Phloem Formation Research Articles

Effect of girdling on phloem and xylem formation in a range of gymnosperms and angiosperms

Summary Girdling is often used as an experimental method to study source/sink controls of cambial growth. However, the phloem responses to girdling have not been well investigated. The aim of this research was to characterize the anatomical changes in the phloem and xylem of different species following trunk girdling. Different species of gymnosperm (Abies sibirica, Pinus sylvestris) and angiosperm (Alnus incana, Populus tremula) woody plants were selected. We girdled trunks during active growth and sampled tissues at two levels (1 cm and 35 cm) above the girdle at the end of the growing season. General responses to girdling were recorded for the studied species, such as increased phloem increments, parenchymatization of conducting tissues, reduction in the size of conducting elements, and an increase in the size of axial parenchyma cells in the phloem and xylem. We observed the suppression of xylogenesis in 3 out of 4 species. Differences in the structure of conducting tissues were found, which are due to species differences in the initial tissue structure. In gymnosperms, noticeable differences in the number of resin ducts in the xylem were observed between control and girdled trees. In angiosperms, we found the formation of cells with thickened cell walls in girdled trees (i.e., the formation of phloem and xylem fibers with thickened cell walls in aspen and sclereids in the phloem of alder). Based on the literature data, the observed responses may be due to both the wounding effect and the influence of the high sugar content above the girdle.

Conserved autophagy and diverse cell wall composition: unifying features of vascular tissues in evolutionarily distinct plants.

The formation of multifunctional vascular tissues represents a significant advancement in plant evolution. Differentiation of conductive cells is specific, involving two main pathways, namely protoplast clearance and cell wall modification. In xylogenesis, autophagy is a crucial process for complete protoplast elimination in tracheary elements, whose cell wall also undergoes strong changes. Knowledge pertaining to living sieve elements, which lose most of their protoplast during phloemogenesis, remains limited. We hypothesized that autophagy plays a crucial role, not only in complete cytoplasmic clearance in xylem but also in partial degradation in phloem. Cell wall elaborations of mature sieve elements are not so extensive. These analyses performed on evolutionarily diverse model species potentially make it possible to understand phloemogenesis to an equal extent to xylogenesis. We investigated the distribution of ATG8 protein, which is an autophagy marker, and cell wall components in the roots of ferns, gymnosperms and angiosperms (monocots, dicot herbaceous plants and trees). Furthermore, we conducted a bioinformatic analysis of complete data on ATG8 isoforms for Ceratopteris richardii. The presence of ATG8 protein was confirmed in both tracheary elements and sieve elements; however, the composition of cell wall components varied considerably among vascular tissues in the selected plants. Arabinogalactan proteins and β-1,4-galactan were detected in the roots of all studied species, suggesting their potential importance in phloem formation or function. In contrast, no evolutionary pattern was observed for xyloglucan, arabinan or homogalacturonan. Our findings indicate that the involvement of autophagy in plants is universal during the development of tracheary elements that are dead at maturity and sieve elements that remain alive. Given the conserved nature of autophagy and its function in protoplast degradation for uninterrupted flow, autophagy might have played a vital role in the development of increasingly complex biological organizations, including the formation of vascular tissues. However, different cell wall compositions of xylem and phloem in different species might indicate diverse functionality and potential for substance transport, which is crucial in plant evolution.

Stem anatomy and ontogeny of phloem wedges in some species of Gymnema (Apocynaceae)

