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1. Hydraulic integrity of plant organs during drought stress and recovery in herbaceous and woody plant species

   https://doi.org/10.1093/jxb/erac451  

   Huber, Annika E; Melcher, Peter J; Bauerle, Taryn L (November 2022, Journal of Experimental Botany)

   Lunn, John (Ed.)

   Abstract The relationship between root, stem, and leaf hydraulic status and stomatal conductance during drought (field capacities: 100–25%) and drought recovery was studied in Helianthus annuus and five tree species (Populus×canadensis, Acer saccharum, A. saccharinum, Picea glauca, and Tsuga canadensis). Measurements of stomatal conductance (gs), organ water potential, and vessel embolism were performed and the following was observed: (i) cavitation only occurred in the petioles and not the roots or stems of tree species regardless of drought stress; (ii) in contrast, all H. annuus organs exhibited cavitation to an increasing degree from root to petiole; and (iii) all species initiated stomatal closure before cavitation events occurred or the expected turgor loss point was reached. After rewatering: (i) cavitated vessels in petioles of Acer species recovered whereas those of P. ×canadensis did not and leaves were shed; (ii) in H. annuus, cavitated xylem vessels were refilled in roots and petioles, but not in stems; and (iii) despite refilled embolisms in petioles of some species during drought recovery, gs never returned to pre-drought conditions. Conclusions are drawn with respect to the hydraulic segmentation hypothesis for above- and below-ground organs, and the timeline of embolism occurrence and repair is discussed. 

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2. Exploring within‐plant hydraulic trait variation: A test of the vulnerability segmentation hypothesis

   https://doi.org/10.1111/pce.14649  

   Wilkening, Jean V.; Skelton, Robert P.; Feng, Xue; Dawson, Todd E.; Thompson, Sally E. (September 2023, Plant, Cell & Environment)

   Abstract Observations show vulnerability segmentation between stems and leaves is highly variable within and between environments. While a number of species exhibit conventional vulnerability segmentation (stem leaf ), others exhibit no vulnerability segmentation and others reverse vulnerability segmentation (stem leaf ). We developed a hydraulic model to test hypotheses about vulnerability segmentation and how it interacts with other traits to impact plant conductance. We do this using a series of experiments across a broad parameter space and with a case study of two species with contrasting vulnerability segmentation patterns:Quercus douglasiiandPopulus trichocarpa. We found that while conventional vulnerability segmentation helps to preserve conductance in stem tissues, reverse vulnerability segmentation can better maintain conductance across the combined stem‐leaf hydraulic pathway, particularly when plants have more vulnerable s and have hydraulic segmentation with greater resistance in the leaves. These findings show that the impacts of vulnerability segmentation are dependent upon other plant traits, notably hydraulic segmentation, a finding that could assist in the interpretation of variable observations of vulnerability segmentation. Further study is needed to examine how vulnerability segmentation impacts transpiration rates and recovery from water stress. 

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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)

   Abstract 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. Seeing is believing: what visualising bubbles in the xylem has revealed about plant hydraulic function

   https://doi.org/10.1071/FP21326  

   Cardoso, Amanda A.; Kane, Cade N.; Rimer, Ian M.; McAdam, Scott A. (January 2022, Functional Plant Biology)

   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. 

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5. Transcriptome analyses of leaf architecture in Sansevieria support a common genetic toolkit in the parallel evolution of unifacial leaves in monocots

   https://doi.org/10.1002/pld3.511  

   Golenberg, Edward M; Popadić, Aleksandar; Hao, Weilong (August 2023, Plant Direct)

   Abstract Planar structures dramatically increase the surface‐area‐to‐volume ratio, which is critically important for multicellular organisms. In this study, we utilize naturally occurring phenotypic variation among threeSansivieriaspecies (Asperagaceae) to investigate leaf margin expression patterns that are associated with mediolateral and adaxial/abaxial development. We identified differentially expressed genes (DEGs) between center and margin leaf tissues in two planar‐leaf speciesSansevieria subspicataandSansevieria trifasciataand compared these with expression patterns within the cylindrically leavedSansevieria cylindrica. TwoYABBYfamily genes, homologs ofFILAMENTOUS FLOWERandDROOPING LEAF, are overexpressed in the center leaf tissue in the planar‐leaf species and in the tissue of the cylindrical leaves. As mesophyll structure does not indicate adaxial versus abaxial differentiation, increased leaf thickness results in more water‐storage tissue and enhances resistance to aridity. This suggests that the cylindrical‐leaf inS. cylindricais analogous to the central leaf tissue in the planar‐leaf species. Furthermore, the congruence of the expression patterns of theseYABBYgenes inSansevieriawith expression patterns found in other unifacial monocot species suggests that patterns of parallel evolution may be the result of similar solutions derived from a limited developmental toolbox. 

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