Divergent pathways across vascular plant lineages drive reduced nighttime transpiration during drought

Stomatal closure in response to water deficit is crucial for maintaining plant water balance. While the mechanisms driving daytime stomatal closure under drought are well studied, the mechanism driving progressive declines in nighttime transpiration (Enight) during drought remains less understood. To investigate whether either abscisic acid (ABA) or declining leaf water status drives progressive declines in Enight during drought in vascular plants, we conducted experiments using representative fern, gymnosperm, and angiosperm species, including a severe ABA-deficient mutant and tree species. These species span a spectrum of stomatal control by ABA, ranging from insensitive to endogenous ABA in the fern to reliance on ABA for stomatal closure in the herbaceous angiosperm. We found that reductions in Enight during drought are driven by hydropassive stomatal closure in ferns and gymnosperms, transitioning to ABA regulation in gymnosperms under severe stress, and are triggered by ABA in herbaceous angiosperms. In all species, the proportion of total transpiration occurring at night increased as stomata closed during the drought. The reduction of Enight during drought appears to be a convergent stomatal response across vascular land plants but is driven by diverse regulatory mechanisms linked to evolutionary history and ecological strategy.