Parenchyma underlies the interspecific variation of xylem hydraulics and carbon storage across 15 woody species on a subtropical island in Japan

Tree height and not climate influences intraspecific variations in wood parenchyma fractions of angiosperm species in a mountain forest of eastern China

PREMISE

Parenchyma is an important cell type in the secondary xylem of angiosperm trees, with considerable variability in its abundance. However, the functional significance of these variations and their roles in plant ecological strategies is poorly understood at the inter- and intraspecific levels.

METHODS

For this study, fractions of axial parenchyma cells (AP) and ray parenchyma cells (RP) in xylem tissue were quantified for 156 individuals of 45 tree species along an elevational gradient from 600 to 1600 m a.s.l. in eastern China. Environmental factors and height of sampled trees at each sampling site were also measured. Linear mixed models were employed to assess the relative extent of both intraspecific and interspecific variations in parenchyma cells in xylem tissue fractions and to identify intraspecific variations along environmental gradients (e.g., temperature and humidity) and tree heights.

RESULTS

Interspecific variations explained the large diversity in AP fractions. Conversely, intraspecific variations accounted for ~50% of the overall variations in RP fractions. Further, intraspecific variations in the RP and the total AP and RP fractions exhibited negative correlations with tree heights but showed no significant relationship with climate.

CONCLUSIONS

Intraspecific variations in parenchyma cells in xylem tissue fractions are not necessarily an adaptation or acclimation to changes in the environment but are coordinated with tree heights.

Physiological mechanisms and drought resistance assessment of four dominant species on the Loess Plateau under drought stress

The escalating frequency and severity of droughts have caused growth decline and increased mortality risk for plantations on the Loess Plateau. The main aim of this study was to explore the physiological mechanisms of four native dominant species during drought-induced mortality and evaluate their drought resistance capabilities. Drought was induced by withholding water from potted seedings, and we compared patterns in pit membrane damage, hydraulic function, and non-structural carbohydrates (NSC) dynamics across four tree species with distinct anatomical features. Our findings reveal species-specific vulnerability thresholds: Pinus tabulaeformis (-2.86 Mpa), Quercus liaotungensis (-1.92 Mpa), Robinia pseudoacacia (-0.109 Mpa), and Syringa reticulata (-0.93 Mpa). Additionally, drought stress was found to compromise pit membrane integrity, with water potential thresholds identified as R.pseudoacacia (-1.37 Mpa), S. reticulata (-2.20 Mpa), Q. liaotungensis (-2.39 Mpa), and P. tabulaeformis (-1.85 Mpa). The study concludes that R. pseudoacacia and S. reticulata exhibit greater susceptibility to hydraulic failure under severe drought conditions, leading to increased mortality risks. In contrast, Q. liaotungensis and P. tabulaeformis demonstrate enhanced drought tolerance and survival capacity. Our research elucidates the physiological mechanisms of drought-induced mortality, emphasizing the critical role of pit membrane damage in this process. These findings not only provide valuable insights into the drought resistance of native dominant species but also establish a scientific foundation for future artificial forest transformation initiatives on the Loess Plateau.

Mistletoes have higher hydraulic safety but lower efficiency in xylem traits than their hosts

Both mistletoes and their hosts are challenged by increasing drought, highlighting the necessity of understanding their comparative hydraulic properties. The high transpiration of mistletoes requires efficient water transport, while high xylem tensions demand strong embolism resistance, representing a hydraulic paradox. This study, conducted across four environments with different aridity indices in Yunnan, China, examined the xylem traits of 119 mistletoe-host species pairs. Mistletoes showed lower water use efficiency, indicating a more aggressive water use. They also showed lower hydraulic efficiency (lower vessel diameter and theoretical hydraulic conductivity) but higher safety (lower vulnerability index and higher conduit wall reinforcement, vessel grouping index, and wood density) compared with their hosts, supporting the trade-off between efficiency and safety. Environmental variation across sites significantly affected xylem trait comparisons between mistletoes and hosts. Additionally, the xylem traits of mistletoes were strongly influenced by host water supply efficiency. The overall xylem trait relationships in mistletoes and hosts were different. These findings stress the impact of host and site on the hydraulic traits of mistletoes, and suggest that mistletoes may achieve high transpiration by maintaining high stomatal conductance under low water potentials. This study illuminates the distinctive adaptation strategies of mistletoes due to their parasitic lifestyle.

