Respons morfofisiologi, agronomi, dan studi mekanisme toleransi padi gogo mutan terhadap cekaman kekeringan

Abstract

Perubahan iklim secara global menyebabkan kenaikan temperatur udara dan perubahan pola curah hujan. Hal ini akan menimbulkan kekeringan pada sebagian besar areal pertanian sehingga dapat menurunkan produktivitas tanaman budidaya. Peningkatan produksi padi dengan memanfaatkan lahan kering menjadi tantangan di masa depan mengingat keterbatasan daya dukung lahan, terutama sumber daya air. Selain kondisi kesuburan lahan kering yang tidak menguntungkan dan ketersediaan air yang tergantung dari hujan, kendala lain yang dihadapi adalah masih kurang tersedianya varietas padi yang toleran terhadap cekaman abiotik, terutama kekeringan. Oleh karena itu perlu pengembangan varietas padi gogo toleran kekeringan untuk mengantisipasi perubahan iklim serta optimalisasi lahan kering yang banyak tersebar di Indonesia salah satunya dengan pemuliaan tanaman berbasis mutasi induksi. Untuk keperluan pengembangan varietas padi gogo toleran, diperlukan informasi menyeluruh berbagai respons pertumbuhan tanaman padi gogo mutan akibat cekaman kekeringan. Tujuan umum penelitian ini adalah untuk memperoleh informasi respons morfologi, fisiologi, dan agronomi padi gogo mutan serta mekanisme toleransinya pada cekaman kekeringan. Penelitian terdiri atas tiga topik penelitian dengan lima percobaan. Topik penelitian pertama berkaitan dengan toleransi Situgintung pada cekaman kekeringan fase vegetatif dan reproduktif. Topik penelitian kedua adalah seleksi padi gogo mutan M3 dan M4 pada cekaman kekeringan fase vegetatif dan reproduktif di lapangan. Seleksi M1 dan M2 sudah dilakukan oleh peneliti sebelumnya. Topik ketiga adalah studi mekanisme padi gogo mutan M5 pada cekaman kekeringan fase reproduktif. Situgintung merupakan varietas padi gogo yang dilepas Badan Tenaga Nuklir Nasional (BATAN) pada tahun 1992. Hasil penelitian menunjukkan, Situgintung mempunyai tingkat toleransi kekeringan pada fase vegetatif maupun reproduktif yang lebih baik dibandingkan IR20 (cek peka). Situgintung mampu mengurangi dampak negatif dari kondisi kekurangan air dengan cara mengurangi kerusakan klorofil dan mengurangi penurunan jumlah anakan. Kandungan prolin pada Situgintung tidak berbeda nyata pada kontrol dan kekeringan. MDA pada genotipe Situgintung meningkat saat kekeringan. Situgintung menunjukkan kemampuannya mengurangi penurunan jumlah anakan, sehingga penurunan relatif hasil panen Situgintung yang ditunjukkan dari bobot gabah per rumpun lebih kecil dibanding IR20, namun masih lebih besar dibanding Salumpikit (cek toleran). Pada kekeringan fase reproduktif, penurunan relatif CO2 interseluler, konduktansi stomata, dan transpirasi Situgintung lebih kecil jika dibandingkan dengan IR20. Seleksi padi gogo mutan M3 dilakukan dengan metode indeks seleksi terboboti. Hasil penelitian menunjukkan karakter yang dapat dijadikan kriteria seleksi padi gogo mutan toleran kekeringan fase reproduktif adalah bobot gabah per rumpun (BG), kerapatan malai (KM), panjang malai (PM), dan jumlah gabah isi per malai (GI). Skrining padi gogo mutan M3 fase reproduktif di lapangan melalui metode seleksi indeks terboboti menghasilkan rumus I = (5*0.44BG) + (2*0.41KM) + (0.23PM) + (0.42GI). Hasil penelitian didapatkan 89 genotipe mutan M4 berdaya hasil tinggi dengan karakter agronomi baik sehingga akan dilanjutkan pada penelitian selanjutnya. Padi mutan M4 diambil 20 nomor terbaik hasil perangkingan nilai seleksi indeks terboboti untuk diseleksi kembali pada kekeringan fase vegetatif di lapangan dengan metode DTD (Drought Tolerance Degree). Dari hasil dendrogram didapatkan empat nomor mutan (G197, G77, G82 dan G107) yang mempunyai kedekatan sifat dengan Salumpikit (cek toleran). Nilai DTD mempunyai korelasi positif dan nyata dengan daya tumbuh kembali, kandungan klorofil, kadar air relatif, jumlah anakan produktif, panjang malai, jumlah gabah isi, bobot 100 butir dan bobot gabah per rumpun. Nilai DTD menggambarkan kemampuan tanaman dalam mempertahankan kehijauan daun (stay green). Pada evaluasi mekanisme toleransi kekeringan fase reproduktif digunakan tiga genotipe mutan M5 (G77, G107 dan G82). Penelitian dilakukan dengan ember di rumah kaca BATAN Jakarta Selatan. Cekaman kekeringan fase reproduktif mengakibatkan penurunan yang tinggi pada hasil gabah padi gogo. Genotipe G107, G82, dan G77 mengalami penurunan hasil sebesar 69,25%, 70,12%, dan 74,98%. Salumpikit juga mengalami penurunan hasil 71,48%. Penurunan hasil IR20 mencapai 93,13%. Peubah morfofisiologi dan agronomi yang berkorelasi nyata dengan hasil adalah prolin, katalase, askorbat peroksidase, potensial air daun, klorofil total, laju transpirasi, laju fotosintesis, CO2 interseluler, konduktansi stomata, bobot kering tajuk, tinggi tanaman, jumlah anakan, panjang malai, jumlah gabah hampa dan isi per malai, persentase gabah hampa, dan bobot 100 butir. Hasil penelitian menunjukkan mekanisme toleransi padi mutan pada cekaman kekeringan melalui dua bentuk yaitu avoidance dan tolerance. Drought avoidance ditunjukkan dengan penggulungan daun, penutupan stomata, rendahnya transpirasi, mengurangi kerusakan klorofil (stay green), dan meningkatkan rasio akar tajuk. Hal ini dilakukan tanaman untuk mengurangi kehilangan air selama kekeringan. Genotipe padi yang toleran kekeringan mampu menjaga potensial air daun tetap tinggi. Pemeliharaan potensial air yang tinggi pada tanaman selama pra-pembungaan dikaitkan dengan mengurangi keterlambatan pembungaan dan menurunkan sterilitas bulir, yang berkontribusi pada hasil yang lebih tinggi. Mekanisme tolerance ditunjukkan dengan kemampuan tanaman melakukan penyesuaian osmotik sel. Penyesuaian osmotik melalui akumulasi zat terlarut seluler, seperti prolin, telah dibuktikan sebagai salah satu cara untuk mengatasi cekaman osmotik yang disebabkan oleh hilangnya air. Mekanisme dengan mengurangi dampak negatif cekaman oksidatif selama kekeringan memainkan peran penting selama toleransi kekeringan. Aktivitas enzim antioksidan (CAT dan APX) pada genotipe toleran relatif lebih tinggi dibanding genotipe peka. Hal ini menunjukkan bahwa aktivitas enzim antioksidan yang lebih tinggi berperan dalam memberikan toleransi terhadap cekaman kekeringan. Konsentrasi CAT dan APX yang lebih tinggi mampu mengatasi radikal O2 dan produknya H2O2 yang disebabkan oleh cekaman kekeringan. Tanaman toleran mampu mengatur laju fotosintesis dan konduktansi stomata, sehingga mengurangi penurunan hasil.

