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Many insects feed on xylem or phloem sap of vascular plants. Although physical damage to the plant is minimal, the process of insect feeding can transmit lethal viruses and bacterial pathogens. Disparities between insect-mediated pathogen transmission efficiency have been identified among xylem sap-feeding insects; however, the mechanistic drivers of these trends are unclear. Identifying and understanding the structural factors and associated integrated functional components that may ultimately determine these disparities are critical for managing plant diseases. Here, we applied synchrotron-based X-ray microcomputed tomography to digitally reconstruct the morphology of three xylem sap-feeding insect vectors of plant pathogens: Graphocephala atropunctata (blue-green sharpshooter; Hemiptera, Cicadellidae) and Homalodisca vitripennis (glassy-winged sharpshooter; Hemiptera, Cicadellidae), and the spittlebug Philaenus spumarius (meadow spittlebug; Hemiptera, Aphrophoridae). The application of this technique revealed previously undescribed anatomical features of these organisms, such as key components of the salivary complex. The visualization of the 3D structure of the precibarial valve led to new insights into the mechanism of how this structure functions. Morphological disparities with functional implications between taxa were highlighted as well, including the morphology and volume of the cibarial dilator musculature responsible for extracting xylem sap, which has implications for force application capabilities. These morphological insights will be used to target analyses illuminating functional differences in feeding behavior., (© 2023 The Authors. Journal of Morphology published by Wiley Periodicals LLC.)

Premise: The 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., Methods: Larix occidentalis, Pseudotsuga menziesii, and Pinus ponderosa (all ≤6 weeks old) were imaged using x-ray computed microtomography during desiccation to assess seedling biomechanical responses with concurrently measured hydraulic conductivity (k s ) and water potential (Ψ) to assess vulnerability to xylem embolism formation and other tissue damage., Results: In non-stressed samples for all species, pith and cortical cells appeared circular and well hydrated, but they started to empty and deform with decreasing Ψ which 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 in L. occidentalis by Ψ of -2.1 MPa, but only 47.7% and 62.1% in P. ponderosa and P. menziesii at -4.27 and -6.73 MPa, respectively., Conclusions: Larix occidentalis seedlings 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., (© 2020 Botanical Society of America.)

Premise: Because of its broad range in the neotropical rainforest and within tree canopies, the tank bromeliad Guzmania monostachia was investigated as a model of how varying leaf hydraulic conductance (K leaf ) could help plants resist and recover from episodic drought. The two pathways of K leaf , inside and outside the xylem, were also examined to determine the sites and causes of major hydraulic resistances within the leaf., Methods: We measured leaf hydraulic conductance for plants in the field and laboratory under wet, dry, and rewetted conditions and applied physiological, anatomical, and gene expression analysis with modeling to investigate changes in K leaf ., Results: After 7 d with no rain in the field or 14 days with no water in the glasshouse, K leaf decreased by 50% yet increased to hydrated values within 4 d of tank refilling. Staining to detect embolism combined with modeling indicated that changes outside the xylem were of greater importance to K leaf than were changes inside the xylem and were associated with changes in intercellular air spaces (aerenchyma), aquaporin expression and inhibition, and cuticular conductance., Conclusions: Low values for all conductances during drying, particularly in pathways outside the xylem, lead to hydraulic resilience for this species and may also contribute to its broad environmental tolerances., (© 2019 The Authors. American Journal of Botany is published by Wiley Periodicals, Inc. on behalf of the Botanical Society of America.)

Premise: Water deficit and drought conditions are increasing in intensity, frequency, and duration in the Iberian Peninsula. We observed natural variation in leaf traits across the range of Quercus suber L. (cork oak), an ecologically important species within this region. Stomatal traits (e.g., pore length, maximum aperture) and carbon isotope composition (δ 13 C) provide an opportunity to examine the integrative effects of drought and dry-season intensity on leaf development, maximum stomatal conductance, and adaptation to precipitation regimes., Methods: Gross leaf traits (e.g., area, thickness), stomatal traits (e.g., pore length, size, aperture), and carbon isotope discrimination were measured in Q. suber leaves, and maximum stomatal conductance to water vapor (g wmax ) was calculated. Trees were sampled from nine natural populations across a climate gradient in the Iberian Peninsula, including trees from two genetic lineages. Linear mixed models compared total water deficit to leaf traits, accounting for tree and site as random effects., Results: Quercus suber gross leaf morphology remained consistent across the climate gradient, but increasing water deficit was correlated with smaller stomata at the leaf level and low δ 13 C at the tree level. No traits were significantly different between the two genetic lineages., Conclusions: While there were no significant differences in gross leaf morphology across the climate gradient or between the genetic lineages, stomatal traits and δ 13 C responded to climate, suggesting that Q. suber can inhabit a range of environments in the Iberian Peninsula via micro-adjustments of the trait that controls water loss into the atmosphere., (© 2019 Botanical Society of America.)

