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1. Seasonal patterns of increases in stem girth, vessel development, and hydraulic function in deciduous tree species

   https://doi.org/10.1093/aob/mcac032  

   Valdovinos-Ayala, Jessica ; Robles, Catherine ; Fickle, Jaycie C. ; Pérez-de-Lis, Gonzalo ; Pratt, R. Brandon ; Jacobsen, Anna L. ( March 2022 , Annals of Botany)

   The onset of spring growth and vessel formation were examined within three deciduous woody plant species, Acer rubrum, Populus balsamifera ssp. trichocarpa and Quercus rubra. We were broadly interested in the lag between the onset of girth expansion and the formation of mature and hydraulically conductive vessels within the new xylem.

   Dendrometers were installed on 20 trees (6–7 per species), and expansion of both bole and distal stems was monitored throughout the growing season in a common garden. For each species, four to six distal stems were harvested every other week for anatomical examination of vessel formation. Additionally, for Populus and Quercus, hydraulic conductivity measurements and active xylem staining were completed on all stem samples.

   For all three species, the timing of girth expansion was similar. Expansion of distal branches occurred 12–37 d earlier than that of the bole. Vessel formation initiated several weeks prior to leaf-out, but no new earlywood vessels were mature at the time of bud break for Acer and Populus and only a few were present in Quercus. Initial stem girth expansion occurred 2 to >6 weeks before the maturation of the first current-year vessels, and there was an additional delay of up to 4 weeks before mature vessels became hydraulically functional. Hydraulic conductivity was strongly correlated with the number and diameter of stained vessels.

   Bud break and leaf expansion relied predominantly on water supplied by vessels formed during prior seasons. Early-season activity is likely affected by the function of older xylem vessels and the environmental factors that influence their structure and function. Understanding the functional lifespan of vessels and the varying contributions of new and older vessels to conductivity are critical to understanding of the phenology and vascular function of long-lived woody plants in response to changing climates.

    

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2. What is the fate of xylem‐transported CO 2 in Kranz‐type C 4 plants?

   https://doi.org/10.1111/nph.15908  

   Stutz, Samantha S. ; Hanson, David T. ( June 2019 , New Phytologist)

   High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C₃plants exit leaves in the dark. In the light, C₃species use a small portion of xylem‐transported CO₂for leaf photosynthesis. However, it is not known if xylem‐transported CO₂will exit leaves in the dark or be used for photosynthesis in the light in Kranz‐type C₄plants.

   Cut leaves ofAmaranthus hypochondriacuswere placed in one of three solutions of [NaH¹³CO₃] dissolved in KCl water to measure the efflux of xylem‐transported CO₂exiting the leaf in the dark or rates of assimilation of xylem‐transported CO₂* in the light, in real‐time, using a tunable diode laser absorption spectroscope.

   In the dark, the efflux of xylem‐transported CO₂increased with increasing rates of transpiration and [¹³CO₂*]; however, rates of¹³C_effluxinA. hypochondriacuswere lower compared to C₃species. In the light,A. hypochondriacusfixed nearly 75% of the xylem‐transported CO₂supplied to the leaf.

   Kranz anatomy and biochemistry likely influence the efflux of xylem‐transported CO₂out of cut leaves ofA. hypochondriacusin the dark, as well as the use of xylem‐transported CO₂* for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz‐type C₄species over C₃species.

    

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3. Stem and leaf xylem of angiosperm trees experiences minimal embolism in temperate forests during two consecutive summers with moderate drought

   https://doi.org/10.1111/plb.13384  

   Guan, X. ; Werner, J. ; Cao, K.‐F. ; Pereira, L. ; Kaack, L. ; McAdam, S. A. ; Jansen, S. ( December 2022 , Plant Biology)

   Drought events may increase the likelihood that the plant water transport system becomes interrupted by embolism. Yet our knowledge about the temporal frequency of xylem embolism in the field is frequently lacking, as it requires detailed, long‐term measurements.

