Localized heating of the stem during the dormant season induced the division of phloem and cambial cells in a deciduous diffuse-porous hardwood poplar. The results reveal that an increase in temperature of the stem is one of the most important factors in cambial reactivation in deciduous trees. Division of phloem cells started prior to cambial reactivation in both heated and non-heated control portions of stems. Heat treatment for at least 2 weeks was necessary to produce new cell plates in the phloem. New cell plates were formed in phloem cells that had been produced during the previous growing season both in heated and in non-heated control portions of stems. Therefore, the type and locations of first-dividing cells were the same in heated and control portions of stems. This observation suggests that phloem cells might respond to heating more rapidly than cambial cells in hybrid poplar.
Localized heating has been reported to induce cambial reactivation earlier than natural cambial reactivation in several evergreen conifers, such as Pinus contorta
(Savidge and Wareing, 1981
), Picea sitchensis
(Barnett and Miller, 1994
), Cryptomeria japonica
(Oribe and Kubo, 1997
), Abies sachalinensis
(Oribe et al., 2001
) and Picea abies
(Gričar et al., 2006
). Oribe et al. (2001
) observed, in the evergreen conifer Abies sachalinensis
, that heating for 5–6 d was necessary for cambial reactivation. By contrast, in the diffuse-porous hardwood poplar studied here, approximately 4 weeks was required for cambial reactivation. Therefore, we suggest that, in poplar, the state of dormancy is deep and that lengthy heating is needed to break cambial dormancy. In the deciduous conifer Larix leptolepis
, 2 weeks of heating failed to reactivate the division of cambial cells (Oribe and Kubo, 1997
). Longer heat treatment of deciduous trees, as compared with evergreen conifers, might be needed for cambial reactivation from the quiescent dormant state.
The present results also showed that, during the second heating treatment in December and January, more time was required for cambial reactivation than during the first heating treatment in February and March. Previous studies have shown that resting cells in the cambium are unable to divide, even when exposed to favourable conditions, while cambium in the quiescent state retains the potential to generate new cells (Savidge and Wareing, 1981
; Barnett and Miller, 1994
; Oribe and Kubo, 1997
; Oribe et al., 2001
; Rensing and Samuels, 2004
). Therefore, it is likely that the cambium of poplar in December and January might be in a transitional state, between resting and quiescence, at our experimental site and, therefore, no rapid response to heating was possible.
found, in the deciduous hardwood Aesculus hippocastanum
, that the first cambial cell division occurred on the phloem side of cambial cells. Similarly, Oribe et al. (2003)
and Gričar et al. (2006)
reported that, in the evergreen conifers Abies sachalinensis
and Picea abies
, the first cambial cell division started on the phloem side of cambial cells. By contrast, here the first cell division in the cambial zone occurred in the second layer of fusiform cambial cells from the previous year's xylem side. This observation is in agreement with the site of first cambial cell division in the deciduous hardwoods Fraxinus excelsior
(Funada and Catesson, 1991
) and Robinia pseudoacacia
(Farrar and Evert, 1997a
). Therefore, the pattern of cambial reactivation appears to depend on species.
In heated poplar stems, xylem differentiation started after 2 months of heating and well-developed xylem cells were produced within 1 week. The structure of the newly formed secondary xylem cells was more or less similar to that of the normally formed xylem cells in hybrid poplar stems. Oribe and Kubo (1997)
and Gričar et al. (2006)
observed, in the evergreen conifers Cryptomeria japonica
and Picea abies
, that localized heating induced the formation of earlywood tracheids with a large radial diameter and secondary walls during cambial dormancy in a temperate zone. By contrast, Oribe et al. (2003)
observed, in heated Abies sachalinensis
, that only minimal differentiation of xylem occurred after cambial reactivation in a cool temperate zone. Their results are different from those of the present investigation of heated poplar stems. The results here show clearly that heating can induce the formation of large volumes of normal secondary xylem according to the normal pattern found in the temperate zone. The timing and extent of induction of xylem differentiation by heating might depend on climatic conditions.
It has been reported that bud burst and the development of new leaves are related to cambial reactivation and xylem differentiation (Aloni, 1991
; Suzuki et al., 1996
). Barnett (1992)
showed, in Aesculus hippocastanum
, that the first division of cells in the cambium started when buds started to swell. Here bud burst and cambial reactivation in non-heated portions of stems started simultaneously during our first heating experiment. However, cambial reactivation in heated portions of stems started without bud burst, indicating that no close relationship exists between the timing of bud burst and cambial reactivation. Therefore, we suggest that, in deciduous hardwood hybrid poplar, bud burst is not a prerequisite for cambial reactivation from the quiescent state. Xylem differentiation in heated portions of stems started after bud flushing, suggesting that some factor(s) from expanding new leaves might be required for xylem differentiation.
Heat treatment changed the amount and localization of storage starch in the phloem and cambium. The amount of storage starch increased near the cambium, as compared with that during the dormant state, when phloem cell division occurred in heated portions of stems. By contrast, during cambial reactivation, a small amount of storage starch was found mainly in phloem parenchyma cells and ray parenchyma cells. After cambial reactivation, storage starch disappeared completely and continuous cell division occurred in the cambium in heated portions of stems. It seems that storage starch, which is a source of sugars, is needed for the division of phloem cells and cambial reactivation. Cambial reactivation occurred in heated portions of the stem of a deciduous hardwood before the development of new leaves, indicating that there was no need for a supply of photosynthate from leaves. Therefore, the utilization of storage starch might be needed for new cell division in the phloem and cambium.
Oribe et al. (2003)
demonstrated, in Abies sachalinensis
, that cells of heat-reactivated cambium stopped dividing with the disappearance of starch from the storage tissues around the cambium and little or no new xylem was formed. Therefore, they concluded that continued division of cells in the cambium, after cambial reactivation, requires a continuous supply of sucrose. In the current study, starch grains reappeared in the phloem parenchyma cells and ray parenchyma cells near the cambium after cambial reactivation, although the source of this additional starch was not identified. Consequently, a large volume of secondary xylem cells was produced. We can postulate that the differentiation of secondary xylem cells needs considerable energy, which can be obtained from stored carbohydrate. Increases in temperature might induce the enzyme-mediated conversion of storage starch to sucrose for the continuous division of cells in the cambium and the production of new xylem cells in deciduous hardwood poplar.
It is clear that an increase in the temperature of the stem can directly induce the breaking of cambial dormancy in deciduous hardwood hybrid poplar, a model tree. The pattern of cambial reactivation and secondary xylem differentiation induced by heating was the same as that in non-heated control stems. Therefore, cambial reactivation by heat treatment in winter might be a good model system with which to investigate the dynamics of cambial biology because it is easy to follow the process from the division of phloem and cambial cells to secondary xylem development. Using this system, it should be possible to identify the endogenous factors, such as specific genes and proteins and plant growth regulators, that are involved in reactivation of the cambium.