（別紙様式 14）
2017年 8月24日
論文の内容の要約
氏 名 MD HASNAT RAHMAN
学位の種類 博士（農学）
学府又は研究科・専攻 大学院連合農学研究科環境資源共生科学専攻 指導を受けた大学 東京農工大学
学位論文名 Cell biology of mechanism of wood formation in trees under different climatic conditions
【論文の内容の要約】
Introduction
Wood is a renewable resource that has been used as a raw material for construction, furniture, pulp and paper, chemicals and fuel. Wood is formed through the activity of vascular cambium (cambium). Cambial activity is controlled through a complex interaction between environmental and internal factors. Although wood has great economic importance, the precise process of wood formation through cambial activity under different climatic conditions is not fully understood. Therefore, the main purpose of the present cell biological study was to identify the factors that regulate cambial activity and xylem differentiation in trees under different climatic conditions.
Materials and methods
Several conifers and hardwoods growing in Japan (temperate zone), Bangladesh (sub-tropical zone) and Indonesia (tropical zone) were used in this study. Samples were collected as small blocks from the main stem of adult trees or small portion of the main stem of seedlings, those contained phloem, cambium and xylem. Samples were collected from the locally heated stem and non-heated stem under natural conditions at different seasons. Cambial cell division and differentiation of its derivatives were investigated by light microscopy and transmission electron microscopy.
Results and discussion
In temperate zone, cambial activity ceases in autumn or winter (cambial dormancy) and resumes again (cambial reactivation) in late winter or early spring. The timing of cambial reactivation affects the duration of wood formation. Localized heating (20-22˚C) in winter or an increase in temperature under natural condition in spring induces cambial reactivation in Chamaecyparis pisifera tree. However, Chamaecyparis tree requires longer period of heating for cambial reactivation than other conifers. Such
differences are closely related to the cambial sensitivity to temperatures from quiescent to active state. By contrast, under natural condition, the timing of cambial reactivation can be predicted from the sum of the maximum daily temperatures, in degrees, above a threshold value (cambial reactivation index; CRI). These observations suggest that an increase in temperature might be a limiting factor in the onset of cambial reactivation during the quiescent dormant state of trees.
After formation of a few layers of earlywood tracheids with large diameters and thin walls, tracheids with narrow diameters and thick walls that are similar to latewood were formed with heating in Chamaecyparis tree. Moreover, after cambial reactivation, stored starch started to decrease from cambial region and disappeared during formation of tracheids. The results suggest that an increase in temperature might induce the conversion of stored starch to soluble sugars for the activation of cambial cell division and earlywood formation, but, the shortage of starch might be responsible for the formation of latewood tracheids.
Furthermore, it is also revealed that an increase in temperature on the stem can induce cambial reactivation which is independent of needles and buds or basipetal polar transport of auxin in Abies homolepis. However, after cambial reactivation, initiation of earlywood tracheids is delayed or suppressed when the transport of polar auxin is inhibited. In contrast, exogenous application of auxin (IAA) on the stem initiates the formation of earlywood tracheids, indicating that continuous supply of adequate level of auxin to the cambium is needed for continuous cell division and cell expansion. In addition, another plant hormone gibberellin might be also important for cell division and earlywood tracheid formation.
In tropical zones, the cambium of trees shows often no obvious seasonal cycles of activity and dormancy. However, no or little precipitation (dry season) induces the temporal cessation of cambial activity. In contrast, the continuous precipitation (no dry season) induces no obvious cessation of cambial activity. In sub-tropical zones, cambial activity initiates during hot-summer season with the supply of high irrigation water.
Therefore, the precipitation or the availability of water on the soil might be the critical factor in regulation of cambial activity on tropical trees.
Finally, it is concluded that changes in temperature and precipitation might control the process of wood formation directly through the regulation of cambial activity or indirectly through changes in the supply of internal factors such as soluble sugars, auxin and gibberellin under different climatic conditions. Therefore, the future climate changes such as global warming and severe drought will affect the production of wood and the environmental adaptivity of trees.
