An official website of the United States government
PremiseThe young seedling life stage is critical for reforestation after disturbance and for species migration under climate change, yet little is known regarding their basic hydraulic function or vulnerability to drought. Here, we sought to characterize responses to desiccation including hydraulic vulnerability, xylem anatomical traits, and impacts on other stem tissues that contribute to hydraulic functioning. MethodsLarix occidentalis,Pseudotsuga menziesii, andPinus ponderosa(all ≤6 weeks old) were imaged using x‐ray computed microtomography during desiccation to assess seedling biomechanical responses with concurrently measured hydraulic conductivity (ks) and water potential (Ψ) to assess vulnerability to xylem embolism formation and other tissue damage. ResultsIn non‐stressed samples for all species, pith and cortical cells appeared circular and well hydrated, but they started to empty and deform with decreasingΨwhich resulted in cell tearing and eventual collapse. Despite the severity of this structural damage, the vascular cambium remained well hydrated even under the most severe drought. There were significant differences among species in vulnerability to xylem embolism formation, with 78% xylem embolism inL. occidentalisbyΨof −2.1 MPa, but only 47.7% and 62.1% inP. ponderosaandP. menziesiiat −4.27 and −6.73 MPa, respectively. ConclusionsLarix occidentalisseedlings appeared to be more susceptible to secondary xylem embolism compared to the other two species, but all three maintained hydration of the vascular cambium under severe stress, which could facilitate hydraulic recovery by regrowth of xylem when stress is relieved.
more »
« less
Using heat plumes to simulate post-fire effects on cambial viability and hydraulic performance in Sequoia sempervirens stems
Abstract Injury to the xylem and vascular cambium is proposed to explain mortality following low severity fires. These tissues have been assessed independently, but the relative significance of the xylem and cambium is still uncertain. The goal of this study is to evaluate the xylem dysfunction hypothesis and cambium necrosis hypothesis simultaneously. The hot dry conditions of a low severity fire were simulated in a drying oven, exposing Sequoia sempervirens (Lamb. ex D. Don) shoots to 70 and 100 °C for 6–60 min. Cambial viability was measured with Neutral Red stain and water transport capacity was assessed by calculating the loss of hydraulic conductivity. Vulnerability curves were also constructed to determine susceptibility to drought-induced embolism following heat exposure. The vascular cambium died completely at 100 °C after only 6 min of heat exposure, while cells remained viable at 70 °C temperatures for up to 15 min. Sixty minutes of exposure to 70 °C reduced stem hydraulic conductivity by 40%, while 45 min at 100 °C caused complete loss of conductivity. The heat treatments dropped hydraulic conductivity irrecoverably but did not significantly impact post-fire vulnerability to embolism. Overall, the damaging effects of high temperature occurred more rapidly in the vascular cambium than xylem following heat exposure. Importantly, the xylem remained functional until the most extreme treatments, long after the vascular cambium had died. Our results suggest that the viability of the vascular cambium may be more critical to post-fire survival than xylem function in S. sempervirens. Given the complexity of fire, we recommend ground-truthing the cambial and xylem post-fire response on a diverse range of species.
