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ヒノキ着生枝葉の光合成速度の日変化を，同化箱法によって野外で測定した．さらに同一試料の切断枝葉を用いて，実験室内において光強度・葉温と光合成速度との関係を求め，野外着生枝葉で測定された光強度，葉温条件から光合成速度の日変化曲線を近似した．両曲線を比較したところ，曇天や雨天日においては対応がよく，両曲線はほぼ同じ日変化を示したが，晴天日や土壌の乾燥した日には測定値は近似値よりも低い値となり、日変化曲...線は異なっていた．このような測定値と近似値の差は水ストレスにより着生枝葉の光合成速度が低下したことにあると考えられた．水ストレス存在下での光合成速度は木部圧ポテンシャル（Ⅹylem pressure potential：ⅩPP）を水分指標とし，ⅩPPと光合成速度の関係を用いて補正することにより算出した．この結果，近似値から水ストレスによる光合成速度の低下分を差し引いた晴天日における計算値は，乾燥から回復した一時期を除いて，測定値とよく一致した．このことから，晴天日において光合成速度の測定値と近似値に差が生じた大きな要因は水ストレスであると判断された．これらの結果から，ヒノキにおいては切枝法の測定値をもとに着生枝葉の野外での光合成速度を近似する場合，水ストレスの存在下では近似値は実際よりも高く計算される可能性のあることが指摘された．
An average rate of CO_2, uptake (net photosynthesis) for every 6 minutes in daytime, leaf temperature, light intensity and xylem-pressure potential were measured with attached shoot of 3-year-old hinoki (Chamaecyparis obtusa Endl.) between September 19 and October 3 under ambient conditions in the field. After the measurement the shoot were excised, and rates of net photosynthesis at steady states (Pn) were measured with the detached shoot by changing leaf temperature and light intensity. The values of Pn were plotted as a function of the factors affecting on photosynthesis, leaf temperature(LT) and light intensity(LI). We found that the plots of Pn versus LT at different light intensities are able to be approximated by the equation Pn=a(LT-b)^2+c The approximation allows us to calculate the values of Pn corresponding to the values of LT and LI in the field. By using the approximation method, we simulated time courses in daytime of the net photosynthesis observdd in the attached shoot in the field. When the simulation was attempted under the assumption that photosynthesis is mainly restricted by LT and LI, coincidence between the "observed" and "simulated" time courses occured only in the cases where water content of soil was high and vapour pressure gradient(VPG) from leaf to air was low. The result suggest that photosynthesis under dried soil and high VPG is suppressed. On the other hand, under the assumption that photosynthesis is restricted by the three factors LT, LI and XPP, the curves corrected based on the relation between Pn and XPP showed close agreement with the "observed" time courses in all except the case where dried soil became abruptly wet. These results are discussed in relation to the effect of water stress on net photosynthesis.show more
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