摘葉, 摘芽, 輪截, 光の遮斷等の處理が常綠針葉樹の成長 特に肥大成長に及ぼす影響

Abstract

本報告は常綠針葉樹中主としてクロマツを材料として行つた摘葉, 摘芽, 輪截, 光の遮斷等の處理が成長, 特に肥大成長に及ぼす影響に就ての實驗の結果と, 之に關連して形成層活動に對する條件, 特に成長素の關係に就て考察したものである｡ 實驗の結果の主要なる點は次の通りである｡ 1. 實驗に用いたクロマツ幼齡木の無處理の標準狀態に於ける成長は凡そ次の如き經過をとる｡ 卽ち年によつて多少の相違はあるが, 成長の開始は4月上旬にあり, 伸長, 肥大殆んど同時と見られる｡ 而して前者は5月上 ･ 中旬に最大, 6月中旬に略々完了し, 其後7月中旬以後に僅乍ら新芽の伸長を見る｡ 後者の最大は4月下旬に1度現れ, 其後低下して6, 7月の間は衰え, 8, 9月に第二の最大が現れ, 10月下旬に略々停止する｡ 2. 成長開始前にすべての芽を摘去しても, 舊葉を殘す時は肥大成長は起る｡ 芽の存否は肥大成長開始の條件とはならない｡ 併し頂部の成長は遲滯し, 特に成長期の後半に成長著しく減退する｡ 3. 芽を殘し, すべての舊葉を除くときは, 芽の伸長と共に肥大成長も起るが, 新葉の伸びる迄は成長量は微々たるものである｡ 以上によつて見ると肥大成長は成長期の前半に於ては主として舊葉によつて支えられ, 後半に於ては新葉によつて維持されると思われる｡ 4. すべての芽及び舊葉を除くときは肥大成長は全く起らない｡ 貯藏物質は存在してもそれのみを以て肥大成長を起す事は出來ない｡ しかし貯藏物質を消費して潜伏芽が伸長し, 新しく葉を生ずるときは之に伴つて肥大成長が現れる｡ 尤も芽が伸長しても其新葉を摘去すれば肥大成長は止む｡ 結局肥大成長には葉の存在を必要とすると言う事になる｡ 5. 輪截によつて上部に存する葉と皮部の連絡を斷つときは其部分の肥大成長は停止する｡ 輪截の上部にては其處に存する葉の量により夫が少なければ肥大成長は減少し, 夫が或程度以上に存在すれば正常以上にもなる｡ しかし伸長成長, 特に葉の伸長は常に制限される｡ 6. 主軸と側枝は皮部の連絡ある限り互に同化物質を交流して其成長を支えるが, 其間の距離が大なる程, 又經路が曲折せる程それは困難になる｡ 7. 光を遮斷せる狀態に於て伸長せる芽も其下の軸の肥大成長を促す効果がある｡ 舊葉に於ても同樣の効果はあるが極めて微弱である｡ 8. 成長しつゝある毬果もその着ける軸の肥大成長を促すが其効果は甚しく僅少である｡ 9. 芽の伸長とその軸の肥大成長は成長に要する構成物質を互に競合う｡ 從て芽の數を減ずれば軸の肥大成長は大となるが, 芽の減少は新葉の減少を意味するから成長期の後期には全體としての成長量が劣る樣になる｡ 10. 葉の減少は肥大成長の減少を來すが, それは單に失われたる葉の量に比例するものではなく葉のつける位置に關係し, 上位の葉の喪失は下位の葉の喪失よりも影響が著しい｡ 以上によつて見ると肥大成長には上部に葉の存在を必要とし, しかも夫と皮部の連絡あるを要する｡ 又その葉は必ずしも光に浴するを要しないから, 葉の存在は形成層の活動に對し同化作用以外何等かの意義ある事を示唆する｡ 之は近時成長に對し必須の要素と見られている成長素が葉に生じ皮部を下降して形成層を刺戟するによる事を物語るものであろう｡ しかし樹木に就て成長素の研究は未だ十分でない｡ 筆者はクロマツに就て直接成長素を測定し, 次の樣な事を知つた｡ a. 芽は伸長を開始すると共に成長素を增加し, 伸長の最大なる附近に最も多く見出される｡ しかし伸長を終れる部分にも消失する事なく, 秋末迄多量に檢出される｡ b. 芽に成長素が增加すると共に夫より下の軸にも成長素が現れ, 成長期中多少の增減はあるが秋末迄存續する｡ c. 芽又は新葉を除去すると其下の軸の成長素は著しく減ずる｡ 故に新葉は成長素の主要な給源と目される｡ しかし舊葉が殘れる限り, すべての芽を除去しても成長素は消失しない｡ 從て舊葉も多少は成長素を供給するとせねばならぬ｡ 尤も夏以後, 新葉の伸長の終る頃より後は舊葉を除去しても成長素の量に殆んど變化が見られないから其頃より舊葉の成長素生產能力は著しく減ずると思われる｡ d. 輪截を行つた場合處理部直下にて急に成長素は消失し, 時と共にそれが下方へ波及する｡ 而して其處に葉のない場合は全く見出されなくなる｡ e. 新葉に光を遮斷すると成長素は減ずるが, 葉の生活する限り全く消失する事はない｡ 之等によつて見ると前述の肥大成長を停止せる場合は成長素の消失する場合と一致しているが, もし成長素の缺乏が肥大成長停止の原因であるならば, 之に成長素を加うれば成長が起らなければならぬ｡ 此問題に關し筆者は摘葉, 輪截等によつて肥大成長の停止せる部分へヘテロオーキシ其他の合成成長素劑を塗布して見たが, 其結果局所的乍ら明かに新生假導管の出現を見た｡ 摘葉により全く同化器官を缺く場合に於ても貯藏物質の存する限り形成層の活動が認められた｡ 勿論之は溫度水分其他の環境條件が許す範圍に於てである｡ 本研究は筆者が可なり前に行つたものであるが, とりまとめる機會を失つていた｡ 此處に其機會を專まれたる沼田敎授と, 公表の便宜を與えられたる佐藤並びに上田敎授, 演習林當局の厚意に深謝する｡

