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

Abstract

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.

Keywords: Chamaecyparis obtusa; Cryptomeria japonica; cambial dormancy; false ring; latewood formation; localized cooling of stem.

Fig. 1.

Cooled-water circulation (A) and the system for passing cooled water around the main stem, at breast height, of an adult specimen of Chamaecyparis obtusa (B). Arrows indicate the cooled and non-cooled control portions of the stem.

Fig. 2.

Records of the maximum, average and minimum daily air temperatures at the experimental site in Fuchu, Tokyo, from 1 September 2010 to 30 November 2010 (A) and from 1 June 2011 to 31 October 2011 (B). Black arrows indicate the start of localized cooling of stems and red arrows indicate the cessation of cooling.

Fig. 3.

Light micrographs showing transverse views of cambium, differentiating phloem and xylem collected from cooled and non-cooled control portions of stems of Cryptomeria japonica. Before the start of cooling on 30 September 2010, differentiating tracheids and a few thin cell plates (arrowheads) were observed in the cambium (A). Two weeks after the start of cooling, on 14 October 2010, no new cell plates were observed in the cambial zone and differentiation of tracheids was almost complete in cooled portions of stems (B). By contrast, differentiation of tracheids continued in non-cooled control stems (C). Four weeks after the start of cooling, on 31 October 2010, cambial cells were arranged very compactly and no new cell plates were found in the cambium, indicating that the cambium had entered dormancy (D). On the same day, many differentiating tracheids were observed in non-cooled control portions of stems of Cryptomeria japonica trees (E). C, cambium; Ph, phloem; Dxy, differentiating xylem; Xy, xylem. Scale bars = 200 µm.

Fig. 4.

Light micrographs showing transverse views of cambium, differentiating phloem and xylem, and mature xylem that were collected from cooled and non-cooled control portions of stems of adult specimens of Chamaecyparis obtusa. Active cambial cell divisions with large numbers of differentiating cells were observed before the start of localized cooling, on 8 June 2011, in Chamaecyparis obtusa (A). Two weeks after the start of cooling, on 24 June 2011, active cambial cell divisions with large numbers of differentiating cells were observed both in cooled portions of stems (B) and in non-cooled control portions of stems of Chamaecyparis obtusa (C). Four weeks after the start of cooling, on 8 July 2011, no evident changes to differentiating tracheids were observed in cooled portions of stems (D). On the same date, 8 July 2011, active cambial cell divisions with large numbers of differentiating cells were observed in non-cooled control portions of stems of Chamaecyparis obtusa (E). C, cambium; Ph, phloem; Xy, xylem. Scale bars = 200 µm.

Fig. 5.

Light micrographs showing transverse views of cambium, differentiating phloem and xylem, and mature xylem that were collected from cooled and non-cooled control portions of stems of Chamaecyparis obtusa. Five weeks after the start of cooling, on 15 July 2011, a reduction in diameters of tracheids (black arrowheads) was observed for the first time in Chamaecyparis obtusa (A). On the same date, in non-cooled control portions of stems, active cambial cell divisions with large numbers of differentiating cells were observed when the cambial zone consisted of eight or nine radial layers of fusiform cambial cells (B). Prominent thickening of cell walls of tracheids was observed 6 weeks after the start of cooling, on 22 July 2011, when the cambial zone consisted of four or five radial layers of fusiform cambial cells (C). On the same date, in non-cooled control portions of stems, the cambial zone was consisted of eight or nine radial layers of fusiform cambial cells (D). Eight weeks after the start of cooling, on 5 August 2011, more pronounced reductions in radial diameter and in the thickness of cell walls of tracheids were observed, together with decreased cambial activity (E). On the same day, in non-cooled control portions of stems, reductions in tracheid diameter (black arrowheads) were evident when the cambial zone consisted of eight or nine radial layers of fusiform cambial cells (F). C, cambium; Ph, phloem; Xy, xylem. Scale bars = 200 µm.

Fig. 6.

Bar charts showing changes in numbers of radial layers of fusiform cambial cells and widths of the current year’s xylem in cooled and non-cooled control portions of stems of Chamaecyparis obtusa over the course of the experiment (n = 4, i.e. four images were analysed for each of the treatments). For each measurement, after 5 weeks of cooling, there was a significant difference in the number of fusiform cambial cells between cooled and non-cooled stems (A). After 4 weeks of cooling, there was a significant difference in the width of the current year’s xylem between cooled and non-cooled stems (B). Columns and bars show mean values ± s.d. Means with different letters are significantly different at P < 0.05 (Fisher’s LSD test).

Fig. 7.

Light micrographs showing transverse views of cambium and xylem and measurements made after 8 weeks of cooling, on 5 August 2011, of stems of Chamaecyparis obtusa. Radial diameters of tracheids and thicknesses of cell walls of tracheids in stems of Chamaecyparis obtusa trees, from the annual ring boundary towards the cambial zone, are shown after 8 weeks of cooling (n = 4; i.e. for each treatment, four images were analysed) (A). For each measurement, mean values ± s.d. are shown. Means with a different letter are significantly different at P < 0.05 (Fisher’s LSD test). (B) Light micrographs showing radial files of differentiating tracheids, including cambium (Ca). (C) Relative areas as percentage of areas of lumen and cell wall for each differentiating tracheid, as determined by image analysis of transverse sections. The latewood is indicated.

Fig. 8.

Light micrographs showing transverse views of cambium, differentiating phloem and xylem collected from cooled portions of stems 2 months after cessation of cooling of Chamaecyparis obtusa. Cooling was discontinued on 8 August 2011 when latewood formation was evident (arrows indicate cessation of cambial activity as well as formation of false annual ring due to localized cooling) and then cambium entered dormancy. Two months after cessation of cooling, on 14 October 2011, a false annual ring (arrows) was clearly visible and cambium became active again and produced new tracheids, as shown in A. On 1 November 2011, cambium was still active and produced large amounts of tracheids, as shown in B. C, cambium; Ph, phloem; Xy, xylem. Scale bars = 200 µm.