Timothy J. Brodribb

Summary Stomatal closure under high VPD L (leaf to air vapour pressure deficit) is a primary means by which plants prevent large excursions in transpiration rate and leaf water potential (Ψ leaf ) that could lead to tissue damage. Yet, the drivers of this response remain controversial. Changes in Ψ leaf appear to drive stomatal VPD L response, but many argue that dynamic changes in soil‐to‐leaf hydraulic conductance ( K s‐l ) make an important contribution to this response pathway, even in well‐hydrated soils. Here, we examined whether the regulation of whole plant stomatal conductance ( g c ) in response to typical changes in daytime VPD L is influenced by dynamic changes in K s‐l . We use well‐watered plants of two species with contrasting ecological and physiological features: the herbaceous Arabidopsis thaliana (ecotype Columbia‐0) and the dry forest conifer Callitris rhomboidea . The dynamics of K s‐l and g c were continuously monitored by combining concurrent in situ measurements of Ψ leaf using an open optical dendrometer and whole plant transpiration using a balance. Large changes in VPD L were imposed to induce stomatal closure and observe the impact on K s‐l . In both species, g c was observed to decline substantially as VPD L increased, while K s‐l remained stable. Our finding suggests that stomatal regulation of transpiration is not contingent on a decrease in K s‐l . Static K s‐l provides a much simpler explanation for transpiration control in hydrated plants and enables simplified modelling and new methods for monitoring plant water use in the field.

Premise Acmopyle (Podocarpaceae) comprises two extant species from Oceania that are physiologically restricted to ever‐wet rainforests, a confirmed fossil record based on leaf adpressions and cuticles in Australia since the Paleocene, and a few uncertain reports from New Zealand, Antarctica, and South America. We investigated fossil specimens with Acmopyle affinities from the early Eocene Laguna del Hunco site in Patagonia, Argentina. Methods We studied 42 adpression leafy‐shoot fossils and included them in a total evidence phylogenetic analysis. Results Acmopyle grayae sp. nov. is based on heterophyllous leafy shoots with three distinct leaf types. Among these, bilaterally flattened leaves uniquely preserve subparallel, linear features that we interpret as accessory transfusion tissue (ATT, an extra‐venous water‐conducting tissue). Some apical morphologies of A. grayae shoots are compatible with the early stages of ovuliferous cone development. Our phylogenetic analysis recovers the new species in a polytomy with the two extant Acmopyle species. We report several types of insect‐herbivory damage. We also transfer Acmopyle engelhardti from the middle Eocene Río Pichileufú flora to Dacrycarpus engelhardti comb. nov. Conclusions We confirm the biogeographically significant presence of the endangered West Pacific genus Acmopyle in Eocene Patagonia. Acmopyle is one of the most drought‐intolerant genera in Podocarpaceae, possibly due to the high collapse risk of the ATT, and thus the new fossil species provides physiological evidence for the presence of an ever‐wet rainforest environment at Laguna del Hunco during the Early Eocene Climatic Optimum.