Localized cooling of stems induces latewood formation and cambial dormancy during seasons of active cambium in conifers.

BACKGROUND AND AIMS: In temperate regions, trees undergo annual cycles of cambial growth, with periods of cambial activity and dormancy. Environmental factors might regulate the cambial growth, as well as the development of cambial derivatives. We investigated the effects of low temperature by localized cooling on cambial activity and latewood formation in two conifers, Chamaecyparis obtusa and Cryptomeria japonica. METHODS: A plastic rubber tube that contained cooled water was wrapped around a 30-cm-wide portion of the main stem of Chamaecyparis obtusa and Cryptomeria japonica trees during seasons of active cambium. Small blocks were collected from both cooled and non-cooled control portions of the stems for sequential observations of cambial activity and for anatomical measurements of cell morphology by light microscopy and image analysis. KEY RESULTS: The effect of localized cooling was first observed on differentiating tracheids. Tracheids narrow in diameter and with significantly decreased cambial activity were evident 5 weeks after the start of cooling in these stems. Eight weeks after the start of cooling, tracheids with clearly diminished diameters and thickened cell walls were observed in these stems. Thus, localized low temperature induced narrow diameters and obvious thickening of secondary cell walls of tracheids, which were identified as latewood tracheids. Two months after the cessation of cooling, a false annual ring was observed and cambium became active again and produced new tracheids. In Cryptomeria japonica, cambial activity ceased earlier in locally cooled portions of stems than in non-cooled stems, indicating that the cambium had entered dormancy sooner in the cooled stems. CONCLUSIONS: Artificial cooling of stems induced latewood formation and cessation of cambial activity, indicating that cambium and its derivatives can respond directly to changes in temperature. A decrease in the temperature of the stem is a critical factor in the control of cambial activity and xylem differentiation in trees.

The effects of localized heating and disbudding on cambial reactivation and formation of earlywood vessels in seedlings of the deciduous ring-porous hardwood, Quercus serrata.

BACKGROUND AND AIMS: The networks of vessel elements play a vital role in the transport of water from roots to leaves, and the continuous formation of earlywood vessels is crucial for the growth of ring-porous hardwoods. The differentiation of earlywood vessels is controlled by external and internal factors. The present study was designed to identify the limiting factors in the induction of cambial reactivation and the differentiation of earlywood vessels, using localized heating and disbudding of dormant stems of seedlings of a deciduous ring-porous hardwood, Quercus serrata. METHODS: Localized heating was achieved by wrapping an electric heating ribbon around stems. Disbudding involved removal of all buds. Three treatments were initiated on 1 February 2012, namely heating, disbudding and a combination of heating and disbudding, with untreated dormant stems as controls. Cambial reactivation and differentiation of vessel elements were monitored by light and polarized-light microscopy, and the growth of buds was followed. KEY RESULTS: Cambial reactivation and differentiation of vessel elements occurred sooner in heated seedlings than in non-heated seedlings before bud break. The combination of heating and disbudding of seedlings also resulted in earlier cambial reactivation and differentiation of first vessel elements than in non-heated seedlings. A few narrow vessel elements were formed during heating after disbudding, while many large earlywood vessel elements were formed in heated seedlings with buds. CONCLUSIONS: The results suggested that, in seedlings of the deciduous ring-porous hardwood Quercus serrata, elevated temperature was a direct trigger for cambial reactivation and differentiation of first vessel elements. Bud growth was not essential for cambial reactivation and differentiation of first vessel elements, but might be important for the continuous formation of wide vessel elements.

Regulation of cambial activity in relation to environmental conditions: understanding the role of temperature in wood formation of trees.

The timing of cambial reactivation plays an important role in determination of the amount and quality of wood and the environmental adaptivity of trees. Environmental factors, such as temperature, influence the growth and development of trees. Temperatures from late winter to early spring affect the physiological processes 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. The present review focuses on temperature regulation on the timing of cambial reactivation and xylem differentiation in trees, and also highlights recent advances in our understanding of seasonal changes in the cold stability of microtubules in trees. The review also summarizes the present understanding of the relationships between the timing of cambial reactivation, the start of xylem differentiation and changes in levels of storage materials in trees, as well as an attempt to identify the source of energy for cell division and differentiation. 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.

A rapid decrease in temperature induces latewood formation in artificially reactivated cambium of conifer stems.

BACKGROUND AND AIMS: Latewood formation in conifers occurs during the later part of the growing season, when the cell division activity of the cambium declines. Changes in temperature might be important for wood formation in trees. Therefore, the effects of a rapid decrease in temperature on cellular morphology of tracheids were investigated in localized heating-induced cambial reactivation in Cryptomeria japonica trees and in Abies firma seedlings. METHODS: Electric heating tape and heating ribbon were wrapped on the stems of C. japonica trees and A. firma seedlings. Heating was discontinued when 11 or 12 and eight or nine radial files of differentiating and differentiated tracheids had been produced in C. japonica and A. firma stems, respectively. Tracheid diameter, cell wall thickness, percentage of cell wall area and percentage of lumen area were determined by image analysis of transverse sections and scanning electron microscopy. KEY RESULTS: Localized heating induced earlier cambial reactivation and xylem differentiation in stems of C. japonica and A. firma as compared with non-heated stems. One week after cessation of heating, there were no obvious changes in the dimensions of the differentiating tracheids in the samples from adult C. japonica. In contrast, tracheids with a smaller diameter were observed in A. firma seedlings after 1 week of cessation of heating. Two or three weeks after cessation of heating, tracheids with reduced diameters and thickened cell walls were found. The results showed that the rapid decrease in temperature produced slender tracheids with obvious thickening of cell walls that resembled latewood cells. CONCLUSIONS: The results suggest that a localized decrease in temperature of stems induces changes in the diameter and cell wall thickness of differentiating tracheids, indicating that cambium and its derivatives can respond directly to changes in temperature.