The work presented herein has been undertaken to investigate the effects of defoliation, disbudding, girdling, darkening and other treatments upon the growth in trees, with particular reference to evergreen conifers, and special attention has been paid to the relation of growth hormone to cambial activity. The young trees of Pinus Thumbergii growing in the nursery of the Kyoto University Forest were mainly used for as experimental subjects. Under normal conditions in this locality, the radial growth of the pine began in general, at the first week of April, and continued till late in October. The growth-curve showed two maxima, one in April-May and the other in August-September, and some depression between them. As the response of the treatments the following results were observed: 1. The removal of all old leaves before the growing period was followed by the retardation of radial growth until young leaves elongated, whilst, the same treatment after the young leaves fully developed had almost no effect upon the course of growth. 2. By the removal of all buds leaving old leaves the radial growth did not stopped, but its gradual fall was seen in the later period. The removal of young leaves in midsummer also resulted in the marked reduction of autumnal growth. From these facts it may be assumed that the radial growth in evergreen conifers is mainly supported by the activity of old leaves in the early period, and by that of young current leaves in the later season. 3. The complete removal of buds and leaves throughout a tree stopped the growth at once. The similar result was found in the case of girdling. No woody layer was formed below the girdled zone unless dormant buds had arose. In such cases, the reserve substance can not be employed for cambial growth, however abundant it may be. 4. When developing buds or young leaves of a shoot were kept in a dark place the radial growth of the shoot was considerably checked, but it did not ceased as far as the leaves were still alive. Therefore, it is to say that the leaf has, even in dark, some ability to actuate cambial growth. The growing cone showed also similar but slight effect on the bearing shoot. 5. When either a terminal or branches were defoliated above a girdle, the growth of each shoot continued, but its rate was somewhat reduced, especially at the operated shoot. This means that the food-substances used for growth can be transported each other between the terminal and branches, though this become difficult with distance and complexity of the path to travel. 6. The partial excision of buds was beneficial to the growth of the remained parts at least in the beginning, because the competition for building material was relieved. But, afterwards, the gradual fall in growth as the whole was seen according to the reduction in number of new leaves. 7. The decrease in radial growth caused by the partial defoliation was not always proportional to the amount of lost leaves. The removal of leaves at upper part was more effective than at lower part. As the above mentioned, the presence of leaves, and that at upper level, is a matter of special significance for cambial growth. This may be attributable to the effect of growth hormone or auxin, which produced in leaves and transmitted in the morphologically downward direction. In trees, however, the behavior of such substance has not been sufficiently known. Some data obtained from the writer's investigation in pine are as follows: a. In buds, auxin appeared with the inception of height growth and increased with its progress. The maximum was found near the region of the greatest growth, but the amount of auxin did not diminish so much after the elongation ceaced. Considerable quantity could be detected throughout the growing season. b. In stems, the occurrence of auxin was almost simultaneous with in the buds at the upper parts, and spread in downward direction. c. The disappearance of auxin in the late season took place first at the upper part of young shoots and spread also downwards. d. By the removal of all buds or young leaves auxin did not entirely disappear, as far as the old leaves were left on. The same thing was seen in the case of girdling at the parts below the girdle. e. When leaves or buds were brought in a dark place the production of auxin decreased by degrees. In the foregoing experiments, it is to be noted that the decrease or disappearance of auxin always associated with the cessation of radial growth. If the stoppage of radial growth were due to the lack of auxin, the cambial activity must be restored by the use of growth hormones. To test this, attempts were made with heteroauxin and other hormonic reagents. The results have evidently shown that the new woody elements were formed beneath the parts which the hormone paste were applied. Therefore, the presence of auxin is surely considered as a factor of absolute necessity for cambial growth, but it should be remembered, of course, that the effect of auxin does not appear so long as the other factors, such as temperature, water and food supply etc., were not favourable.