Carbon gain is coordinated with enhanced stomatal conductance and hydraulic architecture in coffee plants acclimated to elevated [CO2]: The interplay with irradiance supply.

We recently demonstrated that, under elevated [CO2] (eCa), coffee (Coffea arabica L.) plants grown at high light (HL), but not at low light (LL), display higher stomatal conductance (gs) than at ambient [CO2] (aCa). We then hypothesized that the enhanced gs at eCa/HL, if sustained at the long-term, would lead to adjustments in hydraulic architecture. To test this hypothesis, potted plants of coffee were grown in open-top chambers for 12 months under HL or LL (ca. 9 or 1 mol photons m-2 day-1, respectively); these light treatments were combined with two [CO2] levels (ca. 437 or 705 µmol mol-1, respectively). Under eCa/HL, increased gs was closely accompanied by increases in branch and leaf hydraulic conductances, suggesting a coordinated response between liquid- and vapor-phase water flows throughout the plant. Still under HL, eCa also resulted in increased Huber value (sapwood area-to-total leaf area), sapwood area-to-stem diameter, and root mass-to-total leaf area, thus further improving the water supply to the leaves. Our results demonstrate that Ca is a central player in coffee physiology increasing carbon gain through a close association between stomatal function and an improved hydraulic architecture under HL conditions.

Tip-to-base xylem conduit widening as an adaptation: causes, consequences, and empirical priorities.

In the stems of terrestrial vascular plants studied to date, the diameter of xylem water-conducting conduits D widens predictably with distance from the stem tip L approximating D âˆ Lb , with b ≈ 0.2. Because conduit diameter is central for conductance, it is essential to understand the cause of this remarkably pervasive pattern. We give reason to suspect that tip-to-base conduit widening is an adaptation, favored by natural selection because widening helps minimize the increase in hydraulic resistance that would otherwise occur as an individual stem grows longer and conductive path length increases. Evidence consistent with adaptation includes optimality models that predict the 0.2 exponent. The fact that this prediction can be made with a simple model of a single capillary, omitting much biological detail, itself makes numerous important predictions, e.g. that pit resistance must scale isometrically with conduit resistance. The idea that tip-to-base conduit widening has a nonadaptive cause, with temperature, drought, or turgor limiting the conduit diameters that plants are able to produce, is less consistent with the data than an adaptive explanation. We identify empirical priorities for testing the cause of tip-to-base conduit widening and underscore the need to study plant hydraulic systems leaf to root as integrated wholes.

The interplay between irrigation and fruiting on branch growth and mortality, gas exchange and water relations of coffee trees.

The overall coordination between gas exchanges and plant hydraulics may be affected by soil water availability and source-to-sink relationships. Here we evaluated how branch growth and mortality, leaf gas exchange and metabolism are affected in coffee (Coffea arabica L.) trees by drought and fruiting. Field-grown plants were irrigated or not, and maintained with full or no fruit load. Under mild water deficit, irrigation per se did not significantly impact growth but markedly reduced branch mortality in fruiting trees, despite similar leaf assimilate pools and water status. Fruiting increased net photosynthetic rate in parallel with an enhanced stomatal conductance, particularly in irrigated plants. Mesophyll conductance and maximum RuBisCO carboxylation rate remained unchanged across treatments. The increased stomatal conductance in fruiting trees over nonfruiting ones was unrelated to internal CO2 concentration, foliar abscisic acid (ABA) levels or differential ABA sensitivity. However, stomatal conductance was associated with higher stomatal density, lower stomatal sensitivity to vapor pressure deficit, and higher leaf hydraulic conductance and capacitance. Increased leaf transpiration rate in fruiting trees was supported by coordinated alterations in plant hydraulics, which explained the maintenance of plant water status. Finally, by preventing branch mortality, irrigation can mitigate biennial production fluctuations and improve the sustainability of coffee plantations.

Costs and benefits of gas inside wood and its relationship with anatomical traits: a contrast between trees and lianas.

