The xylem of anisohydric Quercus alba L. is more vulnerable to embolism than isohydric co-dominants

The coordination of plant leaf water potential (Ψ) regulation and xylem vulnerability to embolism is fundamental for understanding the tradeoffs between carbon uptake and risk of hydraulic damage. There is a general consensus that trees with vulnerable xylem regulate Ψ more conservatively than plants with resistant xylem. We evaluated if this paradigm applied to three important eastern US temperate tree species, Quercus alba L., Acer saccharum Marsh., and Liriodendron tulipifera L., by synthesizing 1600 Ψ observations, 122 xylem embolism curves, and xylem anatomical measurements across ten forests spanning pronounced hydroclimatological gradients and ages. We found that, unexpectedly, the species with the most vulnerable xylem (Q. alba) regulated Ψ less strictly than the other species. This relationship was found across all sites, such that coordination among traits was largely unaffected by climate and stand age. Quercus species are perceived to be among the most drought tolerant temperate US forest species; however, our results suggest their relatively loose Ψ regulation in response to hydrologic stress occurs with a substantial hydraulic cost that may expose them to novel risks in a more drought-prone future. We end by discussing mechanisms that allow these species to tolerate and/or recover from hydraulic damage.

Accuracy of the pneumatic method for estimating xylem vulnerability to embolism in temperate diffuse-porous tree species

SummaryThe increasing frequency of global change-type droughts has created a need for fast, accurate and widely applicable techniques for estimating xylem embolism resistance to improve forecasts of future forest changes.We used data from 12 diffuse-porous temperate tree species covering a wide range of xylem safety to compare the pneumatic and flow-centrifuge method for constructing xylem vulnerability curves. We evaluated the agreement between parameters estimated with both methods and the sensitivity of pneumatic measurements to the measurement duration.The agreement between xylem water potentials at 50% air discharged (PAD) estimated with the Pneumatron and 50% loss of hydraulic conductivity (PLC) estimated with the flow-centrifuge method was high (mean signed deviation: 0.12 MPa, Pearson correlation: 0.96 after 15 sec of gas extraction). However, the relation between the estimated slopes was more variable, resulting in lower agreement in xylem water potential at 12% and 88% PAD/PLC. All parameters were sensitive to the duration of the pneumatic measurement, with highest overall agreement between methods after 16 sec.We conclude that, if applied correctly, the pneumatic method enables fast and inexpensive estimations of embolism resistance for a wide range of temperate, diffuse-porous species, which makes it attractive for predicting plant performance under climate change.

A user manual to measure gas diffusion kinetics in plants: Pneumatron construction, operation and data analysis

The Pneumatron device presented measures gas diffusion kinetics in the xylem of plants. The device provides an easy, low-cost, and powerful tool for research on plant water relations. Here, we describe in detail how to construct and operate this device to estimate xylem vulnerability to embolism, and how to analyse pneumatic data. Simple and more elaborated ways of constructing a Pneumatron are shown, either using wires, a breadboard, or a printed circuit board. The instrument is based on an open-source hardware and software system, which allows users to operate it in an automated or semi-automated way. A step-by-step manual and a troubleshooting section are provided. An excel spreadsheet and an R-script are also presented for fast and easy data analysis. This manual should help new users to avoid common mistakes, especially regarding stable measurements of the minimum and maximum amount of gas that can be discharged from xylem tissue.

Plasticity of the xylem vulnerability to embolism in Populus tremula x alba relies on pit quantity properties rather than on pit structure.

