Tissue Structure Changes of Aquilaria sinensis Xylem after Fungus Induction

In this study, we analyzed the mechanism and the process of fungal-induced agarwood formation in Aquilaria sinensis and studied the functional changes in the xylem structure after the process. The microscopic structure of the white zone, transition zone, agarwood zone, and decay zone of 12-and 18-months of inoculation A. sinensis xylem was studied. The distribution of nuclei, starch grains, soluble sugars, sesquiterpenes, fungal propagules, and mycelium in xylem tissues was investigated by histochemical analysis. The results show that the process of agarwood formation was accompanied by apoptosis of parenchyma cells such as interxylary phloem, xylem rays, and axial parenchyma. Regular changes in the conversion of starch grains to soluble sugars, the production of sesquiterpenoids, and other characteristic components of agarwood in various types of parenchyma cells were also observed. The material transformation was concentrated in the interxylary phloem, providing a structural and material basis for the formation of agarwood. It is the core part of the production of sesquiterpenoids and other characteristic products of agarwood. Compared with the A. sinensis inoculated for 12 months, the xylem of the A. sinensis inoculated for 18 months was more vigorous. There were no significant differences between the 12 and 18 months of inoculation in terms of sugars and agarwood characteristic products. In production, harvesting after 12 months of inoculation can improve harvesting efficiency.

Trade-offs among transport, support, and storage in xylem from shrubs in a semiarid chaparral environment tested with structural equation modeling

The xylem in plants is specialized to transport water, mechanically support the plant body, and store water and carbohydrates. Balancing these functions leads to trade-offs that are linked to xylem structure. We proposed a multivariate hypothesis regarding the main xylem functions and tested it using structural equation modeling. We sampled 29 native shrub species from field sites in semiarid Southern California. We quantified xylem water transport (embolism resistance and transport efficiency), mechanical strength, storage of water (capacitance) and starch, minimum hydrostatic pressures (Pmin), and proportions of fibers, vessels, and parenchyma, which were treated as a latent variable representing “cellular trade-offs.” We found that xylem functions (transport, mechanical support, water storage, and starch storage) were independent, a result driven by Pmin. Pmin was strongly and directly or indirectly associated with all xylem functions as a hub trait. More negative Pmin was associated with increased embolism resistance and tissue strength and reduced capacitance and starch storage. We found strong support for a trade-off between embolism resistance and transport efficiency. Tissue strength was not directly associated with embolism resistance or transport efficiency, and any associations were indirect involving Pmin. With Pmin removed from the model, cellular trade-offs were central and related to all other traits. We conclude that xylem traits are broadly governed by functional trade-offs and that the Pmin experienced by plants in the field exerts a strong influence over these relationships. Angiosperm xylem contains different cell types that contribute to different functions and that underpin trade-offs.

Variation in Tracheid Dimensions of Conifer Xylem Reveals Evidence of Adaptation to Environmental Conditions

Background: Globally distributed extant conifer species must adapt to various environmental conditions, which would be reflected in their xylem structure, especially in the tracheid characteristics of earlywood and latewood. A comparative study of conifer species might shed light on how xylem structure responds to environmental conditions. With an anatomical trait dataset of 79 conifer tree species growing throughout China, an interspecific study within a phylogenetic context was conducted to quantify variance of tracheid dimensions and their response to climatic and soil conditions. Results: There was a significant difference in tracheid diameter between early- and latewood while no significant difference was detected in tracheid wall thickness through a phylogenetically paired t-test. Most of the tracheid dimensional traits were positively related to each other based on phylogenetic independent contrast (PIC) analyses, and tracheid structure could be accounted for by the first and second PCA axes. Through a phylogenetic principle component analysis (pPCA), Pinaceae species were found to be strongly divergent in their tracheid structure in contrast to a conservative tracheid structure in species of Cupressaceae, Taxaceae and Podocarpaceae. Meanwhile, tracheid wall thickness decreased from high to low latitudes in both earlywood and latewood, with tracheid diameter decreasing for latewood only. According to the most parsimonious phylogenetic general least square models (PGLS), environment and phylogeny together could explain about 21%~56% of tracheid structure variance, suggesting both genetics and the environment contribute to tracheid characteristics. Conclusions: The large variability of tracheid traits observed along an environmental gradient across China suggests that xylem structure was strongly constrained by the environmental conditions in temperate monsoonal climates and thus could be regarded as an ecological strategy for adapting to environmental stresses, especially freezing and drought. Our results provide insights into the effects of climate and soil on the xylem structure of conifer species thus furthering our understanding of the trees’ response to global change and guiding forest management.

