms tended to increase from predawn to mid-day and decrease during the first half of the night.  During the period of embolism repair at night, stomatal conductance was high (>0.1 mmol m^-2 s^-1), while stem and leaf water potentials remained below -1 MPa.  To our knowledge this is the first time embolism repair has been observed in association with nighttime stomatal conductance.  The mechanisms underlying embolism repair under tension remain unclear, but our observations suggest that it may require a continuous water flux during repair.  The observations shed new light on two of the major unresolved questions of plant water relations research, nighttime transpiration and embolism repair under tension.