The anatomy of climbing plants shows a fascinating arrangement of vascular tissues, which may vary from species to species. In the present study, stem anatomy of Gymnema cuspidatum, G. latifolium, G. montanum and G. sylvestre (Apocynaceae) and the occurrence of vascular variants are described here for the first time. They were compared histologically to understand the structural variations in the secondary xylem and ontogeny of vascular variants if any within the species. All four species increase their stem diameter by a single ring of vascular cambium like most of the eudicots. In thick stems, periderm forms distinct ridges and furrows in all the species except G. sylvestre, in which it was minimal. Differential activity of the cambium resulted in the formation of distinct and deep phloem wedges in G. latifolium and G. sylvestre while in other species they were either shallow or almost absent. The secondary xylem was diffuse-porous with distinct growth rings in G. cuspidatum and G. montanum while they were indistinct in G. latifolium and G. sylvestre. Besides regular xylem elements, the presence of perforated ray cells and radially oriented laticifers in the xylem rays was observed frequently in all the investigated samples. The formation of the intraxylary primary phloem was observed concomitant to the regular protoxylem and protophloem at the pith margin. Subsequently, an additional intraxylary phloem was developed from the adjacent pith cells and by the activity of intraxylary/medullary cambium at the pith margin. In thick stems, parenchymatous cells situated between the protoxylem and intraxylary protophloem led to the initiation of intraxylary cambium. In conclusion, three species showed distinct ridges and furrows due to the formation of periderm except G. sylvestre, phloem ray parenchyma showed the formation of sclerenchyma only in the thick stems of G. sylvestre. The formation of intraxylary phloem in all four species may serve as an additional way for the transport of organic nutrients.

Effect of different stand densities on xylem and phloem formation in Norway spruce plantations

Summary Preliminary results of an investigation of the thinning effect on Norway spruce tree growth in terms of xylogenesis and phloemogenesis are presented. Three plots were selected where the stand densities were reduced by pre-commercial thinning to 1800 trees/ha (plot A; mild thinning) and 1300 trees/ha (plot C; heavy thinning) in February 2020. Plot B had no silvicultural intervention and represented a control variant (4500 trees/ha). Three dominant 14-year-old Norway spruce trees were sampled (micro-cored) for studying the xylem and phloem formation in the 2020 growing season. The total differentiation duration was determined to be around 169.7 ± 12.7–179.3 ± 4.0 days. The average number of xylem cells formed in control plot B was only 140 mature cells, plots C and A were determined to have 175 and 200 mature cells, respectively. Plot A had the fastest growth rate of all the plots studied. In all three plots (A, B, C), the beginning of the early phloem, late phloem sieve cells, and axial parenchyma cells coincided. Nonetheless, in terms of total phloem cell increase, plot C displayed the fastest growth rate among the three plots studied. The first results showed that the rate of total increment in both xylem and phloem cells, as well as the total number of produced tracheids and sieve cells, seem to be positively affected after the thinning application.

Anatomy of xylem and phloem in stems and roots of Populus sibirica and Ulmus pumila from semi-arid steppe in Mongolia

The present study focuses on the cambium and the anatomy of secondary tissues (xylem – wood and phloem) of Siberian poplar (Populus sibirica) and Siberian elm (Ulmus pumila) grown in a plantation in the semi-arid Mongolian steppe. Stem and root microcores from both species were collected and subsequently processed to obtain cross-sections for light microscopy by paraffin embedding, sectioning with a rotary microtome, and staining with safranin and astra blue. The results present the anatomy of the secondary xylem and phloem of stems and roots of both species, along with the characteristics of the youngest xylem and phloem annual rings. We discuss the critical aspects which need to be considered when using the microcoring methodology, along with the need for further studies on wood and phloem formation of less-commonly studied tree species and their characteristics when grown in semi-arid environments.

Vascular implications of Dasineura sp. galls' establishment on Peumus boldus stems.

Some chewing larvae are capable of inducing galls in the host vascular cylinder, e.g. Dasineura sp. (Cecidomyiidae) on Peumus boldus stems. Due to the medicinal and economic importance of P. boldus, the anatomical and functional implications of establishment of Dasineura sp. on P. boldus stems were investigated. We asked if establishment of Dasineura sp. in P. boldus stems induces abnormalities at the cellular and organizational level of the vascular system that increase during gall development in favour of the hydric status of the gall. Anatomical alterations induced in the stems during gall development were determined. Cytohistometric analyses in mature galls were compared to non-galled stems, and water potential and leaf area of non-galled stems were compared with galled stems. Dasineura sp. establishes in the vascular cambium, leading to delignification and rupture of xylem cells, inhibiting formation of phloem and perivascular sclerenchyma. Gall diameter increases together with larval feeding activity, producing a large larval chamber and numerous layers of nutritive tissue, vascular parenchyma, and sclerenchyma. These anatomical alterations do not affect the leaf area of galled stems but favour increased water flow towards these stems. The anatomical alterations induced by Dasineura sp. in P. boldus stems guarantee water and nutrient supply to the gall and larva. After the inducer exits stems, some host branches no longer have vascular connections with the plant body.

