Seasonal variation in cambial activity

The timing of cambial reactivation plays an important role in determination of the amount and quality of wood and the environmental adaptavity of trees.
Environmental factors, such as temperatures, influence the growth and development of trees.
Temperatures from late winter to early spring affect the physiological process that are involved in the initiation of cambial cell division and xylem differentiation in trees.
Cumulative elevated temperatures from late winter to early spring result in earlier initiation of cambial reactivation and xylem differentiation in tree stems and an extended growth period.
However, earlier cambial reactivation increases the risk for frost damage because the cold tolerance of cambium decreases after cambial reactivation.
A better understanding of the mechanisms that regulate wood formation in trees and the influence of environmental conditions on such mechanisms should help in efforts to improve and enhance the exploitation of wood for commercial applications and to prepare for climatic change.
Wood is the product of vascular cambium, and the formation of wood depends on the cambial activity of trees.
In temperate and cool zones, the vascular cambium of the stems of trees undergoes seasonal cycles of activity and dormancy, which are collectively known as annual periodicity.
This periodicity plays an important role in the formation of wood and reflects the environmental adaptivity of trees, for example their tolerance to cold in winter in cool and temperate zones.
The quantity and quality of wood depend on the division of cambial cells and the differentiation of cambial derivatives.
Cambial activity in trees is regulated by both internal factors, such as plant hormones, and environmental factors, such as, temperature, rainfall and photoperiod.
Temperature provides the appropriate physical conditions for the growth and development of trees in temperate and cool climates.
Timing of cambial reactivation is controlled by temperature, which influences both the quantity and quality of wood.
During the period from late winter to early spring, new cell plates are formed in the cambium and this springtime phenomenon is referred to as cambial reactivation.

1. SEASONAL VARIATION IN CAMBIAL ACTIVITY ALEN SHAJI P1914015

2. VASCULAR CAMBIUM Lateral meristem forming vascular tissues. Innermost layer of bark, between the xylem and phloem tissues . Grows both to the inside and to the outside, 1] Cells on inside- secondary xylem . 2] Cells to outside- secondary phloem . Secondary differentiates these new tissues from the primary xylem and phloem, which derive from the apical meristem .

3. ORIGIN OF CAMBIUM EPIDERMIS PROTODERM PRIMARY XYLEM 2 XYLEM APICAL PROCAMBIUM VASCULAR MERISTEM CAMBIUM PRIMARY PHLOEM 2 PHLOEM GROUND MERISTEM CORTEX CORK PHELLODERM CAMBIUM PITH CORK

4. ANATOMY OF VASCULAR CAMBIUM A] Technically vascular cambium is equals to cambial initials . 1] Difficult to distinguish between initials and their derivatives. 2] Refer to a cambial zone . B] Vascular cambium meristamatic cells - highly vacuolated. 1] Fusiform initials - vertically elongated. 2] Ray initials - horizontally elongated or squarish .

5. Ray initials , 1] Produces the radial [lateral] transport system cells . 2] In xylem and phloem these are the parenchyma . Fusiform initials , 1] Produces the axial [vertical] transport system cells . 2] In xylem and phloem these are the sieve tube member, companion cells, tracheids , vessels, and fibres . 3] Can be storied [more advanced; less common] or nonstoried .

6. C] Two possible planes of cell division, 1] Periclinal division - new cells in front or behind the other . a] Yields secondary xylem and phloem . b] New cells toward the inside- xylem; cells to the outside- phloem. 2] Anticlinal division - cells side by side . a] Adds new cells to the vascular cambium as stem grows in girth.

7. ACTIVITY OF VASCULAR CAMBIUM Cambium active from spring to fall; inactive in winter . Patterns of yearly activity produces annual rings in the xylem. Generally the xylem producing cells are more active than the phloem producing cells .

8. DEVELOPMENT OF VASCULAR CAMBIUM Function is to produce secondary growth . Vascular cambium must be formed before secondary growth can occur .

9. Two regions of primary stem contribute to the vascular cambium, 1] Fascicular cambium - meristem cells within vascular bundles . 2] Interfascicular cambium - meristem cells between vascular bundles .

10. FUNCTIONAL VASCULAR CAMBIUM Becomes vascular cambium when the fascicular and interfascicular cambium join to form a complete cylinder around the stem. As soon as the cylinder is formed, vascular cambium becomes active . 1] Divides on both the inner and outer surface of the vascular cambium surfaces. Activity of the vascular cambium, 1] New xylem cells are formed inwardly and attached to the previously produced xylem. 2] New phloem cells are formed outwardly and are attached to the previously produced phloem.

11. DURATION OF CAMBIUM Duration of the functional life of the cambium varies greatly in different species and also in different parts of the same plant . In a perennial woody plant, the cambium of the stem lives from the time of the formation untill the death of the plant. In leaves, inflorescences and other deciduos parts, the functional life of the cambium is short .