Premise of the Study: Despite the strong influence of the frequency and distribution of vessel endings on both hydraulic safety and efficiency, detailed anatomical descriptions or measurements of these structures are generally lacking., Methods: Here we used high-resolution x-ray microcomputed tomography (microCT) to identify and describe xylem vessel endings within Acer rubrum root segments (1.0-2.1 mm diameter, ∼2 mm long). We then compared vessel-lumen diameter, pit density, vessel element length, and perforation plate angle between non-ending vessels (those that traverse an entire segment) and those that end within a segment using three-dimensional image analysis., Key Results: We found 214 vessel endings, 37 complete vessels, and 385 non-ending vessels within four A. rubrum root segments. Vessels that ended within the segments tended to have more acute perforation plate angles and had a smaller diameter than those that did not end within the segments. Most vessel diameters tapered within the last few vessel elements, but the perforation plate angle apparently changed over longer distances. Intervessel pit density and vessel element length did not differ between ending and non-ending vessels., Conclusions: Vessel endings were surprisingly frequent in A. rubrum roots despite the common perception that root vessels are longer than vessels in other tissues. MicroCT proved to be a useful tool for studying the three-dimensional arrangement of vessel endings within xylem networks, and these data will be helpful in developing a better understanding of vessel ending microstructure and function., (© 2017 Botanical Society of America.)

Premise of the Study: Xylem network connections play an important role in water and nutrient transport in plants, but also facilitate the spread of air embolisms and xylem-dwelling pathogens. This study describes the structure and function of vessel relays found in grapevine xylem that form radial and tangential connections between spatially discrete vessels., Methods: We used high-resolution computed tomography, light microscopy, scanning electron microscopy, and single-vessel dye injections to characterize vessel relays in stems and compare their distributions and structure in two Vitis species., Key Results: Vessel relays were composed of 1-8 narrow diameter (~25 µm) vessel elements and were oriented radially, connecting vessels via scalariform pitting within a xylem sector delineated by rays. The functional connectedness of vessels linked by vessel relays was confirmed with single-vessel dye injections. In 4.5-cm sections of stem tissue, there were 26% more vessel relays in V. vinifera compared with V. arizonica. •, Conclusions: Because of their spatial distribution within Vitis xylem, vessel relays increase the connectivity between vessels that would otherwise remain isolated. Differences in vessel relays between Vitis species suggest these anatomical features could contribute to disease and embolism resistance in some species.

Many understory plants rely on diffuse light for photosynthesis because direct light is usually scattered by upper canopy layers before it strikes the forest floor. There is a considerable gap in the literature concerning the interaction of direct and diffuse light with leaves. Some understory plants have well-developed lens-shaped epidermal cells, which have long been thought to increase the absorption of diffuse light. To assess the role of epidermal cell shape in capturing direct vs. diffuse light, we measured leaf reflectance and transmittance with an integrating sphere system using leaves with flat (Begonia erythrophylla, Citrus reticulata, and Ficus benjamina) and lens-shaped epidermal cells (B. bowerae, Colocasia esculenta, and Impatiens velvetea). In all species examined, more light was absorbed when leaves were irradiated with direct as opposed to diffuse light. When leaves were irradiated with diffuse light, more light was transmitted and more was reflected in both leaf types, resulting in absorptance values 2-3% lower than in leaves irradiated with direct light. These data suggest that lens-shaped epidermal cells do not aid the capture of diffuse light. Palisade and mesophyll cell anatomy and leaf thickness appear to have more influence in the capture and absorption of light than does epidermal cell shape.

Light propagation and distribution inside leaves have been recognized as important processes influencing photosynthesis. Monochromatic light absorption across the mesophyll was measured using chlorophyll fluorescence generated from illumination of the cut edge (epi-illumination), as well as the adaxial or abaxial surfaces of the leaf. Species were selected that had basic leaf types: laminar leaf with adaxial palisade layer (Rhododendron catawbiense), needle with palisade (Abies fraseri), and needle without palisade (Picea rubens). Fluorescence was more evenly distributed across the mesophyll for adaxially illuminated leaves with a palisade cell layer, as well as for the needles (cylindrical) without palisade, when compared to fluorescence generated by abaxial illumination. Moreover, fluorescence from green light illumination remained high across the mesophyll of adaxially illuminated R. catawbiense, indicating a possible influence of mesophyll structure on internal light distribution beyond that of chlorophyll levels. These data support the idea that light propagation within the mesophyll is associated with asymmetric mesophyll structure, in particular the presence of palisade cell layers. In addition, we propose that the evolution of a more cylindrical leaf form, such as found in conifer species, may be a structural solution to excessive sunlight that replaces the highly differentiated mesophyll found in most laminar-leaved species.