Knowledge on variations of drought resistance traits are needed to predict the potential of trees to acclimate to coming severe drought events. Xylem vulnerability to embolism is a key parameter related to such droughts, and its phenotypic variability relies mainly on environmental plasticity. We investigated the structural determinants controlling the plasticity of vulnerability to embolism, focusing on the key elements involved in the air bubble entry in vessels, especially the inter-vessel pits. Poplar saplings (Populus tremula x alba) grown in contrasted water availability or light exposure exhibited differences in vulnerability to embolism (P50) in a range of 0.76 MPa. We then characterized the structural changes in features related to pit quantity and pit structure, from the pit ultrastructure to the organization of xylem vessels, using different microscopy techniques (TEM, SEM, LM). A multispectral combination of X-ray microtomography and light microscopy analysis allowed measuring the vulnerability of each single vessel and testing some of the relationships between structural traits and vulnerability to embolism inside the xylem. The pit ultrastructure did not change, whereas the vessel dimensions increased with vulnerability to embolism and the grouping index and fraction of inter-vessel cell wall both decreased with vulnerability to embolism. These findings hold when comparing between trees, or between the vessels inside the xylem of an individual tree. These results evidenced that plasticity of vulnerability to embolism in hybrid poplar occurs through changes in the pit quantity properties such as pit area and vessel grouping rather than on the pit structure.

Delayed effect of drought on the xylem vulnerability to embolism in Fagus sylvatica.

Understanding the variation in drought resistance traits is needed to predict the potential of trees to adapt to severe drought events. Xylem vulnerability to embolism is a critical trait related to drought-induced mortality with a large variability between species. Acclimation of this trait to environmental conditions implies changes in the xylem structure and organization, leading previous studies to investigate its variations under conditions preserving growth. In European beech saplings, we assessed the effect of droughts on the vulnerability to embolism in branches developed the next year during recovery. The newly formed branches displayed lower vulnerability to embolism in the plants that underwent the severest droughts leading to native embolism; the pressure that induces 50% loss of conductance being of -3.98 MPa in severely droughted plants whereas it was of ¬3.1 MPa in control plants, respectively. These results argue for a lagged acclimation of this trait to drought events.

SAFETY AND EFFICIENCY OF XYLEM WATER TRANSPORT IN TWO CASHEW (Anacardium occidentale L.) STRAINS AT THE SEEDLING STAGE

<p>ABSTRACT<br />As cashew trees are grown by transplanting seedlings, the seedling<br />often suffers from drought damaged due to prolonged dry season. Previous<br />study found that the ability to maintain water transport in xylem related to<br />drought resistant character. To determine whether there was trade-off<br />between the ability to maintain water transport in xylem and an efficiency<br />of water transport, differences in xylem vulnerability to dysfunction,<br />hydraulic conductance, and the relationship to xylem vessel diameter were<br />examined in two cashew strains. The xylem vulnerability to dysfunction<br />was evaluated by the applied pressure which induced 50% loss of stem<br />hydraulic conductivity (P 50 ). The hydraulic conductance on root, stem, and<br />leaf were determined with High Pressure Flow Meter (HPFM). Variations<br />in the P 50 values were found between A3-1 and Pangkep, whereas the<br />values were 1.75 and 0.50 MPa, respectively. However, since there was no<br />difference in the hydraulic conductance and the vessel diameter, the trade-<br />off between the ability to maintain water transport in xylem and an<br />efficiency of water transport did not occur in cashew. It was suggested that<br />good combination of efficiency and safety of water transport enables A3-1<br />to strongly uptake soil water either in dry or wet season resulting in good<br />adaptation to drought prone environment, and the P 50 value would be<br />suitable parameter for evaluating drought tolerance of cashew at the<br />seedling stage.<br />Key words: cashew strain, vessel, xylem dysfunction, hydraulic<br />conductance, drought</p><p>ABSTRAK<br />Pengembangan jambu mete secara transplanting sering diikuti<br />cekaman kekeringan pada bibit akibat musim kering yang berkepanjangan.<br />Studi awal memperlihatkan bahwa kemampuan xylem mempertahankan<br />fungsi transportasi air merupakan karakter pertahanan penting terhadap<br />cekaman kekeringan. Untuk mengetahui apakah terjadi kompensasi antara<br />kemampuan pertahanan fungsi xylem dan tingkat efisiensi transportasi<br />airnya dilakukan pengujian pada aspek kepekaan fungsi xylem, hantaran<br />hidraulik, dan ukuran vesselnya. Kepekaan fungsi xylem ditentukan dari<br />nilai tekanan udara yang menyebabkan kehilangan 50% hydraulic<br />conductance (P 50 ). Nilai hydraulic conductance pada akar, batang, dan<br />daun ditentukan dengan menggunakan metode High Pressure Flow Meter<br />(HPFM). Hasil pengujian menunjukkan terdapat perbedaan nilai P 50<br />diantara dua strain jambu mete yang diuji, yakni secara berturut-turut 1,75<br />dan 0,50 MPa pada strain A3-1 dan Pangkep. Karena tidak disertai<br />perbedaan pada hydraulic conductance dan ukuran vesselnya, maka<br />disimpulkan tidak ditemukan nilai adanya mekanisme kompensasi antara<br />kemampuan pertahanan fungsi xylem dan tingkat efisiensi pengangkutan<br />air. Hal ini memungkinkan A3-1 tetap dapat menyerap air tanah secara<br />cukup, baik pada musim kering maupun musim basah, dan mampu<br />beradaptasi dengan baik di daerah rawan kekeringan. Dan nilai P 50 dapat<br />dijadikan sebagai parameter representatif untuk evaluasi toleransi bibit<br />jambu mete terhadap cekaman kekeringan.<br />Kata kunci: strain jambu mete, vessel, fungsi xylem, hydraulic<br />conductance, cekaman kekeringan</p>

