Intrinsic Regulation of Diameter Growth in Woody Plants

(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. Full article

The vascular cambium is the main lateral meristem responsible for the secondary growth of trees. There are a number of explicit and implicit assumptions behind this statement which allow questions to be raised about the mechanism underlying the radial growth of trees. Based on the hypothesis of the diurnal strains of plant organs, it is anticipated that the process of radial growth can be understood as an adaptation to the cyclically changing mechanical stress in the radial direction generated by the phloem during the 24 h day cycle. This qualitative hypothesis treats cambium as a tissue subjected to nighttime stretching and daytime compression in the radial direction. The proposed osmo-mechanical hypothesis of the radial growth of vascular cambium links the daily change in water status and the considerable daily strains in the xylem and phloem with the radial net expansion of a tree trunk. We highlight transpiration as a major factor influencing the secondary growth of woody plants. Thus, we indirectly relate all the biotic (e.g., insect infestation, fungi infections, injuries, shadowing, intra- and interspecies competition, parasitism, symbiosis, etc.) and abiotic (e.g., humidity, water availability, wind, injuries, shadowing, day length in a vegetative season, altitude, temperature, insolation, etc.) processes influencing transpiration with radial growth. In the proposed hypothesis, we also infer that differences in the strains in phloem and xylem are the direct source of tensile stress, tensile stress relaxation, compressive stress, and compressive stress relaxation in the vascular cambium. This sequence appears to be crucial in understanding both the process of the radial growth of trees and the formation of differential wood patterns, within the same genotype as well as in different genotypes. It also provides arguments for the discussion on the mechanisms regulating processes in the vascular cambium. It points out the important role of the variable mechanical stresses in the radial, circumferential, and axial directions and their interference in the development of this lateral meristem. Thus, this hypothesis supports the concept of the epigenetic and systemic regulation of intrinsic wood patterns and tree forms by environmental factors. The hypothesis is focused exclusively on broadleaved trees and symplastic growth. This limitation of the scope is due to a concern for clarity. In this form, the hypothesis provides an alternative explanation for a pure process of radial growth and paves the way for a better interpretation of such phenomena as earlywood and latewood formation. At the same time, this approach to the vascular cambium provides answers to many questions related to the generation of the mechanical conditions necessary for the occurrence of intrusive growth between tangential cell walls; this is of fundamental importance for fusiform initials readjustment, vessel element and fibre formation, ring-porous wood formation, etc. Full article

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. Full article

Despite new knowledge in recent years, our understanding of the phenology of wood formation for various species growing in different environments remains limited. To enhance our knowledge of the tree growth dynamics of boreal tree species, we investigated the average seasonal, monthly, daily, and diel patterns of tree growth and water status from 11 years of observations with the 15 min and 1.5 µm resolved stem radial size variation data of 12 balsam fir (Abies balsamea (L.) Mill.) trees growing in a cold and humid boreal environment. Growth only occurred above an air temperature threshold of 9–10 °C, and the maximal growth rate over the year (23–24 June) was synchronous with the maximal day length (20–21 June) and not with the maximal air temperature, which occurred on average about 2 weeks later (4–5 July). Tree growth was mostly restricted by air temperature and solar radiation under these cold and wet boreal conditions, but our results also highlight a turgor-driven growth mechanism. Diel dynamics reveal that tree growth is minimal during the day when the stem dehydrates, and higher past midnight when the stem is fully rehydrated. This pattern suggests that carbon assimilation through photosynthesis occurs primarily during the day, while energy production and carbon allocation to woody tissues occur primarily at night via cellular respiration. Overall, our results show that the temporal patterns of the growth and water status of balsam fir growing in cold and humid boreal environments are controlled by a set of environmental factors that influence various physiological processes and mechanisms, many of which still need to be documented. Full article

Gišogenesis, otherwise known as secondary-xylem development, was investigated in an old-growth upland population of white spruce (Picea glauca (Moench) Voss) trees having morphologically diverse crowns and growing on a south slope north of East Fork Creek bordering never-glaciated Yukon Beringia. After tree felling, trunks were segmented into one-metre lengths. In the laboratory, widths of xylem layers were measured across the four cardinal directions at each height, followed by Pearson’s product momentum correlations to evaluate variation in historical gišogenetic vigour within and between trees. Substantial variation was found, and it cannot readily be explained in terms of differences in extrinsic environment. Physiological differences in intrinsic gišogenetic regulation within a genetically diverse population, comprising both refugia and recent recruits, is proposed as a probable explanation, thus emphasizing the individuality of each tree’s internal control over how it responds to the extrinsic environment. Further investigations within Yukon Beringia may yield insight into evolutionary diversification of gišogenesis. Full article

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. Full article

Seasonal phenology is expressed in the annual rhythms of growth and quiescence, which may range from being sharply defined to weakly quantitative. These rhythms, both vegetative and reproductive, are a key aspect of a plant’s survival strategy. They enable the plant to both survive seasonal stresses and take advantage of favourable conditions, while making pollination efficient through synchronised flowering. Maturation in woody perennials causes some ontogenetic modulation of seasonal phenology. Shoot phenology is driven by various environmental cues, notably temperatures and daylength, with shoot extremities often being prime receptors. The phenology of shoot extremities is in turn seen as a strong driver of cambial activity, which itself is an aspect of shoot phenology and the basis of stem diameter growth. The aspects of cambial activity reflecting hoot–tip phenology primarily involve the timing of xylem formation and the anatomical and physical properties of xylem cells. The actual amount of diameter growth, however, is governed much more by other factors, notably the growth potential of the species, the space for the tree to grow in, and how favourable local conditions are for growth. Somehow, all tree species allocate resources to shoot extension and diameter growth to represent viable adaptive strategies, although there is no identified role of phenology per se in such allocation. Among species, Pinus radiata is extremely important commercially, largely through a high growth potential that reflects distinctive phenology. Much research on the species provides a basis for linking its phenology and shoot–tip architecture to seasonal timing of diameter growth and fine details of within-ring variation in xylem cell properties. This provides a baseline for comparison with many other tree species, conifers and hardwoods. Selected examples are considered to illustrate the range of adaptive strategies. Regulation of diameter growth and internal variation in wood properties reflects the response to competitive pressures. Modifying the regulation to optimise wood production and quality poses a great challenge for future domestication. Full article

Phloem is one of the vital tissues of the vascular system that plays a crucial role in the conduction of photosynthates. In vascular plants, it develops external to the vascular cambium but in a small fraction of eudicots (formerly known as dicots), it occurs within (interxylary) and inside (intraxylary) the secondary xylem. Ontogenetically, it is classified as Strychnos, Combretum, Azima, and Calycopteris types. In all four cases, phloem islands remain enclosed within the secondary xylem but each has unique origins. Similarly, the deposition of the phloem at the pith margin is common in several plants. It develops from procambial derivatives or adjacent pith cells or by initiating an intraxylary phloem cambium. Functionally, this cambium can produce only phloem or both secondary xylem and phloem. In some instances, the deposition of the secondary xylem and phloem in the same direction has also been documented. Some experimental evidence is available on the role of phloem but is it applicable to inter- and intraxylary phloem? The presence of inter- and intraxylary phloem is attributed to a defence mechanism against insects or plants that show sudden and enormous flowering or it can correlate with high temperatures or an unconducive climate in a desert region where sieve tube elements have become nonfunctional due to high temperatures. The present review is an attempt to analyse the role of interxylary and intraxylary phloem. Full article