Phloem development.

The evolution of the plant vascular system is a key process in Earth history because it enabled plants to conquer land and transform the terrestrial surface. Among the vascular tissues, the phloem is particularly intriguing because of its complex functionality. In angiosperms, its principal components are the sieve elements, which transport phloem sap, and their neighboring companion cells. Together, they form a functional unit that sustains sap loading, transport, and unloading. The developmental trajectory of sieve elements is unique among plant cell types because it entails selective organelle degradation including enucleation. Meticulous analyses of primary, so-called protophloem in the Arabidopsis thaliana root meristem have revealed key steps in protophloem sieve element formation at single-cell resolution. A transcription factor cascade connects specification with differentiation and also orchestrates phloem pole patterning via noncell-autonomous action of sieve element-derived effectors. Reminiscent of vascular tissue patterning in secondary growth, these involve receptor kinase pathways, whose antagonists guide the progression of sieve element differentiation. Receptor kinase pathways may also safeguard phloem formation by maintaining the developmental plasticity of neighboring cell files. Our current understanding of protophloem development in the A. thaliana root has reached sufficient detail to instruct molecular-level investigation of phloem formation in other organs.

Formation of inter-and intraxylary phloem in some species of Argyreia Lour. (Convolvulaceae)

The development of inter-and intraxylary phloem was investigated in nineteen species of Argyreia Lour. Among all species, differentiation of intraxylary protophloem was observed concomitant to the regular external protophloem from the procambial derivatives (except A. nervosa). However, A. nervosa, showed the formation of medullary bundles along with the regular protoxylem while the development of intraxylary phloem was observed only after the establishment of a complete ring of the vascular cambium and deposition of a few secondary xylem elements. In all the species (except A. nervosa), additional intraxylary phloem differentiated from the adjacent parenchyma and the intraxylary cambium initiated at the periphery of the pith. It divided uni-directionally and showed deposition of only phloem elements. In the transverse view, sieve elements of intraxylary phloem were oval to circular, rectangular, or multi-angled or sometimes in a cluster of three to several cells with simple and compound sieve plates oriented obliquely on their terminal ends. In the thick stems of A. cuneata, A. elliptica, A. lawii, A. mastersii, A. nervosa, A. sericea, A. sharadchandrajii and A. splendens, thin-walled unlignified axial parenchyma strands embedded within the secondary xylem divide and differentiated into isolated or groups of interxylary phloem elements. In the transverse view, these sieve elements were like regular phloem (oval to circular) in appearance but occasionally they may be narrow in diameter, tangentially flattened and arranged in clusters. In thick stems, earlier-formed sieve elements of the interxylary phloem collapse and get crushed due to the expansion of adjacent cells. The remaining species though showed the presence of parenchyma strands within the secondary xylem, no interxylary phloem was observed even in thick stems. Such parenchyma cells within islands always showed the presence of druses.

Seasonal Cambial Activity and Formation of Secondary Phloem and Xylem in White Oaks (Quercus alba L.)

