Hydraulic integrity of plant organs during drought stress and recovery in herbaceous and woody plant species

Drought-induced stomatal closure precedes leaf turgor loss and embolism in leaf xylem. Root xylem is embolism resistant and rehydrates after drought, while recovery of above-ground organs is species dependent The relationship between root, stem, and leaf hydraulic status and stomatal conductance during drought (field capacities: 100-25%) and drought recovery was studied in Helianthus annuus and five tree species (Populusxcanadensis, Acer saccharum, A. saccharinum, Picea glauca, and Tsuga canadensis). Measurements of stomatal conductance (g(s)), organ water potential, and vessel embolism were performed and the following was observed: (i) cavitation only occurred in the petioles and not the roots or stems of tree species regardless of drought stress; (ii) in contrast, all H. annuus organs exhibited cavitation to an increasing degree from root to petiole; and (iii) all species initiated stomatal closure before cavitation events occurred or the expected turgor loss point was reached. After rewatering: (i) cavitated vessels in petioles of Acer species recovered whereas those of P. xcanadensis did not and leaves were shed; (ii) in H. annuus, cavitated xylem vessels were refilled in roots and petioles, but not in stems; and (iii) despite refilled embolisms in petioles of some species during drought recovery, g(s) never returned to pre-drought conditions. Conclusions are drawn with respect to the hydraulic segmentation hypothesis for above- and below-ground organs, and the timeline of embolism occurrence and repair is discussed.