12. STRUCTURE OF CAMBIUM There are two concepts; 1] It consists of a uniserate layer of permanent initials with derivatives which may divide a few times had seen becomes converted into permanent tissues . 2] There are several rows of initiating cells which forms a cambial zone , a few individual rows of which persists a cell forming layers for sometimes.

13. During growing periods the cells mature continuosly on both sides of the cambium it becomes quite obvious that only a single layer of cells have permanent existence as cambium .

14. SINGLE CAMBIAL CELL

15. Other layers if present, function only temporarily and become completely transformed into permanent cells . In a strict sense, only the initials constitue the cambium , but frequently the term is used with reference to the cambial zone , because it is difficult to distinguish the initials from their recent derivatives .

16. CAMBIUM GROWTH ABOUT WOUNDS When wounds occurs on plants, a large amount of soft parenchymatous tissue is formed on or below the injures surface, this tissue is known as callus . Callus develops from the cambium and by the division of parenchyma cells in the phloem and the cortex .

17. CAMBIUM IN BUDDING AND GRAFTING Cambium of both stock and scion gives rise to callus which unites and develops continous cambium layer that gives rise to normal conducting tissues.

18. CAMBIUM IN MONOCOTYLEDONS In dracaena, aloe, yucca, veratrum , Cambium layers develops from the meristamatic parenchyma of the pericycle or the innermost cells of the cortex . In the case of roots, the cambium of this develops in the endodermis , the initials of cambium strands tiers to forms a storied cambium as found in the normal cambium of some dicotyledons .

19. SEASONAL VARIATION IN CAMBIAL ACTIVITY INTRODUCTION The timing of cambial reactivation plays an important role in determination of the amount and quality of wood and the environmental adaptavity of trees. Environmental factors, such as temperatures, influence the growth and development of trees . Temperatures from late winter to early spring affect the physiological process that are involved in the initiation of cambial cell division and xylem differentiation in trees . Cumulative elevated temperatures from late winter to early spring result in earlier initiation of cambial reactivation and xylem differentiation in tree stems and an extended growth period .

20. However, earlier cambial reactivation increases the risk for frost damage because the cold tolerance of cambium decreases after cambial reactivation . A better understanding of the mechanisms that regulate wood formation in trees and the influence of environmental conditions on such mechanisms should help in efforts to improve and enhance the exploitation of wood for commercial applications and to prepare for climatic change . Wood is the product of vascular cambium, and the formation of wood depends on the cambial activity of trees .

21. In temperate and cool zones, the vascular cambium of the stems of trees undergoes seasonal cycles of activity and dormancy , which are collectively known as annual periodicity .

22. This periodicity plays an important role in the formation of wood and reflects the environmental adaptivity of trees, for example their tolerance to cold in winter in cool and temperate zones . The quantity and quality of wood depend on the division of cambial cells and the differentiation of cambial derivatives .

23. Cambial activity in trees is regulated by both internal factors , such as plant hormones , and environmental factors , such as, temperature, rainfall and photoperiod . Temperature provides the appropriate physical conditions for the growth and development of trees in temperate and cool climates . Timing of cambial reactivation is controlled by temperature , which influences both the quantity and quality of wood.

24. INDUCTION OF CAMBIAL REACTIVATION BY LOCALIZED HEATING During the period from late winter to early spring , new cell plates are formed in the cambium and this springtime phenomenon is referred to as cambial reactivation .

25. Studies failed to identify the factors that regulate the timing of cambial reactivation in trees . Winter cambial dormancy in trees consists of two stages , namely, the resting and the quiescent stages . During the first 2–4 weeks of dormancy, the cambium is unable to produce new cells even when IAA is supplied under favorable environmental conditions . This stage of dormancy, which is regulated by internal factors, is referred to as the resting stage . After exposure to natural or artificial chilling, the cambium gradually regains the ability to produce new xylem cells in response to IAA under appropriate environmental conditions .

26. When fully responsive to IAA in this way , the cambium is deemed to be in the quiescent stage of dormancy , which is imposed by adverse external factors. The transition from rest to quiescence involves structural, histochemical and functional changes in cambial cells . An increase in temperature might be a limiting factor in the onset of cambial reactivation during the quiescent dormancy of trees . Localized heating for 4 weeks induced cambial reactivation that occurred earlier than natural cambial reactivation .

27. Two weeks of localized heating of stems of the deciduous conifer Larix leptolepis failed to activate the division of cambial cells , while localized heating of stems of C. japonica induced cambial reactivation 6 days after the start of heating , with xylem differentiation starting after heating for 14 or 21 days .

28. Longer localized heating of the stems of deciduous trees, as compared to those of evergreen conifers, might be required for conversion of cambium from a quiescent to an active state . Increase in temperature of the stem might be a direct trigger for cambial reactivation in trees .

29. It has been reported that bud burst and the development of new leaves are related to cambial reactivation and xylem differentiation . Xylem differentiation in heated portions of stems started only after bud flushing , suggesting that some factors from expanding new leaves might be required for the differentiation of xylem .