(1) Background: the cambium has seasonal activity, forming earlywood and early phloem with relatively wide conducting cells, which will function during the most favorable season, and latewood and late phloem with narrower conducting cells, which typically function during the less favorable season. However, few studies have focused on when these two contrasting tissue types are formed in relation to climatic conditions. (2) Methods: the senior author of this paper made weekly collections for an entire year of four specimens per collection back in the 1960s, using traditional anatomical methods to study in detail what the cambium was producing progressively. (3) Results: annual growth rings are evident in both secondary xylem and secondary phloem. The cambium resumes activity in early April, with simultaneous formation of wood and secondary phloem. Both latewood and late phloem production are initiated in early June, the peak of the favorable season. The cambium ends its activity in early August. Phloem growth rings are marked by radially narrow sieve elements interspersed among a band of axial parenchyma with dark contents. Most specimens produce only one fiber band per season. This feature may be used as an indirect phloem growth ring marker. Wood growth rings are marked by very wide vessels and thick-walled, radially narrow fibers. (4) Conclusions: growth rings are evident in both secondary xylem and secondary phloem. The trees produce their latewood and late phloem long before the beginning of autumn, indicating that they prepare ahead of the selective regime, a phenomenon most likely dependent on the photoperiod. Living sieve elements are present yearlong.

Distribution of Plant Hormones and Their Precursors in Cambial Region Tissues of Quercus myrsinifolia and Castanopsis cuspidata var.sieboldii after Bending Stems or Applying Ethylene precursor

The role of plant hormones in tension wood (TW) formation has been studied but is still unclear. IAA, ABA, ACC, tZ, tZR, iP, and iPR in cambial region tissues were identified and quantified by liquid chromatography/mass spectrometry (LC/MS). We examined the distribution of plant hormones and their precursors in the stems of Quercus myrsinifolia Blume and Castanopsis cuspidata var.sieboldii Nakai after bending the stems or applying an ethylene precursor (ACC). After 3 weeks of bending, though not after 1 week of bending, the auxin (IAA) and abscisic acid (ABA) amounts were larger on the TW side than on the opposite wood (OW) side and in upright trees. After 2 weeks of bending, the peak concentrations of IAA in cambium on the TW side were obviously higher than those on the OW side. After 1 week of bending, the ACC amounts on both sides were larger than in upright trees, but after 3 weeks of bending, they were smaller than in upright trees. Applied ACC did not enhance TW formation but induced axical parenchyma and phloem formation in C. cuspidata var.sieboldii. These results indicated that the distribution patterns of IAA and ABA might have important roles in TW formation in these two species. The role of ACC might be limited in the early stages of TW formation.

Identification and Expression Profile of CLE41/44-PXY-WOX Genes in Adult Trees Pinus sylvestris L. Trunk Tissues during Cambial Activity

WUSCHEL (WUS)-related homeobox (WOX) protein family members play important roles in the maintenance and proliferation of the stem cells in the cambium, the lateral meristem that forms all the wood structural elements. Most studies have examined the function of these genes in angiosperms, and very little was known about coniferous trees. Pine is one of the most critical forest-forming conifers globally, and in this research, we studied the distribution of WOX4, WOX13, and WOXG genes expression in Pinus sylvestris L. trunk tissues. Further, we considered the role of TDIF(CLE41/44)/TDR(PXY) signaling in regulating Scots pine cambial activity. The distribution of CLE41/44-PXY-WOXs gene expression in Scots pine trunk tissues was studied: (1) depending on the stage of ontogenesis (the first group of objects); and (2) depending on the stage of cambial growth (the second group of objects). The first group of objects is lingonberry pine forests of different ages (30-, 80-, and 180-year-old stands) in the middle taiga subzone. At the time of selection, all the trees of the studied groups were at the same seasonal stage of development: the formation of late phloem and early xylem was occurring in the trunk. The second group of objects is 40-year-old pine trees that were selected growing in the forest seed orchard. We took the trunk tissue samples on 27 May 2022, 21 June 2022, and 21 July 2022. We have indicated the spatial separation expressed of PsCLE41/44 and PsPXY in pine trunk tissues. PsCLE41/44 was differentially expressed in Fraction 1, including phloem cells and cambial zone. Maximum expression of the PsPXY gene occurred in Fraction 2, including differentiating xylem cells. The maximum expression of the PsCLE41/44 gene occurred on 27 May, when the number of cells in the cambial zone was the highest, and then it decreased to almost zero. The PsPXY gene transcript level increased from May to the end of July. We found that the highest transcript level of the PsWOX4 gene was during the period of active cell proliferation in the cambial zone, and also in the trees with the cambial age 63 years, which were characterized by the largest number of cell layers in the cambial zone. In this study, we have examined the expression profiles of genes belonging to the ancient clade (PsWOXG and PsWOX13) in stem tissues in Scots pine for the first time. We found that, in contrast to PsWOX4 (high expression that was observed during the period of active formation of early tracheids), the expression of genes of the ancient clade of the WOX genes was observed during the period of decreased cambial activity in the second half of the growing season. We found that PsWOX13 expression was shifted to Fraction 1 in most cases and increased from the phloem side, while PsWOXG expression was not clearly bound to a certain fraction. Based on the data, the role of the CLE41/44-PXY-WOX signaling module in regulating P. sylvestris cambial growth is discussed.

