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To acquire a better understanding of whether RS influences the dynamics of in vivo starch digestion and seed vigour, the high-RS rice mutant RS4 (RS ca. 10%) and the wild type R7954 were used to investigate total amylase activity, seedling vigour, starch content and starch granule structure during germination. RS4 exhibited similar seed vigour to R7954. Amylose and amylopectin in R7954 showed synchronous degradation throughout the whole process, while amylopectin was hydrolysed significantly faster than amylose in RS4 during the earlier germination stages. The starch residues of RS4 after germination (GD) lost endotherm peaks and showed a special X-ray diffraction pattern with only two peaks at around 16.90° and 21.62°, probably due to remnants of amylopectin and its tight crosslinking with the cell wall. The remaining starch after 10 GD, primarily amylopectin may make a critical contribution to total resistant starch content. These results indicated that RS had no negative impact on seed vigour in rice lines, although RS cannot be hydrolysed by α-amylase from human and animal in vitro . By appropriately increasing the special amylopectin fraction, a new breeding programme of high RS crops and improvement in the eating quality of high RS rice varieties might be achieved.

The mechanism responsible for the effects of endogenous lipids on starch functionality is not fully understood. In this study, a high-lipid rice mutant (GZ1), a low apparent amylose content (AAC) mutant (GZ93) and parent rice (R7954) were employed to study the effect of the removal of starch lipids (SL) and non-starch lipids (NSL) on the physicochemical properties of starch and resistant starch (RS) and the rice gel texture. GZ1 had a lipid content of 2.52%, of which 61.1% was NSL. Removal of NSL only had a slight impact on the AAC, RS content, gelatinization temperature, swelling power and paste viscosity, but further removal of SL significantly increased the AAC and RS contents. For R7954 and GZ93, removal of SL decreased Tp, ΔH, PKV, CPV, and BDV but significantly increased the swelling power (SP). In GZI, the removal of SL resulted in significant increases in ΔH, PKV, HPV, CPV, SBV and the hardness of the gel texture. These results suggested that SLs had a much greater influence on the starch physicochemical properties and gel texture than NSLs. The results may be helpful in designing breeding programs for new rice varieties with better functionality.

RS2 and RS3 are two of the main resistant starch types in food, and the primary types found in raw and cooked rice respectively. To elucidate the physiochemical determinants for RS2 and RS3 content in rice, this study investigated the fine amylopectin structure, starch granules morphologies and starch pasting properties of nine different rice accessions. The results revealed that not all the accessions showed a decrease in resistant starch after cooking and cooling. Granular size, paste properties, fb1 and fb3 showed a significant negative correlation with RS3, whereas fa showed a significant positive correlation with RS3. RS2 had no significant correlation with granular size, but did have a significant positive correlation with HPV, CPV and fb2. Different rice types could be distinguished by the fine amylopectin structure of DP6-12 and DP13-24 chains. Taken together fa, fb2 and HPV and CPV values may have the potential to be used as indices for distinguishing or characterising rice with different RS2 and RS3 contents.

Through integrated molecular and biochemical investigations and by using a common mutant line in molecular mapping, we have shown that the SSIIa gene, previously reported to be responsible for the starch granule gelatinization temperature (GT) differences between indica and japonica rice varieties, might also control the GT variations among certain indica varieties. This effect is mediated through differences in the amounts of SSIIa protein bound to starch granules leading to differences in the structure of amylopectin molecules synthesized. For the first time it was shown, that the amylopectin type accounted only for the GT differences between varieties with different SSIIa alleles, but not between varieties carrying a common SSIIa allele. In the latter case, another gene, alk2(t) , with a genetic distance of 3.93 cM from the Wx gene, was identified as being responsible for the GT variations. Based on the thermal properties and amylopectin chain length profile characteristics, it is postulated that the SSIIa gene has at least three different alleles, one in japonica rice and two in indica rice varieties, whereas alk2(t) has at least two alleles for either low or high GT. Thus, the rich diversity of the GT character in rice very probably results from various combinations of these alleles.