Upside down and the game of C allocation

Non-structural carbohydrates (NSCs) represent the primary carbon (C) reserves and play a crucial role in plant functioning and resilience. Indeed, these compounds are involved in the regulation between C supply and demand, and in the maintenance of hydraulic efficiency. Non-structural carbohydrates are stored in parenchyma of woody organs, which is recognized as a proxy for reserve storage capacity of tree. Notwithstanding the importance of NSCs for tree physiology, their long-term regulation and trade-offs against growth were not deeply investigated. This work evaluated the long-term dynamics of mature tree reserves in stem and root, proxied by parenchyma features and focusing on the trade-off and interplay between the resources allocation in radial growth and reserves in stem and coarse root. In a Mediterranean beech forest, NSCs content, stem and root wood anatomy analysis and eddy covariance data were combined. The parenchyma fraction (RAP) of beech root and stem was different, due to differences in axial parenchyma (AP) and narrow ray parenchyma (nRP) fractions. However, these parenchyma components and radial growth showed synchronous inter-annual dynamics between the two organs. In beech stem, positive correlations were found among soluble sugars content and nRP and among starch content and the AP. Positive correlations were found among Net Ecosystem Exchange (NEE) and AP of both organs. In contrast, NEE was negatively correlated to radial growth of root and stem. Our results suggest a different contribution of stem and roots to reserves storage and a putative partitioning in the functional roles of parenchyma components. Moreover, a long-term trade-off of C allocation between growth and reserve pool was evidenced. Indeed, in case of C source reduction, trees preferentially allocate C toward reserves pool. Conversely, in high productivity years, growth represents the major C sink.

Adjustment of storage capacity for non-structural carbohydrates in response to limited water availability in two temperate woody species

Reserves of non-structural carbohydrates (NSC) stored in living cells are essential for drought tolerance of trees. However, little is known about the phenotypic plasticity of living storage compartments (SC) and their interactions with NSC reserves under changing water availability. Here, we examined adjustments of SC and NSC reserves in stems and roots of seedlings of two temperate tree species, Acer negundo L. and Betula pendula Roth., cultivated under different substrate water availability. We found that relative contents of soluble NSC, starch and total NSC increased with decreasing water availability in stems of both species, and similar tendencies were also observed in roots of A. negundo. In the roots of B. pendula, soluble NSC contents decreased along with the decreasing water availability, possibly due to phloem decoupling or NSC translocation to shoots. Despite the contrast in organ responses, NSC contents (namely starch) positively correlated with proportions of total organ SC. Individual types of SC showed markedly distinct plasticity upon decreasing water availability, suggesting that water availability changes the partitioning of organ storage capacity. We found an increasing contribution of parenchyma-rich bark to the total organ NSC storage capacity under decreasing water availability. However, xylem SC showed substantially greater plasticity than those in bark. Axial storage cells, namely living fibers in A. negundo, responded more sensitively to decreasing water availability than radial parenchyma. Our results demonstrate that drought-induced changes in carbon balance affect the organ storage capacity provided by living cells, whose proportions are sensitively coordinated along with changing NSC reserves.

Interspecific variation in Rubisco CO2/O2 specificity along the leaf economic spectrum across 23 woody angiosperm plants in the Pacific islands

The coordinated interspecific variation in leaf traits and leaf lifespan is known as the leaf economic spectrum (LES). The limitation of CO2 diffusion to chloroplasts within the lamina is significant in C3 photosynthesis, resulting in a shortage of CO2 for Rubisco. Although Rubisco CO2/O2 specificity (SC/O) should be adaptively adjusted in response to the interspecific variation in CO2 concentrations [CO2] associated with Rubisco, SC/O variations across species along the LES remain unknown. We investigated the coordination among leaf traits, including SC/O, CO2 conductance, leaf protein content, and leaf mass area, across 23 woody C3 species coexisting on an oceanic island through phylogenetic correlation analyses. A high SC/O indicates a high CO2 specificity of Rubisco. SC/O was negatively correlated with [CO2] at Rubisco and total CO2 conductance within lamina, while it was positively correlated with leaf protein across species, regardless of phylogenetic constraint. A simulation analysis shows that the optimal SC/O for maximizing photosynthesis depends on both [CO2] at Rubisco sites and leaf protein per unit leaf area. SC/O is a key parameter along the LES axis and is crucial for maximizing photosynthesis across species and the adaptation of woody plants.