Structure and development of phloem in some species of Merremia Dennst. ex Endl. sensu lato (Convolvulaceae)

Convolvulaceae is known for the occurrence of cambial variants but studies on phloem development are rare. In the present study, the structure and development of primary and secondary phloem in seven species of Merremia Dennst. ex Endl. s.l. (Convolvulaceae) were investigated. All the examined species were characterized by the presence of inter‐ and intraxylary phloem, except M. emarginata (Burm.f.) Hallier f. which showed an absence of interxylary phloem. The secondary xylem of all the species was characterized by the presence of phloem wedges and thin‐walled parenchyma strands/islands. Among them, all the species showed the formation of interxylary phloem from unlignified parenchyma cells located in the phloem wedges, except Distimake dissectus (Jacq.) A.R.Simões & Staples (syn: M. dissecta (Jacq.) Hallier f.) and M. emarginata. Its development was also observed in the parenchyma strands/pockets of M. hederacea (Jacq.) Hallier f. and Camonea vitifolia (Burm.f.) A.R.Simões & Staples (syn: M. vitifolia (Jacq.) Hallier f.), while in the rest of the species such pockets were present but no interxylary phloem was observed in them. In relatively thick stems, a quantifiable amount of intraxylary secondary phloem is accumulated by the activity of the intraxylary phloem (internal) cambium. The structure and dimensions of the sieve tube elements significantly vary in all three different types of phloem (i.e. regular external, inter‐ and intraxylary). Sieve elements may possess simple or compound sieve plates and may be wide or narrow in diameter within the same species. The presence of inter‐ and intraxylary phloem and phloem wedges is a characteristic feature of all the investigated species while the absence of interxylary phloem in M. emarginata may be associated with its habit as it spreads on the ground. More studies are needed on species that show the presence of wide and narrow sieve elements, which may contribute to understating the functional ecology and evolution of climbing habits. Further studies are recommended on thick stems of all the investigated species along with other species of Merremia to understand their taxonomic position because some of them were recently transferred to other genera.

Critical steps and troubleshooting in sample preparation for wood and phloem formation: from sampling to microscopic observation

We present a technical note that supplements published procedures on optimal sample preparation for performing wood and phloem formation analyses. Before beginning sampling, it is important to learn about the characteristics of the tree or shrub species to be investigated. Some tips are given how to use the Trephor tool in the best way, how to remove the outer hard bark (periderm), how microcores should be handled after removal from the tree, and how they should be oriented for embedding in paraffin, and cutting thin sections for microscopy. Possible defects that may result from improper handling are illustrated and discussed. We also present optimal images to accurately identify different cell development stages in phloem and xylem, which is particularly challenging in hardwoods and Mediterranean tree and shrub species.

