POSISI CONTOH DAUN UNTUK ANALISIS STATUS FOSFOR (P) PADA BIBIT JARAK PAGAR (Jatropha curcas L.) DAN KADAR P TERSEDIA PADA DAERAH PERAKARANNYA

ABSTRAK Status P pada tanaman dapat diduga dengan menganalisis kadar P pada daun, karena daun merupakan suatu bagian tanaman yang sangat aktif. Untuk itu, diperlukan informasi posisi daun yang sesuai untuk dijadikan contoh daun untuk analisis status P tanaman. Percobaan ini bertujuan untuk menentukan posisi daun yang sesuai untuk analisis status P dan mempelajari perbedaan antara kadar P tersedia di tanah dan di rhizosphere, serta hubungannya dengan kadar P daun bibit jarak pagar. Percobaan dilakukan di rumah kaca Balai Penelitian Tanaman Rempah dan Aneka Tanaman Industri (Balittri) Pakuwon, Sukabumi, Jawa Barat, pada September 2006 sampai dengan Juli 2007, disusun dalam rancangan acak lengkap (RAL) dengan 3 (tiga) ulangan. Setiap unit percobaan terdiri atas 20 tanaman. Perlakuan yang diuji adalah: (a) daun ke-1 atau daun yang terletak tepat di bawah kuncup daun yang belum mekar sempurna, (b) daun ke-2 atau daun yang terletak setelah/di bawah daun ke-1, (c) daun ke-3, dan (d) daun ke-4. Parameter yang diamati adalah kadar P daun (%), kadar P tersedia di tanah dan rhizosphere yang diukur dengan metode Bray-1 P. Hasil penelitian menunjukkan bahwa daun ke-2 adalah posisi daun yang sesuai sebagai contoh daun untuk analisis status P pada bibit jarak pagar (Jatropha curcas L.). Kadar P di rhizosphere lebih rendah daripada kadar P di tanah (Bray-1P) yang jauh dari akar jarak pagar. Pada daun ke-2, keeratan hubungan yang ditunjukkan oleh koefisien regresi (R2) antara kadar Bray-1 P di rhizosphere dan kadar P daun jarak pagar (R2 = 0.394) lebih tinggi daripada hubungan antara kadar Bray-1 P di tanah dan kadar P daun (R2 = 0.371), sehingga untuk keperluan menggambarkan ketersediaan P tanah atau untuk melihat hubungan P tanah tersedia dengan kadar ataupun serapan P tanaman jarak pagar, akan lebih tepat bila contoh tanah yang diambil berasal dari rhizosphere akar tanaman. Kata kunci: Jarak pagar (Jatropha curcas L.), posisi daun, status P, P tanah tersedia ABSTRACT Determination of leaf sample position for analysis of P status in physic nut (Jatropha curcas L.) seedlings and the available P in the root zone Status of P in plants can be quantified by analyzing concentrations of the nutrient in leaf as the leaf is the most active plant part. For this, information on appropriate leaf position as samples for analysis of P status in the plants is needed. A research was conducted with aims to determine an appropriate leaf position of physic nut seedlings and to study differences between the concentrations of available P (Bray-1 P) in the bulk soil and the rhizosphere, along with their relationships with the concentrations of leaf P. The experiment was carried out in a glasshouse of Indonesian Spices and Other Industrial Crops Research Institute, Sukabumi, West Java, from September 2006 to July 2007. The experiment was arranged in a completely randomized design with three replications. Each experimental unit consisted of 20 plants. The treatments were: (a) 1st leaf or a leaf located exactly below the shoot, (b) 2nd leaf or a leaf located below the 1st leaf, (c) 3rd leaf, and (d) 4th leaf. All the leaves were taken from the primary branch of the plants. Parameters measured were P concentrations in the leaf, P concentrations in the bulk soil and the rhizosphere (Bray-1 P). The results showed that the 2nd leaf position was the appropriate leaf position to be taken as samples for the leaf analysis of P status in physic nut (Jatropha curcas L.) seedlings. The concentrations of Bray-1 P in the rhizosphere were lower than that in the bulk soil, which is further away from the roots. The R2 values for the relationships between the Bray-1 P concentrations in the rhizosphere and the the 2nd leaf P concentrations were higher than that between the Bray-1 P concentrations in the bulk soil and the 2nd leaf P concentrations, hence, for the objectives to show the soil P availability or to show the relationships between the available soil P and the concentrations or the P uptake by the physic nut, it will be more accurate if the soil samples are taken from the rhizosphere. Key words: Physic nut (Jatropha curcas L.), leaf position, P status, available soil P