Gas inside wood plays an important role in plant functioning, but there has been no study examining the adaptive nature of gas inside wood across plants differing in biomechanical demands. Using a comparative approach, we measured gas volumetric content, xylem's anatomical traits and wood density of 15 tree and 16 liana species, to test whether gas content varies between these plant types strongly differing in their biomechanical demands. We asked (i) whether trees and lianas differ in gas content and (ii) how anatomical traits and wood density are related to gas content. Lianas had significantly less gas content in their branches compared with tree species. In tree species, gas content scaled positively with fiber, vessel and xylem cross-sectional area and fiber and vessel diameter, and negatively with dry-mass density. When pooling trees and lianas together, fiber cross-sectional area was the strongest predictor of gas content, with higher xylem cross-sectional area of fiber associated with higher gas content. In addition, we showed, through a simple analytical model, that gas inside wood increases the minimum branch diameter needed to prevent rupture, and this effect was stronger on trees compared with lianas. Our results support the view that gas inside wood plays an important role in the evolution of biomechanical functioning in different plant forms. Gas inside wood may also play an important role in physiological activities such as water transport, storage, photosynthesis and respiration, but it is still unknown whether these roles are or are not secondary to the mechanical support.

The Pneumatron: An automated pneumatic apparatus for estimating xylem vulnerability to embolism at high temporal resolution.

Xylem vulnerability to embolism represents an important trait to determine species distribution patterns and drought resistance. However, estimating embolism resistance frequently requires time-consuming and ambiguous hydraulic lab measurements. Based on a recently developed pneumatic method, we present and test the "Pneumatron", a device that generates high time-resolution and fully automated vulnerability curves. Embolism resistance is estimated by applying a partial vacuum to extract air from an excised xylem sample, while monitoring the pressure change over time. Although the amount of gas extracted is strongly correlated with the percentage loss of xylem conductivity, validation of the Pneumatron was performed by comparison with the optical method for Eucalyptus camaldulensis leaves. The Pneumatron improved the precision of the pneumatic method considerably, facilitating the detection of small differences in the (percentage of air discharged [PAD] < 0.47%). Hence, the Pneumatron can directly measure the 50% PAD without any fitting of vulnerability curves. PAD and embolism frequency based on the optical method were strongly correlated (r2 = 0.93) for E. camaldulensis. By providing an open source platform, the Pneumatron represents an easy, low-cost, and powerful tool for field measurements, which can significantly improve our understanding of plant-water relations and the mechanisms behind embolism.

Lignin composition is related to xylem embolism resistance and leaf life span in trees in a tropical semiarid climate.

Wood properties influence the leaf life span (LL) of tree crowns. As lignin is an important component of wood and the water transport system, we investigated its relationship with embolism resistance and the LL of several tree species in a seasonally dry tropical ecosystem. We determined total lignin and the monomer contents of guaiacyl (G) and syringyl (S) and related them to wood traits and xylem vulnerability to embolism (Ψ50 ) for the most common species of the Brazilian semiarid, locally known as Caatinga. Leaf life span was negatively related to Ψ50 and positively related to S : G, which was negatively related to Ψ50 . This means that greater S : G increases LL by reducing Ψ50 . Lignin content was not correlated with any variable. We found two apparently unrelated axes of drought resistance. One axis, associated with lignin monomeric composition, increases LL in the dry season as a result of lower xylem embolism vulnerability. The other, associated with wood density and stem water content, helps leafless trees to withstand drought and allows them to resprout at the end of the dry season. The monomeric composition of lignin (S : G) is therefore an important functional wood attribute affecting several key functional aspects of tropical tree species in a semiarid climate.

Plant water use responses along secondary forest succession during the 2015-2016 El Niño drought in Panama.

Tropical forests are increasingly being subjected to hotter, drier conditions as a result of global climate change. The effects of drought on forests along successional gradients remain poorly understood. We took advantage of the 2015-2016 El Niño event to test for differences in drought response along a successional gradient by measuring the sap flow in 76 trees, representing 42 different species, in 8-, 25- and 80-yr-old secondary forests in the 15-km2 'Agua Salud Project' study area, located in central Panama. Average sap velocities and sapwood-specific hydraulic conductivities were highest in the youngest forest. During the dry season drought, sap velocities increased significantly in the 80-yr-old forest as a result of higher evaporative demand, but not in younger forests. The main drivers of transpiration shifted from radiation to vapor pressure deficit with progressing forest succession. Soil volumetric water content was a limiting factor only in the youngest forest during the dry season, probably as a result of less root exploration in the soil. Trees in early-successional forests displayed stronger signs of regulatory responses to the 2015-2016 El Niño drought, and the limiting physiological processes for transpiration shifted from operating at the plant-soil interface to the plant-atmosphere interface with progressing forest succession.

