提高CO_2濃度對不同耐陰等級樹苗光合作用性狀的影響

Effects of Elevated CO_2 Concentrations on the Photosynthetic Traits of Tree Seedlings with Different Shade-Tolerance Levels

許子淳(Tzu-Chun Hsu) ； 郭耀綸(Yau-Lun Kuo)

《台灣林業科學》 36卷4期 (2021/12) Pp. 309-326

https://doi.org/10.7075/TJFS.202112_36(4).0004

摘要

本研究測定人工林下層大氣CO_2濃度的變化，並比較不同耐陰性樹苗的光合作用性狀在不同光量條件下，對CO_2濃度提高的反應有何差異。選用的材料為五類耐陰等級闊葉樹種的天然更新苗各3種。結果發現，林內大氣CO_2濃度於林床表面最高，四季平均可達500 ppm，植株葉片越靠近地表則可利用的CO_2資源越多。五類樹苗的淨光合作用率（P_n）均可因CO_2濃度由400 ppm提高到500 ppm而增大，但不同耐陰等級樹苗的光合作用CO_2利用效率（CUE），於低光或高光條件下有相反的表現。耐陰性較高的三類樹苗在≤ 60 μmol photon m^(-2) s^(-1)低光時有顯著較高的CUE，但陽性及先驅樹苗在≥ 250 μmol photon m^(-2) s^(-1)高光時卻有顯著較高的CUE。當CO_2濃度由400 ppm提高到500 ppm，各類樹苗的光補償點可顯著降低17～45%，但暗呼吸率僅降低11～17%；耐陰性較高的三類樹苗，光量子利用效率（QUE）可因CO_2濃度提高而顯著增加58～92%，然而陽性及先驅樹苗的QUE卻不能顯著增加。土壤呼吸釋出較高的CO_2資源，可使耐陰性較高的樹苗之Pn，在更低的光量時即呈現正值。然而，陽性及先驅樹苗在≤ 15 μmol photon m^(-2) s^(-1)低光時，CO_2濃度需要＞ 500 ppm其P_n才會成為正值。本研究結論為，在低光環境下提高CO_2濃度，耐陰性較高的樹苗其P_n、CUE、QUE均可顯著提高，且其碳固定的促進效應高於較不耐陰的樹苗；但在高光環境下，提高CO_2濃度對較不耐陰樹苗的CUE提升效率較顯著。

關鍵字

CO_2利用效率 ； 光補償點 ； 低光環境 ； 光量子利用效率 ； 耐陰性

並列摘要

This research investigated variations of atmospheric CO_2 concentrations under a plantation and the responses of photosynthetic traits to CO_2 enrichment under different light intensities in seedlings of tree species with various shade-tolerance levels. Naturally regenerated seedlings of 3 broadleaf tree species for each of 5 shade-tolerance levels were chosen to measure photosynthetic responses. Results showed that CO_2 concentrations were the highest at the forest floor, reaching 500 ppm on average in all seasons. The closer the leaves were to the forest ground, the more CO_2 resources were available. Net photosynthetic rates of seedlings of the 5 shade-tolerance levels all increased as CO_2 concentrations were elevated from 400 to 500 ppm. However, the photosynthetic CO_2-use efficiency (CUE) of seedlings among the 5 shade-tolerance levels showed reverse responses under low and high light intensities. Seedlings of species belonging to the 3 higher shade-tolerance levels exhibited significantly higher CUE values under low light intensities (of ≤ 60 μmol photon m^(-2) s^(-1)), yet seedlings of shade-intolerant and very intolerant species showed significantly higher CUE values under high light intensities (of ≥ 250 μmol photon m^(-2) s^(-1)). When CO_2 concentrations increased from 400 to 500 ppm, the light compensation point of seedlings significantly decreased 17~45%, but the dark respiration rate merely decreased 11~17%. The quantum-use efficiencies (QUEs) of seedlings of species belonging to the 3 higher shade-tolerance levels increased 58~92% due to elevated CO_2 concentrations. However, elevated CO_2 concentrations did not significantly increase QUE values of seedlings of shade-intolerant and very intolerant species. The enriched CO_2 emitted by soil respiration could prompt positive photosynthetic rates of shade-tolerant seedlings even under lower light conditions; yet, for seedlings of shade-intolerant and very intolerant species, the CO_2 concentrations needed to be ＞ 500 ppm before their net photosynthetic rates became positive under low light conditions of ≤ 15 μmol photon m^(-2) s^(-1). In conclusion, under low light environments with elevated CO_2 concentrations, shade-tolerant species significantly increased their net photosynthetic rate, CUE, and QUE, showing higher facilitation effects of carbon fixation than shade-intolerant species; yet, under high light environments, the elevated CO_2 concentrations showed more significant effects on increasing CUE values of shade-intolerant species.

並列關鍵字

CO_2-use efficiency ； light compensation point ； low light environment ； quantum-use efficiency ； shade-tolerance