during grain filling period, or relatively low ratio of source to sink (Lu et al., 1994;Yuan, 1994), resulted in Poor grain filling is a major constraint in utilizing the heterosis of poor grain filling of J/IHs. Our recent work, however, japonica/indica hybrid rice (Oryza sativa L.). The objective of this showed that J/IHs had high dry matter accumulation study was to investigate potential causes of poor grain filling of japonica/indica hybrids (J/IHs) by examining the source-sink relations in during grain filling, and their above-ground dry matter 36 J/IHs. Thirty-six J/IHs and their parents and two intervarietal per spikelet was greater than that of IVHs (Wang et hybrids (IVHs) were grown at the Yangzhou University farm during al., 1998;Yang et al., 1999), indicating that poor grain the 1998, 1999, and 2000 rice growing seasons. Growth analyses were filling of J/IHs was not a result of source limitation. We performed at heading and during grain filling period, and yield and hypothesized that poor grain filling might be related to yield components were determined at maturity. Results showed that poor partitioning of assimilates to the grain in J/IHs. J/IHs had 18.9% greater spikelet number per square meter than the The objective of this study was to investigate the two IVHs because of the difference in spikelet number per panicle, potential causes of poor grain filling of J/IHs by examinbut grain yields of the two kinds of hybrids did not differ significantly ing the source-sink relations in 36 J/IHs. as a result of low spikelet filling percentage in J/IHs. Above-ground dry matter accumulation of J/IHs was 73.8% greater than that of IVHs during the grain filling period. The above-ground dry matter MATERIALS AND METHODS per spikelet of the J/IHs was 12.7 and 4.1% higher than that of their Plant Materials and Field Design parents and IVHs, respectively, indicating that source limitation was not the cause of poor grain filling of J/IHs. Translocation of assimilates Thirty-six J/IHs (F 1 ) were grown in the field of Yangzhou and remobilization of stored assimilates from the straw to the grains University farm, Jiangsu Province, China (32Њ30ЈN, 119Њ25ЈE), during the grain filling period in J/IHs was about 64%, which was during the rice growing season (May-October) of 1998, and significantly less than that of IVHs. At maturity, only 44.1% of 14 C the experiment was repeated in 1999 and 2000. All J/IHs were fed to the flag leaves of J/IHs was partitioned into grains and the rest made from six japonica lines with wide compatibility genes remained in stems and leaves. Poor translocation and partitioning of (female parents-PC311, Lunhui 422, Jw-8, Ce01, 02428, and assimilates to the grain of J/IHs resulted in low harvest index. The Ce03) crossed with six indica lines (male parents-Miyang 23, results suggest that poor transport of assimilates to grains account 3037, Zaoxian-dang 18, IR36, Minghui 63, and Yangdao 4). for poor grain filling of J/IHs.Six genotypes (Ce03/Yangdao 4, Ce01/Miyang 23, PC311/ Zaoxian-dang 18, PC31...

A pot trial was conducted with 52 rice cultivars of different types collected from different origins. The results showed that there were great differences in Cd concentrations in straw, brown rice and grain chaff among the rice cultivars grown in a soil containing a Cd concentration of 100 mg kg −1 ; the Cd concentrations in brown rice ranged from 0.22 to 2.86 mg kg −1 . The great genotypic differences in Cd concentrations indicated that it is possible to lower the Cd content of rice through cultivar selection and breeding. Significant differences were found in the Cd concentrations of the rice types of Indica consanguinity and those of Japonica consanguinity, but not between their origins. There were significant correlations between straw and brown rice in Cd concentration and in the total amount of Cd accumulated. The distribution ratios of the Cd accumulated in brown rice to the total Cd accumulation in the aboveground rice plant varied greatly from 12.9 to 137.8 g kg −1 , and there was significant correlation between the distribution ratios and Cd concentrations in brown rice. These indicated that Cd concentration in rice grain is governed by the transport of Cd from root to shoot and also from shoot to grain. Cd concentrations in brown rice also correlated significantly with some important agronomic traits, as well as with nitrogen concentrations, one of the most important criteria for rice quality.

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Sucrose transporters (SUTs) are known to play critical roles in the uptake of sucrose from the apoplast in various steps of sugar translocation. Because developing pollen is symplastically isolated from anther tissues, it is hypothesized that SUTs are active in the uptake of apoplastic sucrose into pollen. To investigate this possibility, a comprehensive expression analysis was performed for members of the SUT gene family in the developing pollen of rice (Oryza sativa L.) using real-time RT-PCR combined with a laser microdissection technique. Among the five SUT genes, OsSUT1 and OsSUT3 were found to be preferentially expressed and had temporal expression patterns that were distinct from each other. Expression of OsSUT1 in pollen was confirmed by a promoter–GUS fusion assay. The physiological function of OsSUT1 in pollen was further investigated using retrotransposon insertion mutant lines. While the homozygote of disrupted OsSUT1 (SUT1–/–) could not be obtained, heterozygote plants (SUT1+/–) showed normal grain filling. Their progeny segregated into SUT1+/– and SUT1+/+ with the ratio of 1:1, suggesting that the pollen disrupted for OsSUT1 is dysfunctional. This hypothesis was reinforced in vivo by a backcross of SUT1+/– plants with wild-type plants and also by in vitro pollen germination on the artificial media. However, starch accumulation during pollen development was not affected by disruption of OsSUT1, suggesting that the sugar(s) required for starch biosynthesis is supplied by other sugar transporters.

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