Abscisic acid driven stomatal closure during drought in anisohydric Fagus sylvatica

Abstract

Stomatal closure limits transpiration during drought, restricting water potential decline and delaying the onset of embolism. While critical for ensuring survival during drought, the mechanisms driving stomatal closure during drought remain equivocal. The hormone abscisic acid (ABA) will close stomata in seed plants and is synthesized as leaf turgor declines. ABA driven stomatal closure during drought is particularly apparent in species that are more isohydric. In contrast, in species that have a more anisohydric response to drought, like Fagus sylvatica, the importance of ABA in driving stomatal closure during drought is often overlooked or excluded, in place of a hypothesized passive, water potential driven stomatal closure. Here we investigated whether ABA drives stomata closure during a mid-summer drought in field grown F. sylvatica. We found that as leaf water potential declines during a drought, foliage abscisic acid (ABA) levels increase considerably and stomata close. ABA levels in leaves increase as water potentials decline to within 0.3 MPa of turgor loss point, when stomata close. Foliage ABA levels correlate with stomatal conductance throughout a drought and post-drought period. From these results we argue that it is hard to exclude increased ABA levels driving stomatal closure during drought in the anisohydric species F. sylvatica.