Variations in wood anatomy in Afrotropical trees with a particular emphasis on radial and axial parenchyma

BACKGROUND AND AIMS

Understanding anatomical variations across plant phylogenies and environmental gradients is vital for comprehending plant evolution and adaptation. Previous studies on tropical woody plants have paid limited attention to quantitative differences in major xylem tissues, which serve specific roles in mechanical support (fibres), carbohydrate storage and radial conduction (radial parenchyma, rays), wood capacitance (axial parenchyma) and water transport (vessels). To address this gap, we investigate xylem fractions in 173 tropical tree species spanning 134 genera and 53 families along a 2200-m elevational gradient on Mount Cameroon, West Africa.

METHODS

We determined how elevation, stem height and wood density affect interspecific differences in vessel, fibre, and specific axial (AP) and radial (RP) parenchyma fractions. We focus on quantifying distinct subcategories of homogeneous or heterogeneous rays and apotracheal, paratracheal and banded axial parenchyma.

KEY RESULTS

Elevation-related cooling correlated with reduced AP fractions and vessel diameters, while fibre fractions increased. Lower elevations exhibited elevated AP fractions due to abundant paratracheal and wide-banded parenchyma in tall trees from coastal and lowland forests. Vasicentric and aliform AP were predominantly associated with greater tree height and wider vessels, which might help cope with high evaporative demands via elastic wood capacitance. In contrast, montane trees featured a higher fibre proportion, scarce axial parenchyma, smaller vessel diameters and higher vessel densities. The lack of AP in montane trees was often compensated for by extended uniseriate ray sections with upright or squared ray cells or the presence of living fibres.

CONCLUSIONS

Elevation gradient influenced specific xylem fractions, with lower elevations showing elevated AP due to abundant paratracheal and wide-banded parenchyma, securing greater vessel-to-parenchyma connectivity and lower embolism risk. Montane trees featured a higher fibre proportion and smaller vessel diameters, which may aid survival under greater environmental seasonality and fire risk.

Linkages among stem xylem transport, biomechanics, and storage in lianas and trees across three contrasting environments

PREMISE

Stem xylem transports water and nutrients, mechanically supports aboveground tissues, and stores water and nonstructural carbohydrates. These three functions are associated with three types of cells-vessel, fiber, and parenchyma, respectively.

METHODS

We measured stem theoretical hydraulic conductivity (Kt), modulus of elasticity (MOE), tissue water content, starch, soluble sugars, cellulose, and xylem anatomical traits in 15 liana and 16 tree species across three contrasting sites in Southwest China.

RESULTS

Lianas had higher hydraulic efficiency and tissue water content, but lower MOE and cellulose than trees. Storage traits (starch and soluble sugars) did not significantly differ between lianas and trees, and trait variation was explained mainly by site, highlighting how environment shapes plant storage strategies. Kt was significantly positively correlated with vessel diameter and vessel area fraction in lianas and all species combined. The MOE was significantly positively correlated with fiber area fraction, wood density, and cellulose in lianas and across all species. The tissue water content was significantly associated with parenchyma area fraction in lianas. Support function was strongly linked with transport and storage functions in lianas. In trees, transport and support functions were not correlated, while storage function was tightly linked with transport and support functions.

CONCLUSIONS

These findings enhance our understanding of the relationship between stem xylem structure and function in lianas and trees, providing valuable insights into how plants adapt to environmental changes and the distinct ecological strategies employed by lianas and by trees to balance the demands of hydraulic transport, mechanical support, and storage.

Water storage capacity is inversely associated with xylem embolism resistance in tropical karst tree species

Tropical karst habitats are characterized by limited and patchy soil, large rocky outcrops and porous substrates, resulting in high habitat heterogeneity and soil moisture fluctuations. Xylem hydraulic efficiency and safety can determine the drought adaptation and spatial distribution of woody plants growing in karst environments. In this study, we measured sapwood-specific hydraulic conductivity (Ks), vulnerability to embolism, wood density, saturated water content, and vessel and pit anatomical characteristics in the branch stems of 12 evergreen tree species in a tropical karst seasonal rainforest in southwestern China. We aimed to characterize the effects of structural characteristics on hydraulic efficiency and safety. Our results showed that there was no significant correlation between Ks and hydraulic safety across the tropical karst woody species. Ks was correlated with hydraulic vessel diameter (r = 0.80, P < 0.05) and vessel density (r = -0.60, P < 0.05), while the stem water potential at 50 and 88% loss of hydraulic conductivity (P50 and P88) were both significantly correlated with wood density (P < 0.05) and saturated water content (P = 0.052 and P < 0.05, respectively). High stem water storage capacity was associated with low cavitation resistance possibly because of its buffering the moisture fluctuations in karst environments. However, both Ks and P50/P88 were decoupled from the anatomical traits of pit and pit membranes. This may explain the lack of tradeoff between hydraulic safety and efficiency in tropical karst evergreen tree species. Our results suggest that diverse hydraulic trait combination may facilitate species coexistence in karst environments with high spatial heterogeneity.

