Retranslocation of Zn from leaves as important factor for zinc efficiency of rice genotypes R. Hajiboland1), B.Singh2) and V. Römheld2) 1) Botany Department, Tabriz University, Tabriz, Iran 2) Institute für Pflanzenernährung (330), Universität Hohenheim, 70593 Stuttgart Abstract Zinc leaves and its dependence on Zn nutritional (Zn) retranslocation from mature to young growing status were studied in a Zn-efficient and a Zn-inefficient rice genotype. Results indicate that retranslocation of Zn takes place from individual, not yet fully expanded, leaves into just emerged youngest leaves. Zn translocation from source to sink leaves was enhanced under low Zn supply and in the Zn-efficient rice genotype. Further, the transition of a leaf from a sink to a source for Zn retranslocation took place 2-3 days earlier under low compared with adequate Zn supply. These data indicate that besides Zn acquisition by roots, its mobility and retranslocation within the plant have to be considered as important factor in genotypical differences for Zn efficiency Introduction Zinc deficiency is a widespread problem in crop production, in particular for cereals on calcareous soils. Plant genotypes greatly differ in Zn efficiency. Apart from efficiency for acquisition of Zn from soils, its utilisation within a plant could be an important mechanism involved in expression of Zn efficiency in cereal genotypes. Experiments were, therefore, conducted to investigate the extent of Zn retranslocation from older to youngest emerged leaves in two rice genotypes differing in Zn efficiency. Materials and Methods Two rice (Oryza sativus L.) genotypes, which were defined in field and pot experiments as Zn-inefficient (IR26) and Zn-efficient (IR36) were precultured in nutrient solution with low Zn supply (0.1µM) for 14 days. Thereafter, the plants were loaded for two days with Zn supplied either as unlabelled Zn (1µM ZnSO4) or radioactive labelled (65Zn) Zn. These Zn-loaded plants were grown in a chelator-buffered (HEDTA) nutrient solution at low (2pM free Zn) and adequate (130 pM free Zn) Zn supply for up to 16 or 20 days. Individual plants were harvested at 4 day intervals and Zn content was determined in the various leaves depending on the position. Results Results indicate a high Zn retranslocation during early leaf growth (Tab.1). Zn retranslocation was particularly high at low Zn supply and amounted up to 60% (Fig 1.) . Further, the transition of the leaf from a sink to a source for Zn retranslocation took place 2-3 days earlier at low Zn supply than at adequate Zn supply (data not shown).. Among the two rice genotypes, absolute and relative translocation was higher in the Zn-efficient genotype (IR36) than in the Zn-inefficient genotype (IR26). Investigation conducted using radiolabelled Zn (65Zn) support the above observations. Tab. 1: Changes in Zn content (ng leaf-1) of individual leaves during leaf growth of rice genotypes at low Zn supply in nutrient solution ,after 2 days of Zn loading ___________________________________________________ Genotype Leaf number IR 26 IR 36 (Zn-inefficient ) (Zn-efficient) N03: (early stage) as youngest leaf 294 331 as 2nd youngest leaf 155 128 reduction (ng leaf-1) 139 203 N04: (early stage) as youngest leaf 371 376 as 2nd youngest leaf 280 151 reduction (ng leaf-1) 91 225 Fig. 1: Relative translocation of Zn from young emerged leaf to youngest emerging leaf in Zn efficient and Zn inefficient rice genotypes under low and adequate Zn supply Conclusion Zinc is highly mobile within leaves during early growth stage. Up to 2/3rd of the original Zn content of young leaves could be mobilised and retranslocated to just emerging leaves. Adequate Zn supply to roots delayed retranslocation of Zn from older to the youngest emerging leaves. Zn-efficient rice genotype revealed a significantly higher Zn retranslocation than the inefficient genotype. The study underlines the importance of Zn retranslocation as an important mechanism affecting Zn efficiency in cereals.