水筆仔樹林上、下層葉光合作用相關特徵比較

Photosynthesis Related Characteristics of Upper and Lower Canopy Leaves of Kandelia obovata, a Mangrove Species in Taiwan

水筆仔是北台灣西岸主要紅樹林樹種。水筆仔樹林的上層葉和下層葉可能因為接受到的光量不同，導致其在形態和生理反應上有所調整。本文比較該樹林上、下層葉的葉面角度、葉面積大小、氣孔密度、單位重葉面積、葉綠素螢光等與光合作用相關特徵，以了解水筆仔樹林對光使用特性。結果發現上、下層葉在形態上有顯著差異：上層葉葉面角度較大、面積較小、氣孔密度較高以及單位重葉面積較小。雖然兩者單位面積葉綠素含量和葉綠素a/b比沒有顯著差異，但上層葉單位面積有顯著較高的氮含量。葉綠素螢光測量顯示：相較於下層葉，上層葉其電子傳遞鏈速率在較高光量下才會達到飽和且有顯著較高的光飽和值。這些特徵差異顯示水筆仔為因應不同光環境，在葉形態和生理上有所調整。

To understand the light use properties of canopy leaves in K. obovata, the dominant mangrove species in the west coast of northern Taiwan, I compared photosynthesis related characters, such as leaf angles, leaf area, stomatal density, specific leaf area, and chlorophyll fluorescence, of upper and lower canopy leaves of the species. Leaves that had developed in exposed positions (upper canopy) exhibited significantly steeper leaf angles, smaller leaf area, higher stomatal density and lower specific leaf area than those in shaded positions (lower canopy). No significant difference was found in chlorophyll (Chl) content per unit leaf area and Chl a/b ratio between the two types of leaves. In contrast, compared to lower canopy leaves, upper canopy leaves had higher nitrogen content per unit leaf area indicating a higher light saturated CO2 assimilation. Chlorophyll fluorescence measurement revealed that photosaturated electron transport rate (ETR) and the photon flux required to saturate ETR were significantly higher in upper canopy leaves than in lower canopy leaves. The plasticity in morphological and physiological response to variation in light regimes may represent an important mechanism for K. obovata to adapt to the saline and nitrogen limiting environment.