木材の細胞膜構造と収縮異方性 (I) : 仮道管の径接細胞膜におけるリグニン分布

Abstract

木材横断面における収縮異方性の原因としては, 種々のオーダーの構造単位に関連してのものが考えられるが, 細胞膜の非晶構造, とくにリグニン分布にその要因を求めた例は少ない。そこで, 本報では針葉樹材スギ・モミについて, フロログルシン-塩酸によるリグニン呈色反応を利用し, 細胞膜中のリグニンをミクロフォトメーターで定量化して, これと微小木口切片 (厚さ30μ) の収縮異方度との関連を追求した。これによって, 細胞膜構造のうち, リグニン分布状態が収縮異方性におよぼす程度を検討して, 2, 3の知見を得た。(1) 形成過程の早材および晩材仮道管において, 接線膜よりも半径膜の呈色濃度が, 生材でとくに大きい。(2) いったん乾燥した成熟材の早材および晩材仮道管においても, 半径膜の呈色濃度が, 接綜膜のそれよりも大である。(3) 呈色濃度の接線膜に対する半径膜の比 (I_r1/I_t1>1) は, 同一年次で晩材の方が大きい。(4) いったん乾燥したスギの辺材および心材の微小木口切片について, 接線収縮の半径収縮に対する比 (βt/βr) と補正した呈色濃度比 (I_r_1/I_t_1) の関係を求めた (Fig. 5)。この結果から次のことが理解できる。すなわち, βt/βrとI_r_1/I_t_1の関係は, いずれも明らかに直線的で, 関係直線は, (ii) 早材仮道管においてはいずれも45°の線に近く位置するが, 晩材仮道管においてはいずれもI_r_1/I_t_1軸側に偏る。したがって, これらのもつ意味を考えると, 心辺材とも早材仮道管の横断面における収縮異方性に, 径接細胞膜におけるリグニン分布の差がおよぼす影響は大きく, 晩材仮道管では, リグニン分布の差も関与するが, ほかにβtを小さくするか, βrを大きくする他の因子の影響が著しいものと思われる。

This paper is concerned with the relationship between the transverse anisotropic shrinkage of wood and the lignin distribution in the cell walls. The experiment was made by use of two conifers, namely Sugi (Cryptomeria japonica) and Momi (Abies firma). The lignin content in the cell walls of tracheids was shown by the intensity of color reaction with phloroglucinol-hydrochloric acid on the cell wall. The absorption of light passing through color films of the stained cross section taken at a magnification of 400 times was measured by a microphotometer to determine the coloring intensity of the cell walls. In order to indicate the difference in the lignin content between the radial and tangential walls, a coloring intensity ratio (Ir1/It1) was proposed. On the other hand, the transverse shrinkage of small serial cross sections of 30 micron in thickness, including 10 to 20 tracheids free from ray cells, was microscopically measured, and the degree of anisotropic shrinkage was shown by an anisotropic shrinkage ratio (βt/βr). During the shrinkage measurement the sections were kept conditioned in a specially designed microchamber (Fig. 1). The results obtained are as follows: 1) The coloring intensity measured by the microphotometer is higher in the radial walls than in the tangential walls in either of green and air-dried mature wood tracheids (Photo 2, 3 and 4, and Fig. 3 and 4). 2) In green wood, the difference in the lignin content estimated from the coloring intensity is evident even in the eighth or ninth cells from the cambial zone for Sugi and the fourth or fifth cells for Momi, where the staining is more prominent in the radial walls than in the tangential walls (Photo 1). 3) The value of the coloring intensity ratio (Ir1/It1), being always greater than 1, is higher for late wood than for early wood in the same annual ring (Fig. 3 and 4). 4) In the small cross section cut from once air-dried sapwood and heartwood of Sugi, the relationship between the anisotropic shrinkage ratio (βt/βr) and the coloring intensity ratio corrected (Ir'/It') are diagrammatically illustrated (Fig. 5). The relationship is shown to be linear. In the early wood the linear lines are drawn with a slope of about 45°, and this suggests that the anisotropic shrinkage in early wood is closely connected with the difference of lignin content between the radial and tangential walls throughout the sapwood and heartwood. In contrast, the linear lines obtained in the late wood lie with an easy slope. This means that the dependence of differential distribution of lignin in the cell walls on the anisotropic shrinkage of wood is less in late wood than in early wood, and in this case the effect of another factor should also be considered.