The experiments described herein have demonstrated that heating or cooling can cause alterations in regular cambial activity. This is in agreement with previous observations of cambial reactivation in locally heated stems of several conifers in winter (Oribe and Kubo, 1997
; Oribe et al., 2001
). Applying heat stimulated divisions in the cambium and led to xylem and phloem formation. Cambial reactivation in the stem of Norway spruce occurred on the phloem side first, which supports the observations of Oribe et al.
(2001, 2003, 2004) in other evergreen conifer species. In locally heated regions of stems of Abies sachalinensis
during cambial dormancy, a few fusiform cambial cells on the phloem side of the cambium differentiated into phloem cells prior to the re-initiation of cambial cell divisions, and only minimal differentiation of xylem occurred after cambial reactivation. According to Larson (1994)
, there are no significant differences among fusiform cambial cells. Furthermore, storage starch is not accumulated in fusiform cambial cells during dormancy (Larson, 1994
; Oribe et al.
, 2001). One of the reasons for earlier cambial reactivation on the phloem side in the heated part of the stem might be a perception of a gradient of external triggers in the cambium (Oribe et al.
, 2001). During heat treatment, there might be two main pathways for translocation of nutrients: radial, from phloem parenchyma cells; and tangential, from ray cambial cells. In this respect, the supply of nutrients to the cambial cells adjacent to the phloem is greater than on the xylem side, resulting in earlier reactivation of the cambial cell on the phloem side (Oribe et al.
, 2001). That the cambium remained dormant elsewhere in the stem and that its reactivation was restricted to the heated region confirmed the observations of Barnett and Miller (1994)
regarding the non-transference of temperature along the stem from the site of its application.
During winter cambial dormancy, cambial growth potential varies with species, evergreen and deciduous habit, and habitat of conifers (Oribe and Kubo, 1997
; Oribe et al., 2001
). Cambial cells of evergreen conifers at the quiescent stage of cambial dormancy, a stage that is imposed by external factors, can re-initiate cell division independently of the growth of new shoots and the development of buds in spring (Oribe and Kubo, 1997
; Oribe et al., 2001
. Localized heating of stems of Abies sachalinensis
during late winter induced localized reactivation of the cambium. However, the effect of localized heating on the extent of cell division in heat-reactivated cambium was not as distinct as that in naturally reactivated cambium. In addition, heated reactivated cambium ceased cell division soon after a few cells had been produced. Oribe et al.
(2003) suggested that in Abies sachalinensis
, continuous cell divisions in heated-reactivated cambium require additional conditions, which appear to be satisfied in naturally reactivated cambium. In the evergreen Cryptomeria japonica
, cambial reactivation often occurred in the heated portion of the stem (Oribe and Kube, 1997). This might reflect a dormancy stage of the cambium, showing that reactivation is more likely during environmentally imposed dormancy. No cambial response to heat treatment in the deciduous Larix leptolepis
indicated that cambial reactivation in this species is limited by several factors associated with bud break (Oribe and Kubo, 1997
The response of the cambium to the drop in temperature in our experiment was less pronounced and was not visible on the xylem side until 30 d of cooling. Cooling part of the Norway spruce stem caused earlier formation of latewood. Adjacent to the cambium, only 1–2 radially expanding xylem cells were observed. In addition, the number of cells in the cambium decreased to four or five, which was typical of dormant cambium. The drop in air temperatures presumably shortened cambial activity, resulting in a lower portion of latewood in the current xylem increment. It appears that the heating and cooling treatments did not influence the widths or the structure of the phloem growth increments. Moreover, at the ultrastructural and topochemical levels, no alterations were observed in the pattern of secondary cell wall formation and lignification or in lignin structure among cooled, heated and control samples.
The results presented herein and the results obtained by others confirm the importance of external factors on cambial activity and corresponding cell differentiation. However, internal factors, such as phytohormones and sugars, must also be taken into consideration (Little and Bonga, 1974; Riding and Little, 1984, 1986; Mellerowicz et al., 1992
; Savidge and Barnett, 1993
; Savidge, 1996, 2000
; Kozlowsky and Pallardy, 1997
; Lachaud et al., 1999
; Aloni et al., 2000
; Krabel, 2000; Uggla et al., 2001
; Wodzicki, 2001
; Oribe et al.
, 2003). The extent of cell divisions and cell differentiation in the cambium might depend on the supply of sucrose from the storage tissue to the cambium (Oribe et al.
, 2003). The cessation of cell divisions in the heat-reactivated cambium could be a result of the absence of sucrose. Oribe et al.
(2003) hypothesized that the continuation of cell division in the cambium after cambial reactivation requires a continuous supply of sucrose. Moreover, an inability of cambial derivatives to proceed with differentiation might cause a deficiency of indole acetic acid (IAA) in the cambial region of locally heated stems during cambial dormancy. During regular cambial activity, a continuous supply of IAA from elongating shoots and/or expanding buds can maintain cambial divisions and the subsequent cell development (Oribe et al.
, 2003). Local cooling of the stem could reduce supply and the level of carbohydrates and IAA to differentiating xylem cells, and hence trigger earlier formation of latewood tracheids and thus earlier cessation of cambial activity than under natural conditions. Long-term experiments are required to provide more detail of the effects of low or high temperatures on the rate of cell division, the histochemical response and possible disturbances in cellular differentiation.