| Summary: | 碩士 === 國立臺灣大學 === 園藝學研究所 === 103 === Iceberg lettuce (Lactuca sativa L. var. capitata ), of the family Asteraceae, is an important cold-season crop for exportation in Taiwan. Iceberg lettuce is harvested at the bottom of stem and the cut surface turns from white to reddish-brown after wounding. This phenomenon called butt discoloration and it reduces the quality of lettuce appearance during postharvest. Wounding of plant tissues usually induces the activity of phenylalanine ammonialyase (PAL) and results in the accumulation of phenolic compounds, which are oxidized to quinones and eventually condense spontaneously to form brown pigments. Researchs indicated that heat shock, 1-butanol, acetic acid and sulfhydryl compounds treatments can prevent cut surface from browning. Therefore, inhibiting the activity of PAL is considered as the way to alleviate butt discoloration. The objective of this study is to study the influence of different treatments on wounded stem disk browning and PAL activity. When stem disks are storage at 5℃, PAL activity in untreated stem disks started to increase at 2 hours, and finally reached its peak at 48 hours. At the same time, browning of stem disks appeared one day after the rise of PAL activity. Heat shock treatment at 50℃ for 90 seconds could inhibit disks browning effectively by keeping PAL activity at the low level within 120 hours after wounding. 1-Butanol is the specific inhibitor of Phospholipase D which could suppress the wounding signal. Stem disks treated with 3% 1-butanol for 5 minutes immediately after wounding could delay the browning, whereas the treatment at 2 hours after wounding fail to inhibit tissue browning. The result suggested that wounding signal is triggered in 2 hours and lead to formation of PAL. Stem disks soaked with 1.5% acetic acid solution for 1min, 2% for 30 sec, or 3% for 5 sec could keep appearance as white as original and repress PAL activity in the disks. Stem disks soaked with 3% acetic acid solution for 5 sec at 12 hours after wounding could effectively inhibit browning and maintain low PAL activity in the disks. It reveals that acetic acid directly interact with PAL to inhibit enzyme activity. 10% acetic sodium solution treatment is the same effect as acetic acid solution treatment. The stem disks treated with sulfhydryl compounds, including Cysteine (Cys), L-cysteine hydrochloride (CysH), and N-acetyl cysteine (NAC) at the concentration of 1%, 2%, and 3%, the color of wounded stem disks are associated with PAL activity. Treatment of 3% CysH could maintain the appearance of stem disks and keep the PAL activity at low level. In the treatment of 2% CysH, the color of wounded stem disks turned green-yellowish. The activity of PAL increased at 3 days after wounding and reached its peak at 4 day. In the treatment of 3% CysH at 2 hours after wounding, browning and PAL activity were completely repress. However, cut surface browning could not be inhibit in the treatment of 3% CysH at 6 or 12 hours after wounding. Meanwhile, PAL activity increased at 3 days after wounding. In the treatment of 1% and 2% Cys, the color of wounded stem disks turned green-yellowish at 4 days after wounding and PAL activity increased at 3 days after wounding. In the treatment of 3% Cys, the color of wounded stem disks turned green-yellowish at 5 days after wounding and PAL activity increased at 4 days after wounding. The appearance of wounded stem disks treated with 1% NAC are brown and green-yellowish at 3 days after wounding; besides, the PAL activity rises ahead of color alteration and reach to the highest activity similar to control treatment. In the treatment of 3% NAC at 2,6 or 12 hours after wounding, the effect of browning inhibition decreased as the treatment time lengthened, the initiation time of PAL activity increase shifts earlier. The result of sulfhydryl compounds treatments suggest that they only partially inhibited color alteration of iceberg lettuce cut surface because PAL activity can not be completely inhibited. The appearance of color alteration after wounding is correlated with the increase of PAL activity in stem tissuse. The PAL was purificated by 40-60% ammonium sulfate saturation, and its purification is three times than crude extract. Adding 80mM acetic acid to purified PAL can irreversiblely inhibit the activity of PAL.
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