Evaporative Flux Method of Leaf Hydraulic Conductance Estimation: Sources of Uncertainty and Reporting Format Recommendation

The accurate estimation of leaf hydraulic conductance (Kleaf) is important for revealing leaf physiology characteristics and function. However, there are some uncertain influencing factors in Kleaf measurement by using evaporation flux method (EFM), a widely used method. In this study, we investigated the potential impacts of plant sampling method, measurement setup, environmental factors, recording instrument, and transpiration steady status identification on Kleaf estimation. Our results indicated that the sampling and rehydration time, the small gravity pressure on leaf, and degassing treatment had limited effects on Kleaf values. Transpiration rate (E) was significantly affected by multiple environmental factors including airflow around leaf, light intensity, and leaf temperature. Kleaf values decreased by 40% from 1000 to 500 µmol m-2 s-1 light intensities and by 15.1% from 27 to 37 oC. In addition, the accurate flow rate (F) steady state identification and the leaf water potential measurement were important for Kleaf estimation. Based on the analysis of influencing factors, we provided a format for reporting the details of the EFM-based Kleaf measurement methods and metadata that future studies could interpret the results in method issue.

Leaf hydraulic safety margin and safety–efficiency trade-off across angiosperm woody species

Leaf hydraulic conductance and the vulnerability to water deficits have profound effects on plant distribution and mortality. In this study, we compiled a leaf hydraulic trait dataset with 311 species-at-site combinations from biomes worldwide. These traits included maximum leaf hydraulic conductance ( K leaf ), water potential at 50% loss of K leaf (P50 leaf ), and minimum leaf water potential ( Ψ min ). Leaf hydraulic safety margin (HSM leaf ) was calculated as the difference between Ψ min and P50 leaf . Our results indicated that 70% of the studied species had a narrow HSM leaf (less than 1 MPa), which was consistent with the global pattern of stem hydraulic safety margin. There was a positive relationship between HSM leaf and aridity index (the ratio of mean annual precipitation to potential evapotranspiration), as species from humid sites tended to have larger HSM leaf . We found a significant relationship between K leaf and P50 leaf across global angiosperm woody species and within each of the different plant groups. This global analysis of leaf hydraulic traits improves our understanding of plant hydraulic response to environmental change.

Loss and recovery of leaf hydraulic conductance: Root pressure, embolism, and extra-xylary resistance

Vascular networks in plant leaves must provide for the safe and efficient transport of water, nutrients, and energy; however, the conditions whereby these networks lose and regain conductive capacity are still poorly understood. We measured the loss and recovery of leaf hydraulic conductance (Kleaf) in a tropical monocotyledon (Bambusa vulgaris) and dicotyledon species (Bauhinia blakeana) using Rehydration Kinetics Method (RKM) as well as a recently developed optical method. We found that both species lost ca 88% of their maximal Kleaf (measured by RKM) before any embolization was detected in their conductive elements (via optical observation). This suggests that the majority of loss in Kleaf, as measured with RKM, was associated with resistances other than embolization. Furthermore, embolism in B. vulgaris, a species known to generate root pressure, was reversed when rehydrated under positive pressure (120 kPa), but not under atmospheric pressure. In contrast, embolism was not reversed in B. blakeana under either elevated or atmospheric pressure. However, reductions in Kleaf that was not associated with embolization was recovered by this species when rehydrated under atmospheric conditions, whereas B. vulgaris did not exhibit any recovery under the same conditions. We suggest that root pressure is an adaptive mechanism allowing for the reversal of embolism and the recovery of extraxylary conductance. The absence of embolism reversal in B. blakeana suggests that embolization may be permanent without neutral or positive xylem pressure, but that the recovery of extraxylary conductance can be regained routinely in this species in the absence of root pressure.

The legacy of C4 evolution in the hydraulics of C3 and C4 grasses

Most studies concerning the functional implications of C4 photosynthesis have focused on enhanced carbon fixation under high temperature, low atmospheric CO2, and/or water limitation, yet the biochemical and anatomical reorganization required for optimal C4 function should also impact plant hydraulics and water use. C4 grasses have increased bundle-sheath size and vein density, and these are thought to have been anatomical precursors for the evolution of C4 from C3 ancestors. Paradoxically, these traits should also lead to higher leaf capacitance and higher leaf hydraulic conductance, yet C4 photosynthesis lowers water demand and increases plant water use efficiency. Here, we use phylogenetic analyses, physiological measurements and photosynthetic modeling to examine the reorganization of hydraulic traits in C4 grass lineages and in closely-related C3 grasses. Evolutionarily young C4 lineages have higher leaf hydraulic conductance, capacitance, turgor loss point, and lower stomatal conductance than their C3 relatives. In contrast, species from older C4 lineages show decreased leaf hydraulic conductance and capacitance, indicating that over time, C4 plants have optimized hydraulic investments while maintaining their C4 anatomical requirements. The “overplumbing” of young C4 lineages lead to a reduced positive correlation between maximal assimilation rate and leaf hydraulic conductance, decoupling a key relationship between hydraulic traits and photosynthesis generally observed in vascular plants.