Conductivity of the phloem in mango (Mangifera indica L.)

Mango (Mangifera indica L., Anacardiaceae), the fifth most consumed fruit worldwide, is one of the most important fruit crops in tropical regions, but its vascular anatomy is quite unexplored. Previous studies examined the xylem structure in the stems of mango, but the anatomy of the phloem has remained elusive, leaving the long-distance transport of photoassimilates understudied. We combined fluorescence and electron microscopy to evaluate the structure of the phloem tissue in the tapering branches of mango trees, and used this information to describe the hydraulic conductivity of its sieve tube elements following current models of fluid transport in trees. We revealed that the anatomy of the phloem changes from current year branches, where it was protected by pericyclic fibres, to older ones, where the lack of fibres was concomitant with laticiferous canals embedded in the phloem tissue. Callose was present in the sieve plates, but also in the walls of the phloem sieve cells, making them discernible from other phloem cells. A scaling geometry of the sieve tube elements—including the number of sieve areas and the pore size across tapering branches—resulted in an exponential conductivity towards the base of the tree. These evaluations in mango fit with previous measurements of the phloem architecture in the stems of forest trees, suggesting that, despite agronomic management, the phloem sieve cells scale with the tapering branches. The pipe model theory applied to the continuous tubing system of the phloem appears as a good approach to understand the hydraulic transport of photoassimilates in fruit trees.

Light microscopy of wood using sanded surface instead of slides

Xylem anatomy is fundamental in studies of the evolution of terrestrial plants, tree ecophysiology, forestry, and wood science. Traditional xylem anatomical studies by light microscopy utilize wood sections. However, the procedures are laborious, and high-quality histological sections have been particularly challenging to achieve from recalcitrant wood species and dry wood material. Modern microscopy offers opportunities for speeding up the xylem anatomical preparations. In this regard, the merits of using a sanded surface for wood anatomical research have been largely overlooked. Sanding of wood surfaces is practiced in dendrochronology and wood identification studies exclusively for the investigation of macro features, such as tree rings, wood porosity, or parenchyma patterns. We conducted microscopic level investigations of sanded surfaces of difficult-to-section high-density woods such as Dalbergia and Quercus species by reflected white light and epifluorescence microscopy. Reflected white light or combinations of reflected light and fluorescence could clearly show xylem micro-features in sanded wood surfaces. The resolution of cell types after sanding with 1000-grit was similar to the resolution obtained by transmitted light microscopy in histological slides. The advantages of sanded wood surfaces compared to traditional wood sections can be summarized as cost- and time-effective sample preparation, large sample area, intact cell walls and tissue structure, preservation of chemical content and extractives, and even focus of the field of view. A simple procedure of wood sanding instead of microscopic slides can be used for xylem microscopy and automatic image analysis of xylem structure.

A gene that underwent adaptive evolution, LAC2 (LACCASE), in Populus euphratica improves drought tolerance by improving water transport capacity

Drought severely limits plant development and growth; accordingly, plants have evolved strategies to prevent water loss and adapt to water deficit conditions. However, experimental cases that corroborate these evolutionary processes are limited. The LACCASEs (LACs) family is involved in various plant development and growth processes. Here, we performed an evolutionary analysis of LACs from Populus euphratica and characterized the functions of LACs in Arabidopsis and poplar. The results showed that in PeuLACs, multiple gene duplications led to apparent functional redundancy as the result of various selective pressures. Among them, PeuLAC2 underwent strong positive selection. Heterologous expression analyses showed that the overexpression of PeuLAC2 alters the xylem structure of plants, including thickening the secondary cell wall (SCW) and increasing the fiber cell length and stem tensile strength. Altogether, these changes improve the water transport capacity of plants. The analysis of the physiological experimental results showed that PeuLAC2-OE lines exhibited a stronger antioxidant response and greater drought tolerance than WT. Three genes screened by transcriptome analysis, NAC025, BG1, and UGT, that are associated with SCW synthesis and drought stress were all upregulated in the PeuLAC2-OE lines, implying that the overexpression of PeuLAC2 thickened the SCW, improved the water transport capacity of the plant, and further enhanced its drought tolerance. Our study highlights that genes that have undergone adaptive evolution may participate in the development of adaptive traits in P. euphratica and that PeuLAC2 could be a candidate gene for molecular genetic breeding in trees.

