土壤鹽度對欖李苗木光合作用之影響

Effects of Soil Salinity on the Photosynthetic Responses of the Mangrove Lumnitzera racemosa Seedlings

本研究探討不同土壤鹽分濃度對欖李（Lumnitzera racemosa）苗木光合作用及相關生理反應之影響，並探討本樹種是否具有景天酸代謝（Crassulacean acid metabolism, CAM）植物特性。欖李苗木以0、0.75、1.5、3.0% NaCl 等四種鹽度處理七個月後，葉片光合作用速率、葉片導度、蒸散作用及細胞間隙CO2 濃度（Ci）均以0.75% 處理者最高，鹽度高於1.5% 處理時，這些參數明顯受抑制。苗木之水分利用效率（water use efficiency, WUE）以0、0.75 及1.5% 三處理較高，3.0%高鹽度處理者則已降低。葉綠素a 及葉綠素總濃度則以0.75% 處理者最高，而葉綠素b 及葉綠素a/b 比值在不同處理間並無顯著差異。以0、0.75 及1.5% 等鹽度處理之苗木其光合作用光度反應曲線相似，光飽和點光度約在540~600μmol m-2 s-1時，光補償點約在7~8.5μmol m-2 s-1 ﹔而苗木以3.0% 高鹽度處理110 天後，在光度大於600μmol m-2 s-1 以上時，光合作用速率較其他處理為高；光飽和點大約在800μmol m-2 s-1 時，光補償點約在18μmol m-2 s-1 之光照強度。SEM 觀察結果顯示，0% 處理之葉片保衛細胞較扁平，0、0.75及1.5% 等三種處理之氣孔數目並無顯著差異，但3.0% 處理者氣孔數則明顯增加。而不同鹽度處理之苗木，其葉片日、夜間之可滴定酸濃度並無顯著差異，顯示欖李並不具有CAM 植物之特性。 This study investigated how different salinities affected the photosynthetic responses of mangrove Lumnitzera racemosa seedlings. An attempt was also made to elucidate the metabolism of the Crassulacean acid metabolism (CAM) cycling in this species. Seedlings were grown in different concentrations of salinity containing 0, 0.75, 1.5, 3.0% NaCl for 7 months, gas exchange characteristics were studied for seedlings grown under a variety of salinity conditions. According to experimental results, the CO2 assimilation rate, leaf conductance, transpiration rate and intercellular CO2 concentration were highest in 0.75% treatment. In addition, all photosynthetic characteristics decreased when salinities above 1.5% NaCl. Water-use efficiencies of seedlings were high in 0, 0.75 and 1.5% treatments and inhibited in 3.0% NaCl treatment. Chlorophyll a concentrations and the total amount of chlorophylls were significantly stimulated in 0.75% NaCl treatment, salinity did not affect chlorophyll b concentrations and chlorophyll a/b ratio. Photosynthetic light response curves of seedlings grown in 0, 0.75 and 1.5% treatments resembled each other, the light saturation points were at 540 to 600μmol m-2 s-1 photosynthetic photon flux densities (PPFD), the light compensation points were at 7 to 8.5μmol m-2 s-1. Seedlings grown in 3.0% high salinity condition, and CO2 assimilation rate was higher than other treatments in higher PPFD (above 600μmol m-2 s-1). The light saturation point was at 800μmol m-2 s-1, the light compensation point was at 18μmol m-2 s-1. Electron microscopies revealed that the guard cells of leaves were thin and flat that grown in 0% treatment. Stomatal densities of seedlings cultured at 0 to 1.5% NaCl, although not significantly different, but increased in 3.0% NaCl treatment. In addition to diurnal fluctuations in titratable acidity of leaves not changing significantly, no evidence for CAM-cycling was found for L. racemosa.

This study investigated how different salinities affected the photosynthetic responses of mangrove Lumnitzera racemosa seedlings. An attempt was also made to elucidate the metabolism of the Crassulacean acid metabolism (CAM) cycling in this species. Seedlings were grown in different concentrations of salinity containing 0, 0.75, 1.5, 3.0% NaCl for 7 months, gas exchange characteristics were studied for seedlings grown under a variety of salinity conditions. According to experimental results, the CO2 assimilation rate, leaf conductance, transpiration rate and intercellular CO2 concentration were highest in 0.75% treatment. In addition, all photosynthetic characteristics decreased when salinities above 1.5% NaCl. Water-use efficiencies of seedlings were high in 0, 0.75 and 1.5% treatments and inhibited in 3.0% NaCl treatment. Chlorophyll a concentrations and the total amount of chlorophylls were significantly stimulated in 0.75% NaCl treatment, salinity did not affect chlorophyll b concentrations and chlorophyll a/b ratio. Photosynthetic light response curves of seedlings grown in 0, 0.75 and 1.5% treatments resembled each other, the light saturation points were at 540 to 600μmol m-2 s-1 photosynthetic photon flux densities (PPFD), the light compensation points were at 7 to 8.5μmol m-2 s-1. Seedlings grown in 3.0% high salinity condition, and CO2 assimilation rate was higher than other treatments in higher PPFD (above 600μmol m-2 s-1). The light saturation point was at 800μmol m-2 s-1, the light compensation point was at 18μmol m-2 s-1. Electron microscopies revealed that the guard cells of leaves were thin and flat that grown in 0% treatment. Stomatal densities of seedlings cultured at 0 to 1.5% NaCl, although not significantly different, but increased in 3.0% NaCl treatment. In addition to diurnal fluctuations in titratable acidity of leaves not changing significantly, no evidence for CAM-cycling was found for L. racemosa.