30. In 55-year-old cambium, cambial reactivation and xylem differentiation occurred earlier than in 80-year-old cambium both under heated and under natural conditions . In Larix , Pinus , Picea and Juniperus , the duration of cambial activity and xylem differentiation differed between adult and old trees even when the trees had been grown under the same climatic conditions.

31. Duration of cambial activity appears to differ between adult and old trees . Cambial sensitivity to temperature might be related to cambial age and the state of cambial dormancy in C. japonica stems. CHANGES IN LEVELS AND LOCALIZATION OF STORAGE MATERIALS DURING CAMBIAL ACTIVITY In temperate and cool zones, the low rates of photosynthesis in late winter and early spring might be expected to limit the supply of sucrose to stems . Under natural conditions, cambial reactivation in trees occurs from late winter to early spring, when photosynthesis is minimal or almost non-existent .

32. Stored materials are crucial for radial growth . Utilization of reserve materials at various stages of growth might provide clues to a full understanding of the growth and development of trees . Not only starch but also lipid droplets in the cambium might be used as sources of energy for cambial activity and the initiation of xylem differentiation in locally heated stems of C. japonica .

33. CAMBIAL REACTIVATION UNDER NATURAL CONDITIONS In Pinus leucodermis , the calculated threshold minimum, mean and maximum daily temperatures for wood formation were approximately 5.5 , 8.2 and 11.5◦C , respectively. In Larix decidua , Pinus cembra and P. abies , cambial activity and xylem differentiation occurred above a certain threshold value of mean daily temperature, which ranged from 5.6 to 8.5◦ C . In Abies balsamea , L . decidua , P. cembra , P. sylvestris , P. leucodermis , Pinus uncinata and P. abies , the critical average temperature for onset of cambial activity ranged between 8 and 9◦C .

34. COLD STABILITY OF MICROTUBULES AND ITS RELATIONSHIP TO CAMBIAL ACTIVITY Structure and organelles of cambial cells exhibited seasonal changes that might be related to adaptation to changes in climatic conditions . Microtubules , a major component of the cytoskeleton, play important roles in the division and differentiation of cells . They are sensitive to temperature, and low temperature tends to depolymerize or disassemble microtubules in plant cells . Therefore, plant-science manuals generally suggest that low temperatures are inappropriate for the visualization of microtubules after chemical fixation .

35. Immunofluorescence microscopy reveals the presence of microtubules in cambium, xylem cells and phloem cells that have been fixed at room temperature.

36. By contrast, low-temperature fixation (2–3◦C) depolymerizes microtubules in fusiform and ray cambial cells and in differentiating cells when the cambium is active in Abies firma, A. sachalinensis and L. leptolepis . Thus, low ambient temperatures above 0◦C, namely, chilling temperatures, might be expected to influence and induce the disassembly of microtubules in cambium and cambial derivatives .

37. In A. firma, it was noted that the depolymerization of microtubules did not occur during low-temperature fixation (2–3◦C). It is possible that, during cambial dormancy, microtubules might be resistant to the normally depolymerizing effect of low temperatures . Clearly, the stability during low-temperature fixation of microtubules in cambial cells, xylem cells and phloem cells differs between seasons of dormant and active cambium . This phenomenon might be closely related to the seasonal changes in the cold tolerance of trees because microtubules play such important roles in cell division and differentiation .

38. A SCHEMATIC DIAGRAM SHOWING THE ANNUAL PERIODS OF CAMBIAL DORMANCY AND ACTIVITY AND THE EFFECTS OF AN INCREASE IN TEMPERATURE FROM LATE WINTER TO EARLY SPRING ON WOOD FORMATION IN TREES.

39. CONCLUSION Localized or environmentally induced increase in the temperature of the stem can directly induce the breaking of cambial dormancy in trees but the responses to such increases in temperature differ among species and depend on stage of dormancy. Prominent decreases in the levels of starch and lipid droplets in the cambium, from cambial reactivation to the start of xylem differentiation, indicate that starch and lipid droplets might be utilized as sources of energy for cell division and the biosynthesis of new cell walls in the cambium.

40. Differences in the behavior of microtubules during low-temperature fixation of active and dormant cambium suggest that microtubules might act as sensors of a change in growth conditions. Future climate change , with increases in temperature from late winter to early spring, might induce earlier cambial reactivation and xylem differentiation , resulting in longer periods of cambial growth and increased production of wood biomass . In addition, such increases in temperature might promote the enzymatic conversion of starch to sugar to supply the energy required for the biosynthesis of new cell walls and for continuous cambial activity .

41. By contrast, a sudden decrease in temperature after the onset of cambial reactivation, such as a late spring frost, when the cold tolerance of the cambium is low might induce the depolymerization of microtubules, with a consequent negative impact on tree growth and development . Therefore, the effects of future climate change from late winter to early spring might have some positive impact on the production of wood biomass , but we cannot exclude the possibility that earlier onset of cambial activity might be associated with increased risk of frost damage to the cambium .

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