Konduktivitas Hidrolik Potensial Pembuluh Xilem Akar Kelapa Sawit pada Zona Riparian dan Non-Riparian di Hutan Harapan Jambi

Abstract

Akar merupakan organ yang sangat krusial dalam penyerapan air pada tumbuhan. Melalui akar, air dan mineral diserap dari tanah dan diangkut hingga ke daun melalui jaringan pembuluh xilem. Daya hantar yang dimiliki pembuluh xilem dalam mentranspor masa air dari akar hingga ke tajuk dikenal dengan konduktivitas hidrolik. Karakter anatomi pembuluh xilem menentukan konduktivitas hidrolik yang ditentukan berdasarkan jumlah, luas dan panjang pembuluh xilem. Kelapa sawit sudah banyak diteliti terkait dengan pertumbuhan dan perkembangannya, kebutuhan air, produktivitas dan lain sebagainya. Namun, penelitian tentang konduktivitas hidrolik pada akar kelapa sawit dan mekanisme xilem dalam melakukan transpor air dari akar hingga ke tajuk, terutama untuk kelapa sawit yang hidup di zona riparian penting untuk diteliti. Penelitian ini bertujuan untuk menganalisis pengaruh perbedaan diameter akar dan perbedaan lokasi (riparian dan non-riparian) terhadap konduktivitas hidrolik pembuluh xilem akar kelapa sawit di Hutan Harapan Jambi untuk menduga bentuk adaptasi kelapa sawit pada zona riparian Penelitian dilaksanakan pada bulan April 2019-Juni 2020 di lokasi sekitar Harapan Jambi dan di Laboratorium Fisiologi dan Genetika Tumbuhan Departemen Biologi FMIPA IPB. Sampel akar kelapa sawit diambil dari plot berukuran 50 m x 50 m di zona riparian dan non-riparian dan masing-masing lokasi terdapat 4 plot sebagai ulangan. Dari tiap plot ditentukan 3 pohon kelapa sawit, dari tiap pohon digali 3 lubang dengan kedalaman 20-30 cm dan diambil masing-masing 3 sampel akar dengan diameter <2 mm, 2≤d<5 mm, dan 5≤d<10 mm. Total sampel akar yang dianalisis sebanyak 144 sampel. Sampel dipotong dengan ketebalan 10 μm menggunakan mikrotom geser dan diwarna dengan campuran 0.65% alcian blue: 0.35% safranin (w/v), kemudian sampel difoto dengan photomicroscope. Sampel dianalisis menggunakan bioimaging menggunakan software Adobe Photoshop CS5 dan Image J. Data numerik dianalisis menggunakan perangkat lunak R dan Linear mixed effect model. Hasil analisis menunjukkan bahwa D (diameter xilem), Alumen (luas lumen), dan Kp (konduktivitas hidrolik potensial) meningkat seiring dengan bertambahnya diameter akar. Sebaliknya, VD (kerapatan pembuluh xilem) menurun secara signifikan seiring dengan meningkatnya diameter akar. Konduktivitas hidrolik potensial di zona riparian lebih kecil daripada di lokasi non-riparian pada diameter akar 2≤d<5 mm dan 5≤d<10 mm. Rendahnya konduktivitas hidrolik potensial pembuluh xilem dan penyempitan lumen pembuluh xilem akar kelapa sawit di zona riparian yang mengalami genangan secara musiman diduga sebagai mekanisme adaptasi untuk mempertahankan ketersediaan air dari akar hingga ke daun pada tumbuhan kelapa sawit di lokasi tersebut.

Roots have an important role in water absorption in plants. Through the roots, water and minerals are taken up from the soil and transported to the shoots through the xylem vessel’s network. The conductivity of xylem vessels in absorb and transport water from the ground to the canopy known as potential hydraulic conductivity. The anatomical characters of xylem vessels determine the potential hydraulic conductivity depending on the number, area, and length of the xylem vessels. Oil palm has been widely studied in terms of growth and development, water use, productivity, and other economically relevant functions. However, not much is known about the potential hydraulic of oil palm root systems and how xylem vessels perform their function to transport water from roots to shoots so far. This information is needed to describe oil palm strategies to maintain water status especially in oil palms that grow under various soil hydrological regimes. This study aims to analyze the influences of the differences in root diameter and plantation types on the hydraulic conductivity of oil palm root xylem in Harapan Jambi Rainforest and to predict the adaptation form of oil palm in riparian zone. The study site is carried out on April 2019 – June 2020 in Harapan Jambi forest and Laboratory of Plant Physiology and Genetics Biology Department IPB. Root samples were collected at four 50 m x 50 m plots in riparian and welldrained oil palm plantation areas. We determine three oil palm trees and in each tree, we dug 3 holes in 20-30 cm soil depth and collected three root samples with <2 mm, 2≤d<5 mm, and 5≤d<10 mm in diameters. We analyzed 144 oil palm roots samples. The samples were sectioned in cross-sections using a sliding microtome for 10 µm and stained using a mixture of 0.65% alcian blue and 0.35% safranin (w/v). Then the sample was photographed with a photomicroscope. Samples were analyzed by bioimaging analysis using Adobe Photoshop CS5 and Image J. Numeric data were analyzed by R software and analyzed using linear mixed effect models. The analysis showed that D (mean vessel diameter), Alumen (vessel lumen area), and Kp (potential hydraulic conductivity) increased with increasing root diameter at both plantation types. On the contrary, VD (vessel density) significantly decreased with increasing root diameter. Potential conductivity in riparian sites was smaller than in well-drained sites and significantly different in root diameter 2≤d<5 mm and 5≤d<10 mm and related to both plantation types. The low hydraulic conductivity of root xylem vessels and the narrowing of vessel lumen that occurs in oil palm roots in the seasonally flooded riparian site were presumed as adaptation mechanisms to maintain water supply from the roots to the shoot in oil palm plants in these sites.