Eco-physiology Characteristics of pinus densiflora Sieb. et Zucc. and Pine Wilt Disease

Aerial photographs taken in the southern part of Okayama Prefecture, western Japan, were analyzed and a tree census was conducted for elucidating the relationship between the incident of pine wilt disease and site conditions. Pine wilt damage tended to be less in areas with dry conditions, such as the upper part of a slope and granite rock areas. It was suggested that the drought resistance of pine trees, caused by longterm water stress, was thought to be one of the important factors in resistance to pine wilt disease. Therefore, we examined the charactreristics of water relations of pinus densiflora Sieb. et Zucc. growing in different soil drying and avoided losing water because of the decrease in the leaf pressure potential derived from the high-elastic cell walls and because of the increase in the hydraulic resistance of the branch. Moreover, the P.densiflora under chronic water stress had a high capacity for photosynthesis. The physiological activity of P.densiflora under chronic water stress stayed high even after the inoculation by nematode. It was suggested that P. deinsiflora growing under longterm water stress could resist the influence of the extensive water deficit caused by the shortage of precipitation and by pine wilt disease, and could delay the development of pine wilt disease owing to the drought resistance.マツ材線虫病の発生と立地の関係を検討した結果、集水面積の小さな斜面上部や花崗岩のような乾燥しやすい母岩のマツ林ほど被害が小さく、マツ樹の水ストレスに対する耐性と被害の発生に何らかの関係があるものと考えられた。そこで、生育土壌の水分条件が異なるマツ樹の生理特性について検討した結果，慢性の水ストレスを受けて生育した個体の葉は，わずかな水分の損失に対して葉の圧ポテンシャルを著しく低下させ，また枝部の通水抵抗を高めることで急性の水ストレスに対して速やかに気孔を閉じることができる特性を有していた．その結果，マツ材線虫病被害発生の誘因として作用するストレスフルな状態を回避できることが示唆された．また，そうした耐乾性を有するマツ樹は同時に高いCO固定効率により光合成活性を高く維持できる特性を持ち合わせることで，マツ材線虫病の病徴の進展を抑制できることが示唆された．このように，立地の違いによるマツ材線虫病被害の発生の差が，生育土壌の水分条件の違いによる耐乾性の差異に大きく関与していることが明らかとなり，マツ材線虫病に対する抵抗性のメカニズムとしてマツ樹の耐乾性の重要性が示された