Global climate change causes an increase in air temperature and changes in rainfal patterns. This will cause drought in most agricultural areas so that it can reduce the productivity of cultivated plants. Increasing rice production by utilizing dry land is a challenge in the future given the limited carrying capacity of land, especially water resources. In addition to unfavorable dry land fertility conditions and the availability of water that depends on rain, another obstacle faced is the limited availability of upland rice varieties that are tolerant to abiotic stresses, especially drought. Therefore, it is necessary to develop drought tolerant upland rice cultivars to anticipate climate change and optimize dry land which is wide spread in Indonesia, one of which is with induced mutation-based plant breeding. For the purposes of developing tolerant upland rice varieties, comprehensive information is needed on various growth responses of mutant upland rice mutans due to drought stress. The general objective of this research was to obtain information of the morphological, physiological, and agronomic responses of mutant upland rice and its tolerance mechanism to drought stress. The study consisted of three research topics with five experiments. The first research topic relates to Situgintung's tolerance to drought stress at vegetative and reproductive phases. The second research topic was the selection of M3 and M4 mutant upland rice under drought stress at the vegetative and reproductive phases in the field. The selection of M1 and M2 has been carried out by previous researchers. The third topic was the study on the mechanism of M5 mutant upland rice at the reproductive phase of drought stress. Situgintung is an upland rice variety released by the National Nuclear Energy Agency (BATAN) in 1992. The results showed that Situgintung had a better drought tolerance level at the vegetative and reproductive phases than IR20 (sensitive check). Situgintung was able to reduce the negative impact of water stress conditions by reducing chlorophyll damage and reducing the number of tillers. The proline content in Situgintung under drought stress was not significantly different from those of control. MDA in the Situgintung genotype increased during drought. Situgintung showed its ability to limited reduction the number of tillers, so the relative decrease in yield of Situgintung as indicated by grain weight per hill was smaller than IR20, but still larger than Salumpikit (tolerant check). In the drought at reproductive phase, the relative decrease in intercellular CO2, stomatal conductance, and transpiration of Situgintung was smaller than that of IR20. Selection of M3 mutant upland rice was carried out using the weighted selection index method. The results showed that the characters that could be used as selection criteria for drought tolerant mutant upland rice at the reproductive phase were grain weight per hill (BG), panicle density (PM), panicle length (PM), and number of filled grains per panicle (GI). The screening of M3 mutant upland rice at the reproductive phase in the field was done through the weighted index selection method using the following formula I = (5*0.44BG) + (2*0.41KM) + (0.23PM) + (0.42GI). There were 89 genotypes of high yielding mutant with good agronomic characters so that they were continued in the further study. The twenty mutant genotypes (M4) were re-selected in the dry vegetative phase in the field using the DTD (Drought Tolerance Degree) method. From the results of the dendrogram, it was found that there were four mutant genotypes (G197, G77, G82 and G107) which had similar characters with Salumpikit (tolerant check). The DTD value had a positive and significant correlation with recovery, chlorophyll content, relative moisture content, number of productive tillers, panicle length, number of filled grains, weight of 100 grains and weight of grain per hill. DTD value describes the ability of plants to maintain green leaves (stay green). In the evaluation of the drought tolerance mechanism at the reproductive phase, three mutants (M5) were used (G77, G107 and G82). The research was conducted with a pot in the BATAN greenhouse, South Jakarta. The reproductive phase of drought stress resulted in a high decrease in upland rice grain yields. Genotypes G107, G82, and G77 decreased yield by 69.25%, 70.12%, and 74.98%. Salumpikit also showed a decline in yield of 71.48%. The IR20 decreased yield by 93.13%. The morphophysiological and agronomic variables that were significantly correlated with yield were proline, CAT, APX, leaf water potential, total chlorophyll, transpiration rate, photosynthetic rate, intercellular CO2, stomatal conductance, canopy dry weight, plant height, number of tillers, panicle length, number of unfilled grain, the number of filled grain, the percentage of unfilled grain, and the weight of 100 grains. The results showed tolerance mechanism of mutant rice under drought stress in two forms, namely avoidance and tolerance. Drought avoidance was indicated by leaf curling, stomatal closure, and low transpiration. reduce chlorophyll damage (stay green), and increase the root-shoot ratio. This was done by reducing water loss during drought. Drought-tolerant rice genotypes were able to maintain high leaf water potential. The maintenance of high water potential in plants during pre-flowering was associated with reduced flowering delay and decreased grain sterility, which eventually contributed to higher yields. The tolerance mechanism was indicated by the ability of plants osmotic adjustment at cell level. Osmotic adjustment through the accumulation of cellular solutes, such as proline, has been shown to be one way of dealing with osmotic stress caused by water loss. Mechanisms by reducing the negative impact of oxidative stress during drought played an important role during drought tolerance. The activity of antioxidant enzymes (CAT and APX) in the tolerant genotype was relatively higher than that of the sensitive genotype. Thus higher antioxidant enzyme activity seemed to play a role in providing tolerance to drought stress. Higher concentrations of CAT and APX were able to remove O2 radicals and their products H2O2 caused by drought stress. Tolerant plants were able to regulate the rate of photosynthesis and stomatal conductance, thereby reducing yield losses during drought.