Passive stomatal closure under extreme drought in an angiosperm species

The phytohormone abscisic acid (ABA), synthesized as leaf turgor declines, plays a major role in closing stomata in species from this lineage, but recent reports of some angiosperms having a peaking-type ABA dynamic in which under extreme drought ABA levels decline to pre-stressed levels raises the possibility that passive stomatal closure by leaf water status alone can occur in species from this lineage. To test this hypothesis, we conducted instantaneous rehydration experiments in the peaking-type species Umbellularia californica through a long-term drought in which ABA levels declined to pre-stress levels yet stomata remain closed. We found that when ABA levels were lowest during extreme drought stomata of U. californica were passively closed by leaf water status alone, with stomata reopening rapidly to maximum rates of gas exchange on instantaneous rehydration. This contrasts with leaves early in drought in which ABA levels are highest, where we found stomata do not reopen on instantaneous rehydration. The transition from ABA driven stomatal closure to passively driven stomatal closure as drought progresses in this species occurs at very low water potentials facilitated by highly embolism resistant xylem. These results have important implications for understanding stomatal control during drought in angiosperms.