Vessel-length determination using silicone and air injection: are there artifacts?

Hydraulic traits and photosynthesis are coordinated with trunk sapwood capacitance in tropical tree species

Water stored in trunk sapwood is vital for the canopy to maintain its physiological function under high transpiration demands. Little is known regarding the anatomical properties that contribute to the hydraulic capacitance of tree trunks and whether trunk capacitance is correlated with the hydraulic and gas exchange traits of canopy branches. We examined sapwood capacitance, xylem anatomical characteristics of tree trunks, embolism resistance, the minimal xylem water potential of canopy branches, leaf photosynthesis and stomatal conductance in 22 species from a tropical seasonal rainforest and savanna. The results showed that the mean trunk sapwood capacitance did not differ between the two biomes. Capacitance was closely related to the fiber lumen fraction and fiber wall reinforcement and not to the axial and ray parenchyma fractions. Additionally, it was positively correlated with the theoretical hydraulic conductivity of a trunk and the specific hydraulic conductivity of branches, and showed a trade-off with branch embolism resistance. Species with a high trunk sapwood capacitance maintained less negative canopy water potentials in the dry season, but higher leaf photosynthetic rates and stomatal conductance in the wet season. This study provides a functional link among trunk sapwood capacitance, xylem anatomy, canopy hydraulics and photosynthesis in tropical trees.

Functional xylem characteristics associated with drought-induced embolism in angiosperms

Hydraulic failure resulting from drought-induced embolism in the xylem of plants is a key determinant of reduced productivity and mortality. Methods to assess this vulnerability are difficult to achieve at scale, leading to alternative metrics and correlations with more easily measured traits. These efforts have led to the longstanding and pervasive assumed mechanistic link between vessel diameter and vulnerability in angiosperms. However, there are at least two problems with this assumption that requires critical re-evaluation: (1) our current understanding of drought-induced embolism does not provide a mechanistic explanation why increased vessel width should lead to greater vulnerability, and (2) the most recent advancements in nanoscale embolism processes suggest that vessel diameter is not a direct driver. Here, we review data from physiological and comparative wood anatomy studies, highlighting the potential anatomical and physicochemical drivers of embolism formation and spread. We then put forward key knowledge gaps, emphasising what is known, unknown and speculation. A meaningful evaluation of the diameter-vulnerability link will require a better mechanistic understanding of the biophysical processes at the nanoscale level that determine embolism formation and spread, which will in turn lead to more accurate predictions of how water transport in plants is affected by drought.

The theory behind vessel length determination using gas flow rates and comparison between two pneumatic methods based on seven woody species

In plants, xylem vessel length is important for long-distance water transport; however, the currently used methods for vessel length measurement are inconvenient and time-consuming. The recently developed semi-automated Pneumatron is a device based on the pneumatic theory that is similar to the air-injection method, and can rapidly estimate vessel length. Mean vessel length was compared between the Pneumatron and the air-injection method in seven woody species with a wide range of vessel lengths (2.3-78.7 cm). The results were consistent between the two methods, regardless of whether the same or different samples were used. The theory underlying the gas flow in vessels was improved and expanded, and compared to that underlying the water flow in order to better understand the pneumatic processes within a stem sample. Moreover, a new and simple equation for gas flow in vessels was derived based on the molar gas flow (mol s-1) rather than volume flow, because the former remains constant with distance throughout the stem axis. We strongly recommend using the Pneumatron in future studies owing to its low cost, convenience, rapidity, and simple operation. However, a number of potential issues need to be considered to avoid artifacts during measurements.

Hydraulic vulnerability segmentation in compound-leaved trees: Evidence from an embolism visualization technique

The hydraulic vulnerability segmentation (HVS) hypothesis implies the existence of differences in embolism resistance between plant organs along the xylem pathway and has been suggested as an adaptation allowing the differential preservation of more resource-rich tissues during drought stress. Compound leaves in trees are considered a low-cost means of increasing leaf area and may thus be expected to show evidence of strong HVS, given the tendency of compound-leaved tree species to shed their leaf units during drought. However, the existence and role of HVS in compound-leaved tree species during drought remain uncertain. We used an optical visualization technique to estimate embolism occurrence in stems, petioles, and leaflets of shoots in two compound-leaved tree species, Manchurian ash (Fraxinus mandshurica) and Manchurian walnut (Juglans mandshurica). We found higher (less negative) water potentials corresponding to 50% loss of conductivity (P50) in leaflets and petioles than in stems in both species. Overall, we observed a consistent pattern of stem > petiole > leaflet in terms of xylem resistance to embolism and hydraulic safety margins (i.e. the difference between mid-day water potential and P50). The coordinated variation in embolism vulnerability between organs suggests that during drought conditions, trees benefit from early embolism and subsequent shedding of more expendable organs such as leaflets and petioles, as this provides a degree of protection to the integrity of the hydraulic system of the more carbon costly stems. Our results highlight the importance of HVS as an adaptive mechanism of compound-leaved trees to withstand drought stress.