Changes in the localization and levels of starch and lipids in cambium and phloem during cambial reactivation by artificial heating of main stems of Cryptomeria japonica trees.

BACKGROUND AND AIMS: Cambial reactivation in trees occurs from late winter to early spring when photosynthesis is minimal or almost non-existent. Reserve materials might be important for wood formation in trees. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules in cambium and phloem were examined from cambial dormancy to the start of xylem differentiation in locally heated stems of Cryptomeria japonica trees in winter. METHODS: Electric heating tape was wrapped on one side of the stem of Cryptomeria japonica trees at breast height in winter. The localization and approximate levels of starch and lipids (as droplets) and number of starch granules were determined by image analysis of optical digital images obtained by confocal laser scanning microscopy. KEY RESULTS: Localized heating induced earlier cambial reactivation and xylem differentiation in stems of Cryptomeria japonica, as compared with non-heated stems. There were clear changes in the respective localizations and levels of starch and lipids (as droplets) determined in terms of relative areas on images, from cambial dormancy to the start of xylem differentiation in heated stems. In heated stems, the levels and number of starch granules fell from cambial reactivation to the start of xylem differentiation. There was a significant decrease in the relative area occupied by lipid droplets in the cambium from cambial reactivation to the start of xylem differentiation in heated stems. CONCLUSIONS: The results showed clearly that the levels and number of storage starch granules in cambium and phloem cells and levels of lipids (as droplets) in the cambium decreased from cambial reactivation to the start of xylem differentiation in heated stems during the winter. The observations suggest that starch and lipid droplets might be needed as sources of energy for the initiation of cambial cell division and the differentiation of xylem in Cryptomeria japonica.

Temperature responses of cambial reactivation and xylem differentiation in hybrid poplar (Populus sieboldii x P. grandidentata) under natural conditions.

Several studies have demonstrated that localized heating of tree stems induces localized cambial reactivation. We analyzed by light microscopy the effects of early spring increases in ambient temperature in 2005 and 2007 on the timing of cambial reactivation and xylem differentiation in stems of two trees of a cloned deciduous hardwood hybrid poplar (Populus sieboldii Miquel. x P. grandidentata Michx.) growing under natural conditions. Meteorological data at the study site showed that temperatures in late winter and early spring differed markedly between 2005 and 2007, with trends toward higher temperatures starting around April 3 in 2005 and around March 20 in 2007. Cambial reactivation occurred about 17 days earlier in 2007 than in 2005. The cumulative daily maximum temperature in excess of 15 degrees C (maximum daily temperatures minus 15 degrees C) in late winter and early spring before cambial reactivation was defined as the cambial reactivation index (CRI(15)). Cambial reactivation, which began when the minimum temperature rose above 0 degrees C, occurred when the CRI(15) was 93 and 96 degrees C in 2005 and 2007, respectively. The differentiation of secondary xylem started earlier in 2007 than in 2005. On May 27, we found a wider current-year band of xylem and a higher frequency of small-diameter vessel elements in 2007 than in 2005. We propose that the timing of cambial reactivation is controlled by air temperature and that earlier cambial reactivation induces earlier differentiation of xylem in hybrid poplar under natural conditions. Our results indicate that the CRI might be a useful indicator of the timing of cambial reactivation.

Induction of cambial reactivation by localized heating in a deciduous hardwood hybrid poplar (Populus sieboldii x P. grandidentata).

BACKGROUND AND AIMS: The timing of cambial reactivation plays an important role in the control of both the quantity and the quality of wood. The effect of localized heating on cambial reactivation in the main stem of a deciduous hardwood hybrid poplar (Populus sieboldii x P. grandidentata) was investigated. METHODS: Electric heating tape (20-22 degrees C) was wrapped at one side of the main stem of cloned hybrid poplar trees at breast height in winter. Small blocks were collected from both heated and non-heated control portions of the stem for sequential observations of cambial activity and for studies of the localization of storage starch around the cambium from dormancy to reactivation by light microscopy. KEY RESULTS: Cell division in phloem began earlier than cambial reactivation in locally heated portions of stems. Moreover, the cambial reactivation induced by localized heating occurred earlier than natural cambial reactivation. In heated stems, well-developed secondary xylem was produced that had almost the same structure as the natural xylem. When cambial reactivation was induced by heating, the buds of trees had not yet burst, indicating that there was no close temporal relationship between bud burst and cambial reactivation. In heated stems, the amount of storage starch decreased near the cambium upon reactivation of the cambium. After cambial reactivation, storage starch disappeared completely. Storage starch appeared again, near the cambium, during xylem differentiation in heated stems. CONCLUSIONS: The results suggest that, in deciduous diffuse-porous hardwood poplar growing in a temperate zone, the temperature in the stem is a limiting factor for reactivation of phloem and cambium. An increase in temperature might induce the conversion of storage starch to sucrose for the activation of cambial cell division and secondary xylem. Localized heating in poplar stems provides a useful experimental system for studies of cambial biology.