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ABSTRAK Penelitian ini bertujuan untuk mengkaji mekanisme sintesis menthol pada berbagai periode pencahayaan tanaman M. piperita L. Penelitian dilakukan di Instalasi Penelitian Balai Penelitian Tanaman Obat dan Aromatik, Lembang, Jawa Barat, dari bulan Januari 2000 hingga Juli 2000. Penelitian dilakukan tiga tahap. Tahap pertama membuat variasi lingkungan cahaya, tahap kedua penyulingan dan analisis komponen minyak dengan kromatografi gas spektrometer massa dan tahap ketiga merunut lintasan biosintesis menthol. Penelitian menggunakan rancangan acak kelompok 5 perlakuan, yaitu L 0 (panjang hari normal sebagai kontrol), L 1 (pemutusan periode gelap 1 jam, pukul 21.00-22.00 mulai umur 30 hari), L 2  (pemutusan periode gelap 1 jam, pukul 21.00-22.00 mulai umur 60 hari), L 3 (penambahan cahaya 4 jam, pukul 18.00-22.00 mulai umur 30 hari), dan L 4 (penambahan cahaya 4 jam, pukul 18.00- 22.00 mulai umur 60 hari). Hasil penelitian menunjukkan bahwa perubahan lingkungan mempengaruhi mekanisme sintesis menthol di dalam tanaman Mentha piperita L. Perubahan lingkungan mengubah lintasan menthol yang selanjutnya mengubah komponen minyak dan mutu menthol. Tingginya kadar menthol dan rendahnya menthofuran pada penambahan cahaya 4 jam terjadi melalui penghambatan pembentukan senyawa menthofuran dengan mereduksi pulegon menjadi menthol, sehingga menthol meningkat, sedangkan pada perlakuan kontrol terjadi oksidasi pulegon ke menthofuran sehingga menthol rendah. Penambahan cahaya 4 jam mulai umur 30 hari setelah tanam menghasilkan minyak dengan kadar menthol paling tinggi yaitu 54,89% dan menthofuran paling rendah yaitu 7,83%. Kata kunci :  Mentha, Mentha piperita L., periode pencahayaan, hasil, komposisi minyak, Jawa Barat ABSTRACT The effect of photoperiod on menthol sysnthesis of Mentha piperita L. Research on the effect of photoperiod on menthol synthesis of Mentha piperita L, was carried out at the Experimental Garden of Institute Reseach for Medicinal and Aromatic Crops, Lembang, West Java, from January until July, 2000. The objective is to study the machanism of menthol synthesis in relation with the manipulation of light periode, three steps were taken: The first step was manipulation of environment using TL lamps (two experiments), the second step was distillation and analisis of peppermint oil from their products with gas chromatography and mass spectrometry, and the third step was tracing the pathway on menthol biosynthesis. At the first experiment, 5 treatments were given i.e. (1) control or normal light period, (2) four hours light supplement at the age of 30 days and 60 days after planting, and (3) one hour interruption of dark period at the age of 30 days and 60 days. The result showed that the effect of light period manipulation can change the pathway of menthol biosynthesis and oil component and finally the quality of menthol. Four hours light supplement at the age of 30 days after planting could enhance the menthol content and reduce menthofuran by blocking the reaction from pulegone to menthofuran, so the pulegone was reduced into menthon and menthol. Four hours light supplement at 30 days after planting showed the highest menthol content (54.89%) and the lowest menthofuran (7.83%). Control treatment (normal light period) showed the lowest menthol content, due to no reduction of pulegone into menthon, but pulegone was oxidized into menthofuran. Without additional light the menthol content decreased and the menthofuran content increased. Key words : Mentha, Mentha piperita L., oil composition, photoperiod, yield, West Java

BIOSINTESIS MENTHOL PADA BERBAGAI PERIODE PENCAHAYAAN TANAMAN MENTHA (Mentha piperita L.)