Effects of Erought Stress and Salt Stress on Physiological Metabolism of Mesembryanthemum crystallinum L. (common ice plant)

本實驗以ice plant ( Mesembryanthemum crystallinum L. )為實驗材料 ，探討缺水及鹽處理對其光合形式轉換過程的影響。長期乾旱處理方式， 藉由PVF式自動供水裝置之PVF長度及條數之不同，分為7種水位處理，以 產生7種不同之栽培介質水分含量，並進行加鹽處理及不加鹽分處理。栽 培介質水分含量與PVF長度及條數有關，依序由高至低分別為第1水位（水 分含量最高）、第2水位......至第7水位（水分含量最低）。不加鹽分處 理組7種水位處理結果顯示，隨栽培介質水分含量降低，植株之PEPCase活 性提高，葉片日夜間可滴定酸含量之差異也增大，而白天之氣孔導度也有 下降之趨勢。加鹽處理組之CAM代謝轉換情形較快且明顯，這可能是由於 缺水及鹽分累積所共同造成之效應。此結果亦顯示鹽處理可加強並促進 ice plant光合形式之轉換。斷水處理株，於斷水一週內，雖有可滴定酸 之累積，但白天氣孔導度仍很高，斷水處理之第二週至第三週期間，日夜 間可滴定酸之含量大幅提高，且氣孔導度極低，顯示斷水處理可迅速誘導 ice plant光合形式之轉換。透過葉片水分潛勢、滲透潛勢及膨壓之測定 ，發現長期乾旱處理組在植株CAM活性增加前，葉片膨壓均會先降為負值 ，植株有凋萎(wilting)現象產生，之後，植株之PEPCase活性提高，白天 氣孔導度降低，且葉片日夜間可滴定酸含量有增高的現象。上述現象產生 後，植株之滲透潛勢開始下降，其葉片的膨壓逐漸恢復為正值，且葉片相 對水分含量也提高，顯示植物已完成CAM代謝的轉換，植株體內之水分狀 態也獲得改善。而斷水處理之植株於膨壓降為負值之時，與CAM代謝相關 之現象也開始表現，但由於持續斷水，植物無法由栽培介質中獲得充足水 分，故滲透潛勢雖降低，但膨壓仍為負值，植株呈現嚴重凋萎現象。綜上 所述，本實驗證實ice plant在遭遇缺水及高鹽逆境時，可經由體內各生 理作用之配合，如PEPCase活性之增加、葉片氣孔導度降低、日夜間可滴 定酸含量增高......等，完成光合形式之轉換，將其光合形式由C3路徑轉 為CAM路徑，以減少水分散失，並維持體內水分的平衡，故可幫助其適應 不良環境。the effects of the water stress and the salt treatment on the transition of photosynthetic modes of ice plant ( Mesembryanthemum crystallinum L. ) were studied.By the application of PVF auto-water-supply systems, 7 substrate water contents were distinguished by various water levels, furthermore, the salt treatment were combined in the same series of the experiment. As the medium water content decreased, the PEPCase activity of treated plants without salt increased and also the difference between the day/night titratable acid content, while the stomatal conductance during daytime decreased. However, the salt-treated ice plant tended to show more CAM-like photosynthetic mode that indicated an additive effect of water stress and salt treatment. The results suggested the salt treatment could enhance the transition of photosynthetic mode of ice plant.The titratable acid accumulation of water-withholding plant was observed after one week of treatment, however, the titratable acid contents of ice plant were significantly increased after 2 to 3 weeks of water- withholding treatment, it is suggested that water-withholding can induced the photosynthetic mode of ice plant immediately.Key parameters of the water relations of ice plant after treatment were also investigated, namely, the water potential, osmotic potential, and turgor pressure of the leaf. The results showed that the leaf turgor of ice plant decreased to negative value after long-term drought treatment and wilted appearances were observed, subsequently, the stomatal conductance decreased during daytime, the PEPCase activity and titratable acid content both increased. After all that, the turgor pressure of leaf increased back to positive value and the water balance of plant was also improved. It is suggested that the C3-CAM transition had been reached.The present study indicated that ice plant can switch the photosynthetic mode from C3 to CAM to adapt to stress conditions by means of various physiological modifications, namely, the increase of PEPCase, the decrease of leaf conductance, and increase of diurnal titratable acid content