Changes in the Differentiation Program of Birch Cambial Derivatives following Trunk Girdling

The mechanisms regulating the tree trunk radial growth can be studied in original experiments. One technique for studying cambium activity (the meristem involved in radial growth) under conditions of an increased photoassimilate level is trunk girdling. We girdled the trunks of 17- to 22-year-old silver birch plants (Betula pendula Roth var. pendula) during the active growth period and collected xylem and phloem samples at two height levels (1 cm and 35 cm) above girdle, 10, 20, and 30 days after girdling. We investigated the changes that occurred at the anatomical level, as well as the activities of sucrose-metabolizing enzymes and antioxidant-system enzymes and the expression of genes that encode proteins involved in sucrose and auxin transport and metabolism. A moderate increase in photoassimilates (35 cm above the girdle) resulted in a change in the ratio of phloem to xylem increments and an increase in the proportion of parenchyma in the conducting tissues. The increase of photoassimilates above the level at which they can be used in the processes of normal tissue growth and development (1 cm above the girdle) led to xylogenesis suppression and the stimulation of phloem formation, a significant increase in the parenchyma proportion in the conducting tissues, and formation of large sclereid complexes. The differentiation of parenchyma and sclereid cells coincided with biochemical and molecular markers of abnormal conducting tissue formation in Karelian birch, which are also characterized by high proportions of parenchyma and sclereid near the cambium. The results obtained are important in understanding the cambium responses to the photoassimilate distribution changes and estimating tree productivity and survival under changing environmental conditions.

Modelling seasonal dynamics of secondary growth in R

The monitoring of seasonal radial growth of woody plants addresses the ultimate question of when, how and why trees grow. Assessing the growth dynamics is important to quantify the effect of environmental drivers and understand how woody species will deal with the ongoing climatic changes. One of the crucial steps in the analyses of seasonal radial growth is to model the dynamics of xylem and phloem formation based on increment measurements on samples taken at relatively short intervals during the growing season. The most common approach is the use of the Gompertz equation, while other approaches, such as general additive models (GAMs) and generalised linear models (GLMs), have also been tested in recent years. For the first time, we explored artificial neural networks with Bayesian regularisation algorithm (BRNNs) and show that this method is easy to use, resistant to overfitting, tends to yield s‐shaped curves and is therefore suitable for deriving temporal dynamics of secondary tree growth. We propose two data processing algorithms that allow more flexible fits. The main result of our work is the XPSgrowth() function implemented in the radial Tree Growth (rTG) R package, that can be used to evaluate and compare three modelling approaches: BRNN, GAM and the Gompertz function. The newly developed function, tested on intra‐seasonal xylem and phloem formation data, has potential applications in many ecological and environmental disciplines where growth is expressed as a function of time. Different approaches were evaluated in terms of prediction error, while fitted curves were visually compared to derive their main characteristics. Our results suggest that there is no single best fitting method, therefore we recommend testing different fitting methods and selection of the optimal one.

Sample preparation protocol for wood and phloem formation analyses

This technical note and its corresponding video show the procedure for optimal sample preparation to perform wood and phloem formation analyses.

Cell Fate Decisions Within the Vascular Cambium-Initiating Wood and Bast Formation.

Precise coordination of cell fate decisions is a hallmark of multicellular organisms. Especially in tissues with non-stereotypic anatomies, dynamic communication between developing cells is vital for ensuring functional tissue organization. Radial plant growth is driven by a plant stem cell niche known as vascular cambium, usually strictly producing secondary xylem (wood) inward and secondary phloem (bast) outward, two important structures serving as much-needed CO2 depositories and building materials. Because of its bidirectional nature and its developmental plasticity, the vascular cambium serves as an instructive paradigm for investigating principles of tissue patterning. Although genes and hormones involved in xylem and phloem formation have been identified, we have a yet incomplete picture of the initial steps of cell fate transitions of stem cell daughters into xylem and phloem progenitors. In this mini-review perspective, we describe two possible scenarios of cell fate decisions based on the current knowledge about gene regulatory networks and how cellular environments are established. In addition, we point out further possible research directions.