Tree species differ in plant economic spectrum traits in the tropical dry forest of Mexico

In tropical dry forests, studies on wood anatomical traits have concentrated mainly on variations in vessel diameter and frequency. Recent research suggests that parenchyma and fibers also play an important role in water conduction and in xylem hydraulic safety. However, these relationships are not fully understood, and wood trait variation among different functional profiles as well as their variation under different water availability scenarios have been little studied. In this work, we aim to (1) characterize a set of wood anatomical traits among six selected tree species that represent the economic spectrum of tropical dry forests, (2) assess the variation in these traits under three different rainfall regimes, and (3) determine the relationships between wood anatomical traits and possible functional trade-offs. Differences among species and sites in wood traits were explored. Linear mixed models were fitted, and model comparison was performed. Most variation occurred among species along the economic spectrum. Obligate deciduous, low wood density species were characterized by wood with wide vessels and low frequency, suggesting high water transport capacity but sensitivity to drought. Moreover, high cell fractions of carbon and water storage were also found in these tree species related to the occurrence of abundant parenchyma or septate fibers. Contrary to what most studies show, Cochlospermum vitifolium, a succulent tree species, presented the greatest variation in wood traits. Facultative deciduous, high wood density species were characterized by a sturdy vascular system that may favor resistance to cavitation and low reserve storage. Contrary to our expectations, variation among the rainfall regimes was generally low in all species and was mostly related to vessel traits, while fiber and parenchyma traits presented little variation among species. Strong functional associations between wood anatomical traits and functional trade-offs were found for the six tree species studied along the economic spectrum of tropical dry forests.

Functional trade-offs are driven by coordinated changes among cell types in the wood of angiosperm trees from different climates

Wood performs several functions to ensure tree survival and carbon allocation to a finite stem volume leads to trade-offs among cell types. It is not known to what extent these trade-offs modify functional trade-offs and if they are consistent across climates and evolutionary lineages. Twelve wood traits were measured in stems and coarse roots across 60 adult angiosperm tree species from temperate, Mediterranean and tropical climates. Regardless of climate, clear trade-offs occurred among cellular fractions, but did not translate into specific functional trade-offs. Wood density was negatively related to hydraulic conductivity (Kth ) in stems and roots, but was not linked to nonstructural carbohydrates (NSC), implying a functional trade-off between mechanical integrity and transport but not with storage. NSC storage capacity was positively associated with Kth in stems and negatively in roots, reflecting a potential role for NSC in the maintenance of hydraulic integrity in stems but not in roots. Results of phylogenetic analyses suggest that evolutionary histories cannot explain covariations among traits. Trade-offs occur among cellular fractions, without necessarily modifying trade-offs in function. However, functional trade-offs are driven by coordinated changes among xylem cell types depending on the dominant role of each cell type in stems and roots.

Functional traits and water-transport strategies of woody species in an insular environment in a tropical forest

PREMISE

Plants survive in habitats with limited resource availability and contrasting environments by responding to variation in environmental factors through morphophysiological traits related to species performance in different ecosystems. However, how different plant strategies influence the megadiversity of tropical species has remained a knowledge gap.

METHODS

We analyzed variations in 27 morphophysiological traits of leaves and secondary xylem in Erythroxylum pulchrum and Tapirira guianensis, which have the highest absolute dominance in these physiognomies and occur together in areas of restinga and dense ombrophilous forest to infer water-transport strategies of Atlantic Forest woody plants.

RESULTS

The two species presented different sets of morphophysiological traits, strategies to avoid embolism and ensure water transport, in different phytophysiognomies. Tapirira guianensis showed possible adaptations influenced by phytophysiognomy, while E. pulchrum showed less variation in the set of characteristics between different phytophysiognomies.

CONCLUSIONS

Our results provide essential tools to understand how the environment can modulate morphofunctional traits and how each species adjusts differently to adapt to different phytophysiognomies. In this sense, the results for these species reveal new species-specific responses in the tropical forest. Such knowledge is a prerequisite to predict future development of the most vulnerable forests as climate changes.