Variations in leaf water status and drought tolerance of dominant tree species growing in multi-aged tropical forests in Thailand

Large-scale abandoned agricultural areas in Southeast Asia resulted in patches of forests of multiple successions and characteristics, challenging the study of their responses to environmental changes, especially under climatic water stress. Here, we investigated seasonal variation in leaf water status and drought tolerance of dominant tree species in three multi-aged tropical forests, ranging from 5 to > 200 years old, with contrasting soil moisture in Thailand. Seasonal variation in leaf water status differed among the forests with trees in young and intermediate sites demonstrating larger differences between seasons than the old-growth forest. Although vulnerability to embolism curves revealed that trees in old-growth forest were potentially more sensitive to declining leaf water status than others, they were predicted to lose < 5% of their hydraulic capacity as opposed to 13% for the trees in the younger sites. Our results suggest that the responses to water stress of tree species in different forest ages greatly vary with a tendency of trees in younger sites to be more resilience than those in older sites. Such information would benefit the selection of tree species that could adapt well to specific environments, thus improving the strategies for managing forests of different ages under a warmer future.

Response of four evergreen savanna shrubs to an incidence of extreme drought: high embolism resistance, branch shedding and maintenance of nonstructural carbohydrates

Extreme drought events are becoming frequent globally, resulting in widespread plant mortality and forest dieback. Although savanna vegetation cover ~20% of the earth's land area, their responses to extreme drought have been less studied than that of forests. Herein, we quantified branch dieback, individual mortality and the associated physiological responses of four evergreen shrubs (Tarenna depauperate Hutch., Maytenus esquirolii (H. Lév.) C.Y. Cheng, Murraya exotica L., Jasminum nudiflorum Lindl.) in a savanna ecosystem in Southwest China to an incidence of extreme drought during 2019 and 2020. We found that 80-100% of the individuals of these species exhibited branch dieback, whereas individual mortality was only found in T. depauperate (4.5%). All species showed high resistance to stem embolism (P50, water potential at 50% loss of hydraulic conductivity ranged from -5.62 to -8.6 MPa), whereas the stem minimum water potentials reached -7.6 to ca -10.0 MPa during the drought. The low water potential caused high native embolism levels (percentage loss of hydraulic conductivity (PLC) 23-65%) in terminal branches, and the remaining stems maintained 15-35% PLC at the end of the drought. Large within-individual variations in stem vulnerability to embolism were detected, and shedding of vulnerable branches could be a mechanism for shrubs to reduce water and carbon consumption. Overall, the content of total nonstructural carbohydrates (NSC) and their components in the stem were generally comparable to or higher than those in the rainy season in three of the four species. Because the leaves were turgor-less for most time during the drought, high NSC levels during the drought could be due to recycling of NSC from dead branches or translocation from roots. Our results suggest high tolerance of savanna shrub species to extreme drought, which could be facilitated by high embolism resistance in some stems and shedding of vulnerable branches to maintain individual water and carbon balance.

Quantifying vulnerability to embolism in tropical trees and lianas using five methods: can discrepancies be explained by xylem structural traits?

Vulnerability curves (VCs) describe the loss of hydraulic conductance against increasing xylem tension, providing valuable insights about the response of plant water transport to water stress. Techniques to construct VCs have been developed and modified continuously, but controversies continue. We compared VCs constructed using the bench-top dehydration (BD), air-injection-flow (AI), pneumatic-air-discharge (PAD), optical (OP) and X-ray-computed microtomography (MicroCT) methods for tropical trees and lianas with contrasting vessel lengths. The PAD method generated highly vulnerable VCs, the AI method intermediate VCs, whereas the BD, OP and MicroCT methods produced comparable and more resistant VCs. Vessel-length and diameter accounted for the overestimation ratio of vulnerability estimated using the AI but not the PAD method. Compared with directly measured midday embolism levels, the PAD and AI methods substantially overestimated embolism, whereas the BD, MicroCT and OP methods provided more reasonable estimations. Cut-open vessels, uncertainties in maximum air volume estimations, sample-length effects, tissue cracks and shrinkage together may impede the reliability of the PAD method. In conclusion, we validate the BD, OP and MicroCT methods for tropical plants, whereas the PAD and AI need further mechanistic testing. Therefore, applications of VCs in estimating plant responses to drought need to be cautious.