Root water gates and not changes in root structure provide new insights into plant physiological responses to drought, flooding and salinity

The influence of aquaporin (AQP) activity on plant water movement remains unclear, especially in plants subject to unfavorable conditions. We applied a multitiered approach at a range of plant scales to (i) characterize the resistances controlling water transport under drought, flooding and flooding plus salinity conditions; (ii) quantify the respective effects of AQP activity and xylem structure on root (Kroot), stem (Kstem) and leaf (Kleaf) conductances, and (iii) evaluate the impact of AQP-regulated transport capacity on gas exchange. We found that drought, flooding and flooding-salinity reduced Kroot and root AQP activity in Pinus taeda, whereas Kroot of the flood-tolerant Taxodium distichum did not decline under flooding. The extent of the AQP-control of transport efficiency varied among organs and species, ranging from 35%-55% in Kroot to 10%-30% in Kstem and Kleaf. In response to treatments, AQP-mediated inhibition of Kroot rather than changes in xylem acclimation controlled the fluctuations in Kroot. The reduction in stomatal conductance and its sensitivity to vapor pressure deficit were direct responses to decreased whole-plant conductance triggered by lower Kroot and larger resistance belowground. Our results provide new mechanistic and functional insights on plant hydraulics that are essential to quantifying the influences of future stress on ecosystem function.

Conductivity of the phloem in Mangifera indica L.

ABSTRACTMangifera indica is the fifth most consumed fruit worldwide, and the most important in tropical regions, but its anatomy is quite unexplored. Previous studies examined the effect of chemicals on the xylem structure in the stems of mango, but the anatomy of the phloem has remained elusive, leaving the long distance transport of photo assimilates understudied.In this work, we used a combination of fluorescence and electron microscopy to evaluate in detail the structure of the sieve tube elements composing the phloem tissue in the tapering branches of mango trees. We then used this information to better understand the hydraulic conductivity of the sieve tubes following current models of fluid transport in trees.Our results revealed that the anatomy of the phloem in the stems changes from current year branches, where it was protected by pericyclic fibers, to older ones, where the lack of fibers was concomitant with laticiferous canals embedded in the phloem tissue. Callose was present in the sieve plates, but also in the walls of the phloem conduits, making them discernible from other phloem cells in fresh sections. A scaling geometry of the sieve tube elements, including the number of sieve areas and the pore size across tapering branches resulted in an exponential conductivity from current year branches to the base of the tree.Our measurements of the phloem in mango fit with measurements of the phloem architecture in the stems of forest woody species, and imply that, despite agronomic pruning practices, the sieve conduits of the phloem scale with the tapering branches. As a result, the pipe model theory applied to the continuous tubing system of the phloem appears as a good approach to understand the “long distance” hydraulic transport of photoassimilates in fruit trees.

Isometric scaling to model water transport in conifer tree rings across time and environments

Xylem hydraulic properties determine the ability of plants to efficiently and safely provide water to their leaves. These properties are key to understanding plant responses to environmental conditions and to evaluating their fate under a rapidly changing climate. However, their assessment is hindered by the challenges of quantifying basic hydraulic components such as bordered pits and tracheids. Here we use isometric scaling between tracheids and pits morphology to merge partial hydraulic models of tracheid’s component to upscale properties at the tree-ring level in conifers trees. Our new model output is first cross-validated with literature and then applied to cell anatomical measurements from Larix sibirica tree-rings formed under harsh conditions in southern Siberia to quantify the intra- and inter-annual variability in hydraulic properties. The model provides a means of assessing how different-sized tracheid’s components contribute to the hydraulic properties of the ring. Up-scaled results indicate that natural inter- and intra-ring anatomical variations have a substantial impact on the tree’s hydraulic properties. Our model facilitates the assessment of important xylem functional attributes because it only requires the more accessible measures of cross-sectional tracheid size. This approach, if applied to dated tree-rings, provides a novel way to investigate xylem structure-function relations across time and environmental conditions.

Differences in leaf area, trichome density, and xylem structure between the two types of Theobroma cacao l. cultivation: With or without shade plants

This study aimed to analyze the structure and density of non-glandular trichomes and the area of cocoa leaves, and the differences of xylem vessel structures on various shade tree composition. The leaf area and length, and trichomes were observed. The xylem vessel structure was observed from the root system. The result showed the area of cocoa plots without shade tree has more varied leaves size, in which upper canopy was smaller than the bottom. The area with various shade had a relatively equal size between the upper and the lower of the canopy. The three stellate-shaped non-glandular trichomes were found on the leaf venations only with the density in two plots increased with time. The xylem width area to the whole root width area ratio (k) in various shade trees was lower (k= 0.641) than that of without shade trees (k= 0.718). The higher k values indicated xylem structure without the shade tree had more xylem cells, and the pores was smaller compared to the plot with various shade trees.