Delayed effect of drought on the xylem vulnerability to cavitation in Fagus sylvatica L.

Knowledge on variations of drought resistance traits is needed to predict the potential of trees to adapt to severe drought events expected to be more intense and frequent. Xylem vulnerability to cavitation is among the most important traits related to drought-induced mortality and exhibits a large variability between species. Acclimation of this trait to environmental conditions implies changes in the xylem structure and organization, leading previous studies to investigate its variations under conditions preserving growth. In European beech saplings, we assessed the effect of droughts of on the vulnerability to cavitation in branches that develop during recovery. The newly formed branches displayed lower vulnerability to cavitation in the plants that underwent the severest droughts leading to native embolism; the pressure that induces 50% loss of conductance being of -3.98 MPa in severely droughted plants whereas it was of -3.1 MPa in control plants, respectively. Although unexpected, these results argue for an acclimation, and not a weakening, of this trait to drought events.

Plasticity of the xylem vulnerability to embolism in poplar relies on quantitative pit properties rather than on pit structure

Knowledge on variations of drought resistance traits are needed to predict the potential of trees to acclimate to coming severe drought events. Xylem vulnerability to embolism is a key parameter related to such droughts, and its phenotypic variability relies mainly on environmental plasticity. We investigated the structural determinants controlling the plasticity of vulnerability to embolism, focusing on the key elements involved in the air bubble entry in a vessel, especially the inter-vessel pits. Poplar saplings (Populus tremula x alba) grown in contrasted water availability or light exposure exhibited differences in vulnerability to embolism in a range of 0.76 MPa. We then characterized the structural changes related to qualitative and quantitative pit characteristics, from the pit structure to the organization of xylem vessels, using different microscopy techniques (TEM, SEM, light). X-ray microtomography analysis allowed observing the vessel vulnerability and testing some of the relationships between structural traits and vulnerability to embolism inside the xylem. The pit ultrastructure did not change, whereas the vessel dimensions increased with vulnerability to embolism and the grouping index and fraction of inter-vessel cell wall decreased with vulnerability to embolism. These findings holds when comparing trees or when comparing vessels inside the xylem. These results evidenced that plasticity of vulnerability to embolism occurs through changes in the quantitative pit properties such as pit area and vessel grouping rather than on the pit structure.