An effective way to reduce the risk of cadmium (Cd) entering the food chain is to use low Cd-accumulation rice cultivars, particularly in Asia. The fundamental requirement for breeding low grain Cd-accumulation cultivars is to know the genotypic variation in Cd accumulation and the physiological processes and genetic basis governing the Cd accumulation in rice grain. In this experiment, genotypic variation in Cd accumulation and distribution among rice organs was studied using thirty-five rice varieties. They were grown with irrigation water containing 2 ppm Cd throughout rice growing season under field condition in 2007. At harvest, plants were sampled and analyzed for Cd concentration and accumulation in each rice organ. Significant variation of Cd concentration and accumulation in rice organs were found among thirty-five rice cultivars, revealing more than 8-fold varietal differences in grain Cd concentration and shoot Cd accumulation. Cd concentration and accumulation in grain were significantly different among cultivar groups, showing the highest in indica and the lowest in temperate japonica. Tongil-type and tropical japonica rice showed a Cd concentration intermediate to that of temperate japonica and indica rice. The higher Cd accumulation in grain of indica rice was attributable to the greater ability of Cd uptake. The greater ability of root-shoot translocation in tropical japonica and shoot-grain redistribution in tongil-type resulted in the significantly higher grain Cd concentration in these cultivar groups than in temperate japonica. For over 35 cultivars tested, grain Cd concentration revealed a significant positive correlation with root Cd concentration and shoot Cd concentration and accumulation while no significant correlation with root-shoot translocation factor and shoot-grain redistribution ratio. However, correlation analyses within each cultivar group showed that grain Cd concentration was significantly correlated with root-shoot translocation factor in indica, with root Cd concentration in tongil-type, with shoot Cd concentration and accumulation in tropical japonica, and with shoot Cd accumulation and shoot-grain redistribution ratio in temperate japonica. These results indicate that genotypic variation in grain Cd accumulation, in general, is controlled by all the three physiological processes but the major physiological process governing its genotypic variation within cultivar group is different depending on cultivar groups.

The functional stay-green trait gives leaves a longer duration of greenness and photosynthetic capacity during the grain-filling period. We developed two independent recombinant inbred line populations from the intra- and intersubspecific crosses of Oryza sativa L. subsp. japonica 'Suweon490' (japonica) × O. sativa subsp. japonica 'SNU-SG1' (japonica) and O. sativa subsp. indica 'Andabyeo' (indica) × O. sativa subsp. japonica 'SNU-SG1' (japonica), respectively. The common parental line 'SNU-SG1' was the functional source for the stay-green trait. Quantitative trait locus (QTL) mapping based on simple sequence repeat markers identified a total of six QTLs associated with two stay-green traits across two populations. The two traits were cumulative chlorophyll content (SPAD value) of flag leaf (CSFL) and total cumulative SPAD value of the four upper leaves (TCS). Four QTLs, tcs4, csfl6, csfl9 (or tcs9), and csfl12, located on chromosomes 4, 6, 9, and 12, respectively, were detected simultaneously in both populations. The remaining two QTLs, csfl2 (or tcs2) and tcs5, on chromosomes 2 and 5, respectively, were found to be population specific. Moreover, the functional stay-green trait of 'SNU-SG1' positively correlated with grain yield performance. Two yield QTLs, yld6 and yld9, on chromosomes 6 and 9 found in both populations were positioned at the same locations with the csfl6 and tcs9 QTLs for stay-green traits. Thus, the identified chromosomal regions can be promising targets of marker-assisted introgression of the functional stay-green trait into breeding materials for improvement of rice yield.

Functional stay-green has been regarded as a promising characteristic to be introduced for improving rice yield potential. A functional stay-green rice "SNU-SG1" that was identified from japonica rice collections was compared with two regular high-yielding rice cultivars (HYVs) for the temporal change of leaf chlorophyll, soluble protein, and root activity, and nitrogen accumulation and remobilization during the grain-filling period. SNU-SG1 had slower decreasing rate and maintained higher concentration of chlorophyll and soluble protein in upper four leaves during the grain-filling period than HYVs "Suweon490" and "Andabyeo", revealing a typical stay-green characteristic. Even though SNU-SG1 remobilized almost the same proportion of N accumulated before heading as HYVs to grain, it maintained much higher leaf N concentration due to the significantly higher N accumulation that is ascribable to the higher root activity sustenance during grain-filling period. The functional stay-green trait of SNU-SG1 seems to stem not only from the genetic control preventing chlorophyll degradation but also from the higher capacity to absorb N from soil due to the sustained strong root activity during grain-filling period. SNU-SG1 exhibited higher crop growth rate during late grain-filling period than HYVs, resulting in higher grain-filling percentage and non-structural carbohydrate re-accumulation in the stem at the final stage of grain filling. It is concluded that SNU-SG1 has a promising trait "functional stay-green" contributable to rice yield potential improvement through the improved grain filling.The functional stay-green trait during grain filling that results from a delay in the onset of leaf senescence or a slower decrease of chlorophyll content and photosynthetic activity (Thomas and Howarth 2000) will probably extend the assimilatory capacity of the canopy and might contribute to higher grain yields (Gwathmey et al. 1992). Spano et al. (2003) reported that functional stay-green mutants of durum wheat maintained longer photosynthetic activity, had higher seed weights, and yielded more grain than the parental genotype. Luo (2006) also reported a new stay-green wheat cultivar "CN17" with delayed leaf senescence maintained longer and higher photosynthetic competence compared with the control cultivar and this aspect was correlated with the difference in chloroplast development.Grain filling and leaf assimilation sustenance are usually conflicting processes in monocarpic cereal crops as the amount of N absorbed during grain filling is much smaller than the amount of N accumulated in mature grains. Thus, a large part of grain N is remobilized from vegetative organs especially from leaf blades to the developing grain (Mae 1997), causing leaf senescence and decrease of photosynthesis after flowering. As plant leaves, particularly chloroplasts, are sites of protein accumulation, mobilization of chloroplast protein is a central metabolic activity in leaf senescence and most of the mobilizable protein in a leaf is soluble protein...

Cadmium (Cd) poses a serious risk to human health due to its biological concentration through the food chain. To date, information on genetic and molecular mechanisms of Cd accumulation and distribution in rice remains to be elucidated. We developed an independent F7 RIL population derived from a cross between two japonica cultivars with contrasting Cd levels, 'Suwon490' and 'SNU-SG1', for QTLs identification of Cd accumulation and distribution. 'Suwon490' accumulated five times higher Cd in grain than 'SNU-SG1'. Large genotypic variations in Cd accumulation (17-fold) and concentration (12-fold) in grain were found among RILs. Significant positive correlations between Cd accumulation in grain with shoot Cd accumulation and shoot to grain distribution ratio of Cd signify that both shoot Cd accumulation and shoot to grain Cd distribution regulate Cd accumulation in japonica rice grain. A total of five main effect QTLs (scc10 for shoot Cd accumulation; gcc3, gcc9, gcc11 for grain Cd accumulation; and sgr5 for shoot to grain distribution ratio) were detected in chromosomes 10, 3, 9, 11, and 5, respectively. Of these, the novel potential QTL sgr5 has the strongest effect on shoot to grain Cd distribution. In addition, two digenic epistatic interaction QTLs were identified, suggesting the substantial contribution of nonallelic genes in genetic control of these Cd-related traits.