   We measured xylem embolism resistance and midday xylem water potentials during the consecutive summers of 2019 and 2020 to estimate maximum levels of embolism in leaf and stem xylem of ten temperate angiosperm tree species. We also studied vessel and pit membrane characteristics based on light and electron microscopy to corroborate potential differences in embolism resistance between leaves and stems.

   Apart fromA.pseudoplatanusandQ.petraea, eight species experienced minimum xylem water potentials that were close to or below those required to initiate embolism. Water potentials corresponding to ca. 12% loss of hydraulic conductivity (PLC) could occur in six species, while considerable levels of embolism around 50% PLC were limited toB.pendulaandC.avellana. There was a general agreement in embolism resistance between stems and leaves, with leaves being equally or more resistant than stems. Also, xylem embolism resistance was significantly correlated to intervessel pit membrane thickness (T_PM) for stems, but not to vessel diameter and total intervessel pit membrane surface area of a vessel.

   Our data indicate that low amounts of embolism occur in most species during moderate summer drought, and that considerable levels of embolism are uncommon. Moreover, our experimental andT_PMdata show that leaf xylem is generally no more vulnerable than stem xylem.

    

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4. A temporal shift in resource allocation facilitates flowering before leaf out and spring vessel maturation in precocious species

   https://doi.org/10.1002/ajb2.1222  

   Savage, Jessica A. ( January 2019 , American Journal of Botany)

   New growth in the spring requires resource mobilization in the vascular system at a time when xylem and phloem function are often reduced in seasonally cold climates. As a result, the timing of leaf out and/or flowering could depend on when the vascular system resumes normal function in the spring. This study investigated whether flowering time is influenced by vascular phenology in plants that flower precociously before they have leaves.

   Flower, leaf, and vascular phenology were monitored in pairs of precocious and non‐precocious congeners. Differences in resource allocation were quantified by measuring bud dry mass and water content throughout the year, floral hydration was modelled, and a girdling treatment completed on branches in the field.

   Precocious flowering species invested more in floral buds the year before flowering than did their non‐precocious congeners, thus mobilizing less water in the spring, which allowed flowering before new vessel maturation.

   A shift in the timing of resource allocation in precocious flowering plants allowed them to flower before the production of mature vessels and minimized the significance of seasonal changes in vascular function to their flowering phenology. The low investment required to complete floral development in the spring when the plant vascular system is often compromised could explain why flowers can emerge before leaf out.

    

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5. Xylem form and function under extreme nutrient limitation: an example from California's pygmy forest

   https://doi.org/10.1111/nph.16405  

   Cary, Katharine L. ; Ranieri, Gina M. ; Pittermann, Jarmila ( February 2020 , New Phytologist)

   Xylem anatomy and function have large implications for plant growth as well as survival during drought, but the impact of nutrient limitation on xylem is not fully understood. This study examines the pygmy forest in California, a plant community that experiences negligible water stress but is severely stunted by low‐nutrient and acidic soil, to investigate how nutrient limitation affects xylem function.

   Thirteen key anatomical and hydraulic traits of stems of four species were compared between pygmy forest plants and nearby conspecifics growing on richer soil.

   Resistance to cavitation (P₅₀), a critical trait for predicting survival during drought, had highly species‐specific responses: in one species, pygmy plants had a 26% decrease in cavitation resistance compared to higher‐nutrient conspecifics, while in another species, pygmy plants had a 56% increase in cavitation resistance. Other traits responded to nutrient limitation more consistently: pygmy plants had smaller xylem conduits and higher leaf‐specific conductivity (K_L) than conspecific controls.

   Edaphic stress, even in the absence of water stress, altered xylem structure and thus had substantial impacts on water transport. Importantly, nutrient limitation shifted cavitation resistance in a species‐specific and unpredictable manner; this finding has implications for the assessment of cavitation resistance in other natural systems.

    

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