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We evaluated a two-component transposon iAc/Ds system for generating a library of insertional mutants in rice. The constructs used have gene or enhancer trapping properties, plasmid rescue and T-DNA/Ds launching pad reporter facilities. Mutagenic iAc/Ds lines were produced by three methods: crossing iAc and Ds containing lines; co-transformation with iAc and Ds constructs; and super-transformation of iAc transgenic calli with Ds constructs. First and second generation screening populations, derived from crosses (F2 and F3) or double transformation (DtT1 and DtT2), were analysed for stable insertion lines containing Ds transposed to locations unlinked to iAc. The average frequencies of putative stable insertion (PSI) lines in the F2, DtT1, F3 and DtT2 populations were 6.61, 5.58, 11.47 and 7.05% respectively, with large variations in these frequencies in screening populations derived from different mutagenic lines. Further analyses indicated that 41, 33, 65 and 64% of the PSI lines, respectively, have Ds transposed to locations unlinked to the original Ds launching pad. Using the plasmid rescue system, sequences flanking Ds from 137 PSI lines were obtained. Sixty-eight of these lines had unique insertions in genomic regions, of which 18 were known sequences. Because the average frequency of proven stable insertion lines in any of our screening populations has been less than 5%, we suggest that additional features should be incorporated in this two-component iAc/Ds system to increase the screening efficiency, and to make it suitable for large-scale insertional mutagenesis and determination of gene function in rice.

The Japanese rice cultivar, Akenohoshi, has numerous spikelets in a panicle (extra-heavy panicle type) and achieves a large yield sink capacity. However, this cultivar, as well as other extra-heavy panicle types, does not always produce higher yields because of poor grain filling of the spikelets on the secondary branches in a panicle. To determine whether the poor grain filling found in Akenohoshi was due to source-limited or sink-limited conditions, the present study examined the responses of grain-filling characteristics to several spikelet-removal treatments immediately after heading. Only when the spikelets on secondary branches remained was a significant increase in filled grain percentage in this spikelet position observed. This increase seemed to be associated with the increases in rate and duration of the grain-filling process and in single grain weight in this spikelet position. These results clearly indicate that the poor grain filling in the spikelet on secondary branches of Akenohoshi could mainly be attributed not to sink-limited conditions, but to source-limited conditions probably at specific stages of grain filling.

A new sweet potato breeding line, Kanto 116, was developed, featuring low gelatinization temperature and an altered starch fine structure. Starch granules from Kanto 116 showed an abnormal morphology characterized by cracking into granules. Starch content, amylose content and tuberous root appearance of Kanto 116 were similar to those of the control and the parents. Pasting temperatures of Kanto 116 starch determined by the Rapid Visco Analyser were 51.4 — 52.6 °C, approximately 20 °C lower than those of the control and parents starches. Onset, peak, and conclusion temperature of gelatinization, and gelatinization enthalpy of Kanto 116 starch determined by differential scanning calorimetry were 39.0 °C, 46.9 °C, 64.8 °C, and 8.8 J/g, respectively, and much lower than those of the control and parents starches. The chain‐length distribution of the amylopectin molecules, determined by high‐performance anion‐exchange chromatography, showed that Kanto 116 starch had a higher proportion of short chains (DP 6 — 11) and a lower proportion of chains between DP 12 — 28 than control and parent starches. The debranched β‐limit dextrin of Kanto 116 starch also showed that the proportion of both short and long B1 chains was different from those of the control and parents starches.

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