Xylem hydraulic safety and construction costs determine tropical tree growth.

Faster growth in tropical trees is usually associated with higher mortality rates, but the mechanisms underlying this relationship are poorly understood. In this study, we investigate how tree growth patterns are linked with environmental conditions and hydraulic traits, by monitoring the cambial growth of 9 tropical cloud forest tree species coupled with numerical simulations using an optimization model. We find that fast-growing trees have lower xylem safety margins than slow-growing trees and this pattern is not necessarily linked to differences in stomatal behaviour or environmental conditions when growth occurs. Instead, fast-growing trees have xylem vessels that are more vulnerable to cavitation and lower density wood. We propose the growth - xylem vulnerability trade-off represents a wood hydraulic economics spectrum similar to the classic leaf economic spectrum, and show through numerical simulations that this trade-off can emerge from the coordination between growth rates, wood density, and xylem vulnerability to cavitation. Our results suggest that vulnerability to hydraulic failure might be related with the growth-mortality trade-off in tropical trees, determining important life history differences. These findings are important in furthering our understanding of xylem hydraulic functioning and its implications on plant carbon economy.

Differences in xylem and leaf hydraulic traits explain differences in drought tolerance among mature Amazon rainforest trees.

Considerable uncertainty surrounds the impacts of anthropogenic climate change on the composition and structure of Amazon forests. Building upon results from two large-scale ecosystem drought experiments in the eastern Brazilian Amazon that observed increases in mortality rates among some tree species but not others, in this study we investigate the physiological traits underpinning these differential demographic responses. Xylem pressure at 50% conductivity (xylem-P50 ), leaf turgor loss point (TLP), cellular osmotic potential (πo ), and cellular bulk modulus of elasticity (ε), all traits mechanistically linked to drought tolerance, were measured on upper canopy branches and leaves of mature trees from selected species growing at the two drought experiment sites. Each species was placed a priori into one of four plant functional type (PFT) categories: drought-tolerant versus drought-intolerant based on observed mortality rates, and subdivided into early- versus late-successional based on wood density. We tested the hypotheses that the measured traits would be significantly different between the four PFTs and that they would be spatially conserved across the two experimental sites. Xylem-P50 , TLP, and πo , but not ε, occurred at significantly higher water potentials for the drought-intolerant PFT compared to the drought-tolerant PFT; however, there were no significant differences between the early- and late-successional PFTs. These results suggest that these three traits are important for determining drought tolerance, and are largely independent of wood density-a trait commonly associated with successional status. Differences in these physiological traits that occurred between the drought-tolerant and drought-intolerant PFTs were conserved between the two research sites, even though they had different soil types and dry-season lengths. This more detailed understanding of how xylem and leaf hydraulic traits vary between co-occuring drought-tolerant and drought-intolerant tropical tree species promises to facilitate a much-needed improvement in the representation of plant hydraulics within terrestrial ecosystem and biosphere models, which will enhance our ability to make robust predictions of how future changes in climate will affect tropical forests.

Plant pneumatics: stem air flow is related to embolism - new perspectives on methods in plant hydraulics.

Wood contains a large amount of air, even in functional xylem. Air embolisms in the xylem affect water transport and can determine plant growth and survival. Embolisms are usually estimated with laborious hydraulic methods, which can be prone to several artefacts. Here, we describe a new method for estimating embolisms that is based on air flow measurements of entire branches. To calculate the amount of air flowing out of the branch, a vacuum was applied to the cut bases of branches under different water potentials. We first investigated the source of air by determining whether it came from inside or outside the branch. Second, we compared embolism curves according to air flow or hydraulic measurements in 15 vessel- and tracheid-bearing species to test the hypothesis that the air flow is related to embolism. Air flow came almost exclusively from air inside the branch during the 2.5-min measurements and was strongly related to embolism. We propose a new embolism measurement method that is simple, effective, rapid and inexpensive, and that allows several measurements on the same branch, thus opening up new possibilities for studying plant hydraulics.