more »
« less
- Award ID(s):
- 1925577
- PAR ID:
- 10413078
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Tree Physiology
- Volume:
- 43
- Issue:
- 5
- ISSN:
- 1758-4469
- Format(s):
- Medium: X Size: p. 769-780
- Size(s):
- p. 769-780
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
• Drought-induced xylem embolism is a primary cause of plant mortality. Although ~70% of cycads are threatened by extinction and extant cycads diversified during a period of increasing aridification, the vulnerability of cycads to embolism spread has been overlooked. • We quantified the vulnerability to drought-induced embolism, pressure-volume curves, in situ water potentials, and a suite of xylem anatomical traits of leaf pinnae and rachises for 20 cycad species. We tested whether anatomical traits were linked to hydraulic safety in cycads. • Compared to other major vascular plant clades, cycads exhibited similar embolism resistance to angiosperms and pteridophytes but were more vulnerable to embolism than non-cycad gymnosperms. All 20 cycads had both tracheids and vessels, the proportions of which were unrelated to embolism resistance. Only vessel pit membrane fraction was positively correlated to embolism resistance, contrary to angiosperms. Water potential at turgor loss was significantly correlated to embolism resistance among cycads. • Our results show that cycads exhibit low resistance to xylem embolism and that xylem anatomical traits–particularly vessels–may influence embolism resistance together with tracheids. This study highlights the importance of understanding the mechanisms of drought resistance in evolutionarily unique and threatened lineages like the cycads.more » « less
-
Shabala, Sergey (Ed.)Maintaining water transport in the xylem is critical for vascular plants to grow and survive. The drought-induced accumulation of embolism, when gas enters xylem conduits, causes declines in hydraulic conductance (K) and is ultimately lethal. Several methods can be used to estimate the degree of embolism in xylem, from measuring K in tissues to directly visualising embolism in conduits. One method allowing a direct quantification of embolised xylem area is the optical vulnerability (OV) technique. This method has been used across different organs and has a high spatial and temporal resolution. Here, we review studies using the OV technique, discuss the main advantages and disadvantages of this method, and summarise key advances arising from its use. Vulnerability curves generated by the OV method are regularly comparable to other methods, including the centrifuge and X-ray microtomography. A major advantage of the OV technique over other methods is that it can be simultaneously used to determine in situ embolism formation in leaves, stems and roots, in species spanning the phylogeny of land plants. The OV method has been used to experimentally investigate the spreading of embolism through xylem networks, associate embolism with downstream tissue death, and observe embolism formation in the field.more » « less
-
null (Ed.)Quantitative knowledge of xylem physical tolerance limits to dehydration is essential to understanding plant drought tolerance but is lacking in many long-vessel angiosperms. We examine the hypothesis that a fundamental association between sustained xylem water transport and downstream tissue function should select for xylem that avoids embolism in long-vessel trees by quantifying xylem capacity to withstand air entry of western North American oaks ( Quercus spp.). Optical visualization showed that 50% of embolism occurs at water potentials below −2.7 MPa in all 19 species, and −6.6 MPa in the most resistant species. By mapping the evolution of xylem vulnerability to embolism onto a fossil-dated phylogeny of the western North American oaks, we found large differences between clades (sections) while closely related species within each clade vary little in their capacity to withstand air entry. Phylogenetic conservatism in xylem physical tolerance, together with a significant correlation between species distributions along rainfall gradients and their dehydration tolerance, suggests that closely related species occupy similar climatic niches and that species' geographic ranges may have shifted along aridity gradients in accordance with their physical tolerance. Such trends, coupled with evolutionary associations between capacity to withstand xylem embolism and other hydraulic-related traits, yield wide margins of safety against embolism in oaks from diverse habitats. Evolved responses of the vascular system to aridity support the embolism avoidance hypothesis and reveal the importance of quantifying plant capacity to withstand xylem embolism for understanding function and biogeography of some of the Northern Hemisphere’s most ecologically and economically important plants.more » « less
-
Tognetti, Roberto (Ed.)Abstract Maintaining xylem water transport under drought is vital for plants, but xylem failure does occur when drought-induced embolisms form and progressively spread through the xylem. The hydraulic method is widely considered the gold standard to quantify drought-induced xylem embolism. The method determines hydraulic conductivity (Kh) in cut branch samples, dehydrated to specific drought levels, by pushing water through them. The technique is widely considered for its reliable Kh measurements, but there is some uncertainty in the literature over how to define stable Kh and how that relates to the degree of xylem embolism formation. Therefore, the most common setup for this method was extended to measure four parameters: (i) inlet Kh, (ii) outlet Kh, (iii) radial flow from xylem to surrounding living tissue and (iv) the pressure difference across the sample. From a strictly theoretical viewpoint, hydraulic steady state, where inflow equals outflow and radial flow is zero, will result in stable Kh. Application of the setup to Malus domestica Borkh. branches showed that achieving hydraulic steady state takes considerable time (up to 300 min) and that time to reach steady state increased with declining xylem water potentials. During each experimental run, Kh and xylem water potentials dynamically increased, which was supported by X-ray computed microtomography visualizations of embolism refilling under both high- (8 kPa) and low-pressure (2 kPa) heads. Supplying pressurized water can hence cause artificial refilling of vessels, which makes it difficult to achieve a truly stable Kh in partially embolized xylem.more » « less