Seasonal changes in the width and structure of non-collapsed phloem affect the assessment of its potential conducting efficiency

Summary Quantitative phloem anatomy is increasingly used in dendroecological studies since recent evidence shows that phloem traits are a valuable indicator of the morphological and physiological strategies of tree performance in different environments. To better understand intra-annual variations in non-collapsed phloem (NCPH) width and structure in three temperate species (Picea abies, Fagus sylvatica, and Quercus petraea), we investigated phloem samples repeatedly taken during the growing season, using light microscopy. The diameter and area of sieve elements were measured in early and late phloem to estimate their conducting potential. The seasonal dynamics of phloem formation and the collapse of sieve elements significantly affect the structure and width of the NCPH. In combination with sieve element characteristics, they determine the conducting potential of the NCPH, which is not constant throughout the growing season. Although the three species differed in terms of wood porosity, the seasonal structural variations of the NCPH followed a similar pattern. At the onset of the growing season, the phloem increment of the previous year was crucial for the function and accounted for over 80% of the NCPH, whereas at the end of the growing season, the phloem increment of the current year constituted the majority of the NCPH. These results indicate that sampling time should be considered when comparing quantitative phloem anatomy data from different laboratories, indicating the need for a uniform sampling protocol and methodology. Otherwise, the assessment of the conducting potential of phloem sieve elements is difficult to compare.

Do Growth-Limiting Temperatures at the High-Elevation Treeline Require an Adaptation of Phloem Formation and Anatomy?

Low temperatures during the growing season restrict the growth of trees at high elevations and lead to the formation of the high-elevation treeline. To ensure the survival and growth of trees in such extreme locations, sufficient vascular transport capacity – enabled by vascular anatomical characteristics – is required. However, in contrast to the xylem, only little is known about the effects of low temperatures on the anatomy and formation of the phloem as important nutrient- and signal-conducting tissue. In this review, known findings of cold-induced changes in the anatomical and phenological properties of vascular tissues are used as starting points to discuss how low temperatures might affect phloem formation at the treeline and how this conductive tissue might adaptively respond to this growth-limiting environmental variable. Data currently available suggest that low temperatures lead to changes in the anatomy and phenological development of the phloem. In order to ensure the functionality of the phloem and thus the survival of trees at the high-elevation treeline, appropriate adaptations to the prevailing low temperatures are therefore to be expected and are discussed in this review.

Comparison of Intra-Annual Xylem and Phloem Formation of Picea crassifolia Stands at Two Latitudes in Northwest China

Understanding the changes in xylem and phloem formation of trees and their relationship along latitudes are important for evaluating and predicting how fragile forests may respond to climate change; however, corresponding studies are still relatively scarce. This study investigated the intra-annual dynamics of xylem and phloem formation of Picea crassifolia and their relationship at two latitudes of arid and semi-arid forests in China. The results showed that both xylem and phloem formation varied at different latitudes. Xylem formation at the low-latitude site (Luoshan) started two weeks earlier than that at the high-latitude site (Helanshan) but ended one week later, resulting in an extended growing season at the low-latitude site. Phloem formation preceded cambium activity and xylogenesis at both sites by 24.6 days in Luoshan, which had warmer conditions, and by 17.3 days in Helanshan. In Luoshan, compared to Helanshan, there occurred significantly more enlarging and wall thickening cells, during (relatively wet) June–August, but significantly fewer enlarging and wall thickening cells as well as total xylem cells, during (relatively dry) April–May. Sample trees produced significantly fewer early phloem cells during the early growing season (April–May) in Luoshan, but generated significantly more late phloem cells during the late growing season in Helanshan. Additionally, different trade-offs between xylem and phloem formation were observed at different sites. The longer duration of early phloem formation might have shortened the duration of xylem lignification in Helanshan; in Luoshan, the date that late phloem reached its maximum growth rate was significantly positively correlated with the date when xylem lignification ended. The results revealed the plasticity of xylem and phloem formation under changing environmental conditions and a complex and site-specific relationship between xylem and phloem formation. These findings could help us better understand and predict the future growth of arid and semi-arid forests in China in response to climate change.