Wood Nutrient-Water-Density Linkages Are Influenced by Both Species and Environment

Tropical trees store a large amount of nutrients in their woody tissues, thus triggering the question of what the functional association of these elements with other wood traits is. Given the osmotic activity of mineral elements such as potassium, sodium, and calcium, these elements should be strong candidates in mediating the water storing capacity in tropical trees. We investigated the role of wood nutrients in facilitating wood water storage in trees by using branch samples from 48 tropical tree species in South America and examined their associations with wood density (ρ). Wood density varied from 316 kg/m3 in Peru plots, where the soil nutrient status is relatively higher, to 908 kg/m3 in Brazil plots, where the nutrient availability is lower. Phosphorus content in wood varied significantly between plots with lowest values found in French Guiana (1.2 mol/m3) and plots with highest values found in Peru (43.6 mol/m3). Conversely, potassium in woody tissues showed a significant cross-species variation with Minquartia guianensis in Brazil showing the lowest values (8.8 mol/m3) and with Neea divaricata in Peru having the highest values (114 mol/m3). We found that lower wood density trees store more water in their woody tissues with cations, especially potassium, having a positive association with water storage. Specific relationships between wood cation concentrations and stem water storage potential nevertheless depend on both species' identity and growing location. Tropical trees with increased water storage capacity show lower wood density and have an increased reliance on cations to regulate this reservoir. Our study highlights that cations play a more important role in tropical tree water relations than has previously been thought, with potassium being particularly important.

Parenchyma fractions drive the storage capacity of nonstructural carbohydrates across a broad range of tree species

PREMISE

Nonstructural carbohydrates (NSCs) play a key role in tree performance and functioning and are stored in radial and axial parenchyma (RAP) cells. Whether this relationship is altered among species and climates or is linked to functional traits describing xylem structure (wood density) and tree stature is not known.

METHODS

In a systematic review, we collated data for NSC content and the proportion of RAP in stems for 68 tree species. To examine the relationships of NSCs and RAP with climatic factors and other functional traits, we also collected climatic data at each tree's location, as well as wood density and maximum height. A phylogenetic tree was constructed to examine the influence of species' evolutionary relationships on the associations among NSCs, RAP, and functional traits.

RESULTS

Across all 68 tree species, NSCs were positively correlated with RAP and mean annual temperature, but relationships were only weakly significant in temperate species and angiosperms. When separating RAP into radial parenchyma (RP) and axial parenchyma (AP), both NSCs and wood density were positively correlated with RP but not with AP. Wood in taller trees was less dense and had lower RAP than in shorter trees, but height was not related to NSCs.

CONCLUSIONS

In trees, NSCs are stored mostly in the RP fraction, which has a larger surface area in warmer climates. Additionally, NSCs were only weakly linked to wood density and tree height. Our analysis of evolutionary relationships demonstrated that RAP fractions and NSC content were always closely related across all 68 tree species, suggesting that RAP can act as a reliable proxy for potential NSC storage capacity in tree stems.

Influence of Colder Temperature on the Axial and Radial Parenchyma Fraction of Quercus ciliaris C.C.Huang & Y.T.Chang Wood and Its Relationship with Carbohydrate Reserve (NSC)

Parenchyma in the secondary xylem comprises the main tissue for the storage of non-structural carbohydrates (NSC) in woody plants. Across species, the amount of parenchyma depends on the general environment of the distribution area and determines to a large extent the NSC storage. However, little information is available on the relationship between parenchyma fractions, NSC storage, and the environmental influences within individual species. This information is crucial to assessing the adaptive capacities of tree populations in the context of increasing the frequency and severity of stress-inducing events. In this study, parenchyma fractions and NSC concentrations of the secondary xylem in trunks of a subtropical evergreen oak (Quercus ciliaris C.C.Huang & Y.T.Chang) were quantified along an elevational gradient from 700 m to 1200 m a.s.l. in eastern China. Air temperatures within the distribution area correlated with altitude were recorded. The results showed that the total parenchyma fractions did not covary with the colder temperatures. However, axial parenchyma fractions were lower with a colder climate, while the fractions of multiseriate rays and total ray parenchyma were higher. Higher concentrations of starch and NSC were significantly associated with larger axial parenchyma fractions. The sugar concentration displayed no significant relationship with parenchyma fractions. These findings suggest that the total parenchyma fractions in secondary xylem do not increase in response to a colder climate, while colder temperatures drive changes in the composition of parenchyma for Q. ciliaris.