Studies on the Response of Photosynthesis to Light Intensity in Leaves Attached at Various Positions in Tree Crowns of a Cryptomeria japonica Even-aged Stand－Effects of Shading and Leaf Aging
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In this 17-year-old stand, the photosynthetic rate per unit of light intercepting area was measured and compared at various potitions in the crown during a vigorous growing season. At light saturation, the rates were higher in the upper and the lower surface positions in the conical shaped part of the crown exposed to sunlight (13 to 18 mg CO_2 dm^<-2>hr^<-1>), than in the upper and the lower positions in the cylindrical shaped shaded part (8 to 10 mg). At lower light intensities, a reverse relationship occurred. The seasonal changes in the photosynthetic rates measured at light saturation and at lower light intensities differed from each other. Also different seasonal changes were observed at different positions in the crown. The photosynthetic rate in shaded parts was relatively steady. In seedlings grown under artificial shading and in saplings grown in the open, the photosynthetic rates measured at light saturation and at lower light intensities increased with shading, whereas the rates at light saturation decreased with increased age of leaves (i.e. current year leaves and one-year-old leaves). In dense stands, older leaves in the crown generally grow under heavier shade conditions, and the differences in photosynthetic rates at various positions at light saturation may be due mainly to leaf aging, whereas those at lower light intensities are due to shading.17年生スギ林分における生長期の樹冠各部の葉の光強度-光合成関係を調べた。その際, 葉が光をさえぎる有効表面積をペースにして光合成速度を表わした。また, スギ葉の光合成に及ぼす光環境と葉齢の影響を苗木と幼齢木で調べた。強光域の純光合成速度は, 陽樹冠上部と下部表面で高く, 13〜18mg CO_2 dm^<-2>hr^<-1>の値を示し, 陰樹冠上部と下部で低く, 8〜10mgの値を示した。一方, 弱光域の純光合成速度は, 陰樹冠で高い値を示した。光合成速度の季節変化は, 強光域と弱光域で異なり, また樹冠部位によっても異なるようだ。強光域と弱光域の純光合成速度は, 葉が庇陰下で生育することで上昇し, 強光域の光合成速度は, 当年生から1年生にいたる葉齢の進行で低下した。スギの閉鎖林分では, 概して, 葉齢の進んだ葉ほど強い庇陰下で生育するから, 樹冠部位による強光域の光合成速度のちがいは, 主として葉齢の影響により, 弱光域の光合成速度のちがいは, 光環境の影響によると考えた。
- 1979-06-25
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