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水ストレス下でのヒノキシュートの光合成速度，葉のコンダクタンスを推定するためのシミュレーションモデルを作成した．モデルに用いたパラメーターは光強度（LI），葉温（LT），大気飽差（VPD），明け方の木部圧ポテンシャル（BP），機体通水抵抗（R_{s-t}）の五因子である．モデルでは樹木の水ストレスの指標として木部圧ポテンシャル（XPP）を設定し，以下の仮定のもとでモデルを構築した．（1）葉のコンダ...クタンスはXPPの一因子で律速される．（2）定常状態での光合成速度と葉のコンダクタンスの関係は非定常状態でも変わらない．（3）切断シュートで得られる光合成速度，葉のコンダクタンスは水ストレスを受けていない．このモデルで推定された光合成速度，葉のコンダクタンスは全体的には実測値に近い値となり，本モデルの有効性が示された．しかし，細部においては，（1）午前中に低く推定する．（2）土壌の乾燥した午後には高く推定する．（3）光強度変化の著しい時には葉のコンダクタンスだけを低く推定するなどの不一致点があり，それぞれの原因として，（1）蒸散速度が樹液流速と対応していなかったこと，（2）葉のコンダクタンスの律速因子がXPPだけではなかったこと，（3）気孔の開閉時間を考慮しなかったこと，が考えられた．
A simulation model for CO_2-assimilation and leaf conductance of hinoki (Chamaecyparis obtusa Endl.) shoot under water stress conditions was constructed. Parameters used for this model were light intensity(LI), leaf temperature(LT), vapour pressure deficit(VPD), base potential(BP) and resistance to water flux from soil to leaf(R_{s-t}). For the model construction, xylem pressure potential(XPP) of shoot was assumed for an index of water coditions of hinoki tree, and other three assumptions were prepared. These assumptions were (1) leaf conductance was mainly controlled by XPP, (2) relations between CO_2-assimilation and leaf conductance at steady states were similar to those at non steady states, and (3) CO_2-assimilation and leaf conductance obtained by detached leaves were not restricted by water stress. Simulated time courses of CO_2-assimilation and leaf conductance were similar to the observed time courses, but there were three differences between these two curves. These differences were, (1) in early morning, simulated curves were underestimated, (2) when soil water content was low, simulated curves were overestimated in the afternoon, (3) when light intensity fluctuated hardly, simulated leaf conductance curves were overestimated. The causes of these differences and the processes how to remove them are discussed.show more
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