Sunday, 9 July 2006

Radiotracer Technique in Establishing Genotypic Divergence of Rice (Oryza sativa L. ) Cultivars in Zinc Utilization from Variable Sources.

Chinnappan Sudhalakshmi1, Ramasamy Krishnasamy1, U. Surendran1, and A. RajaRajan2. (1) Tamil Nadu Agricultural Univ, Lawley Road, Coimbatore, 641003, India, (2) Regional Research Station, Tamil Nadu Agricultural Univ, Vridhachalam, India

Radiotracer is a scientific tool for the determination of the nutrient translocation in plant system. It is possible to delineate beyond doubt, the crop uptake from fertilizer source and from native source employing 65Zn labeled fertilizers. Knowledge on zinc utilization from variable sources will throw light on the ability of genotypes to thrive in soils of low external supply of zinc. Hence 65Zn labeled compounds were employed to bring out the genotypic variations in zinc utilization from soil and fertilizer sources of two rice genotypes viz., zinc efficient Norungan and zinc inefficient PMK 3. Two rice genotypes were raised in waterproof ceramic pots containing 10 kg soil (DTPA Zn 0.9 mg kg-1) of clay loam texture (Vertic Ustochrept). The total requirement of zinc was 0.96 g and the tagging level was 1mCi / g of Zn. ZnSO4 was tagged with 65Zn using the principle of isotope dilution technique and five treatments were imposed viz., T1 control; T2 ZnSO4 @ 12.5 kg ha-1; T3 ZnSO4 @ 25 kg ha-1; T4 ZnSO4 @ 37.5 kg ha-1 and T5 ZnSO4 @ 50.0 kg ha-1. The experiment was laid out in factorial completely randomized design with each treatment replicated thrice. At the physiological growth stages (active tillering, flowering and harvest), five grams of the plant material from each treatment was ashed in silica crucible and transferred to the scintillation vial and the radioactivity of the sample was determined by integral counting. The radioassay data were employed in standard formulae and the per cent Zn derived from fertilizer (%Zndff) and the per cent Zn derived from soil (%Zndfs) were determined. The per cent Zn derived from fertilizer showed an elevated trend for PMK 3 (Zn inefficient) with an overall average of 4.78 % than that observed for Norungan (Zn efficient). This apparently implies the inability of PMK 3 to mobilize the nutrient from the soil. The %Zndfs was higher for Norungan but it assumed lesser magnitude in PMK 3. The zinc uptake from the soil (both shoot and root) was higher for Norungan than that observed for PMK 3. The higher zinc efficiency of Norungan could be attributed to the greater Zn uptake capacity from the soil. The Zn inefficient PMK 3 could utilize only the readily available sources of nutrients i.e., that from fertilizer. This establishes the relative ability of two rice genotypes to grow in Zn deficient soil. In the shoot, the Zn uptake from fertilizer shot up with increasing dose of applied Zn (till 37.5 kgha-1) for PMK 3 whereas in Norungan, the maximum uptake was realized at 25 kg ha-1 of ZnSO4. In root with increasing dose of applied zinc, sequestration increased, more prominently visualized in PMK 3. The soil zinc uptake increased with increasing dose of applied zinc in PMK 3, which could probably be attributed to the increased root activity with increased zinc fertilization. In Norungan, the uptake shot up till 25 kg ha-1 and thereafter it tended to decline with increasing dose of applied zinc. Irrespective of the source of zinc, the root zinc accumulation was higher in PMK 3. The binding sites with higher affinity for Ca or other cations in the zinc deficient roots (Hart et al., 1998) or vacuolar sequestration of zinc in the root cells (Rengel and Graham, 1995) would have contributed for the above cause. Keywords : 65Zn, %Zndff, %Zndfs, fertilizer, genotypes, soil, zinc. References: (1) Hart, J.J., W.A.Norvell, R.M.Welch, L.A.Sullivan and L.V. Kochian. 1998. Characterization of zinc uptake, binding and translocation of bread and durum wheat cultivars. Plant Physiol.118 : 219 226. (2) Rengel, Z. and R.D.Graham. 1995. Importance of seed Zn content for wheat growth on Zn-deficient soil. II. Grain yield. Plant Soil 173: 267-274.

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