Madras Agric. J., 104 (4-6): 157-160, June 2017 *Corresponding author’s email: soilsudha@yahoo.co.in Genotypic Divergence in Per Cent Zinc Derived from Variable Sources and on Zinc Uptake in Rice [Oryza sativa (L.)] as Established by Radiotracer Techniques C. Sudhalakshmi* and S.Meena Soil Science and Agricultural Chemistry, Coconut Research Station, Aliyar Nagar A pot culture experiment was conducted at Tamil Nadu Agricultural University, Coimbatore to establish the genotypic divergence in per cent zinc (Zn) derived from fertilizer (% Zndff) or from soil (% Zndfs) and on zinc uptake of two rice genotypes Zn -effcient Norungan and Zn –ineffcient PMK 3. To assess the partitioning of Zn in the shoot and root of the genotypes, radiotracer technique was employed, in which graded levels of Zn (0.0, 12.5, 25.0, 37.5 and 50.0 kg ha -1 of ZnSO 4 ) as 65 Zn labeled ZnSO 4 were applied. Per cent Zndff showed an increase by 50 % for Zn-ineffcient PMK 3 than that for Norungan while reverse was the trend for % Zndfs implying the fact that Zn ineffcient genotypes have the ability to use only readily available sources of Zn. Zinc uptake from fertilizer increased with increasing dose of applied zinc for PMK 3 while in Norungan it showed a peak at 25 kg ha -1 of ZnSO 4 and thereafter it declined. Irrespective of the sources of zinc, root zinc accumulation was higher in PMK 3 than that in Norungan. The Zn-effcient genotype Norungan had better translocation of Zn from root to shoot than PMK 3. Key words : Rice genotypes, Zn – effcient, Zn uptake Zinc deficiency is a ubiquitous problem constraining rice production worldwide. Plethora of evidences have accumulated stating that the resilience of rice production is very often put to acid test due to the prevalence of two major limiting nutrients viz., nitrogen and zinc. Today, than ever before, the importance of zinc in crop production is being felt. It is estimated that about 50 percent of soils used for cereal production in the world have low levels of plant available Zn (Graham and Welch, 1996). Due to Zn defciency about two million hectares of rice production are affected in Asia (Ferno et al., 1970), which is an economic and humanitarian problem. Today, there is a shift in research trend from plant production point to food chain (Slingerland et al ., 2006) or food systems (Graham et al ., 2007). Though potential mechanisms including root Zn uptake and translocation, Zn sequestration in leaves and biochemical utilization of Zn have been studied, multitude of questions concerning zinc efficiency mechanisms between genotypes remain unanswered. Rhizosphere of plants always remains an exploratory venue for the scientists and radiotracer is a scientifc tool for the determination of the nutrient translocation in plant system. It is possible to delineate beyond doubt, the crop uptake from fertilizer or from native source employing 65 Zn labeled fertilizers. Hence in the present study 65 Zn labeled compounds were exploited to establish the genotypic variation in % Zndff or % Zndfs and Zn uptake of the rice genotypes, Norungan (Zn effcient) and PMK 3 (Zn ineffcient) (Sudhalakshmi, 2007). Material and Methods A pot culture experiment was performed at the Radioisotope (Tracer) Laboratory, Tamil Nadu Agricultural University, Coimbatore to study the variation in %Zndff and %Zndfs in the rice genotypes Norungan and PMK 3. About 10 kg of the soil (passed through 2 mm sieve) representing clay loam texture of Noyyal series (Vertic Ustropept) was transferred to the ceramic pots. The soil was neutral in reaction, low in KMnO 4 nitrogen, high in Olsen phosphorus and 1NNH 4 OAc potassium and low in DTPA extractable Zn. The seeds of the rice genotypes viz., Norungan and PMK 3 were sown on the well - puddled soil. The seedlings after establishment were thinned to three plants per pot. The fertilization programme was adopted as per the crop production guide. The experiment was laid out in a factorial completely randomized block design with the following treatments each replicated thrice. T 1 : Control T 2 : ZnSO 4 @ 12.5 kg ha -1 T 3 : ZnSO 4 @ 25 kg ha -1 T 4 : ZnSO 4 @ 37.5 kg ha -1 T 5 : ZnSO 4 @ 50 kg ha -1 Zinc sulphate was labeled with 65 Zn @ 1 mCi / g of Zn. The 65 Zn isotope was obtained from Board of Research and Isotope Technology, Mumbai as ZnCl 2 in dilute HCl. The labeled ZnSO 4 was applied on 24 th day after sowing as per the treatment schedule. The