So far, most of the reported researches on cooking and eating quality in rice have been focused on physicochemical properties, i.e., amylose content (AC), gel consistency (GC) and gelatinization temperature (GT), which are directly related to the rice-eating quality. Meanwhile, a large number of QTL were detected, of which the most major QTL for AC and GC was allelic or tightly linked to the wx gene on chromosome 6 (Tan et al. 1999; Lanceras et al. 2000), and a major QTL for GT were allelic or tightly linked to alk gene on chromosome 6 (He et al.

1999; Lanceras et al. 2000). However, these physicochemical methods have a limitation in only serving as indices to indicate whether a rice variety will cook to be dry and fluffy or soft and sticky. They do not describe the complexity of cooked rice texture or give the evaluation of flavor characteristics. Namely, high physicochemical quality is not fully equivalent to better eating quality. Therefore, palatability properties, which include luster (LT), scent (ST), tenderness (TD), viscosity (VC), elasticity (EL) and integrated value of organoleptic evaluation (IVOE), can appropriately describe eating quality of cooked rice and are more effective than physicochemical properties to evaluate rice-eating quality.

In the present study, a direct method for evaluating eating quality of cooked rice was established with some modification on the basis of Yamamoto et al (1995), NSPRC (2000) and Champagne et al (1999). In a session, each panelist degusted seven chromosome segment substitution lines (CSSLs) and two control samples of which one check was assigned to cooker No.1 and another was randomly assigned to normalize the data. 24 panelists were selected from 60 volunteers based on their ability to correctly identify texture or flavor characteristics of cooked rice in the 20 degustation tests. In addition, each CSSL or parent was degusted for four times in different sessions each year to reduce the experimental error.

Then a CSSLs population, derived from Asominori/IR24 with Asominori as genetic background (Kubo et al. 1999), was planted and phenotyped for the above six palatability properties and three physicochemical characteristics in three successive years. The phenotypic value for each of the nine quality traits showed a continuous distribution and some transgressive lines were also observed. Additionally, a total of 33 QTL for these nine quality traits were identified, and six of them were consistent in three years (Fig. 1). These consistent QTL were qLT-8 for luster, qTD-6 and qTD-8 for tenderness, qIVOE-6a and qIVOE-8 for IVOE, and qAC-8 for amylose content. Phenotypic values were different significantly (p<0.001) between the CSSLs harboring any of the six QTL alleles and Asominori, the genetic background parent. Significant phenotypic correlation (r=0.88, p<0.05) was detected between three years for these CSSLs carrying the same QTL alleles. The result indicated that the six QTL were stably expressed in three years. QTL qTD-6 and qIVOE-6a were located on chromosome 6 because they were in the vicinity of RFLP marker C688 which was tightly linked to wx gene on the chromosome. QTL qLT-8, qTD-8, qIVOE-8 and qAC-8 were mapped between RFLP markers XNpb41 and R727 on chromosome 8. Since the three RFLP markers were closely linked to QTL qTD-6, qIVOE-6a, qLT-8, qTD-8, qIVOE-8 and qAC-8, they would be useful markers for marker-assisted selection in rice quality improvement breeding programs.

References

Champagne ET, Bett KL, Vinyard BT, McClung AM, Barton FE, Moldenhauer K, Linscombe S and McKenzie K, 1999. Correlation between cooked rice texture and Rapid Visco Analyzer measurements. Cereal Chemistry 76: 764-771.

He P, Li SG, Qian Q, Ma YQ, Li JZ, Wang WM, Chen Y and Zhu LH, 1999. Genetic analysis of rice grain quality, Theor Appl Genet 98: 502-508.

Kubo T, Nakamura K and Yoshimura A, 1999. Development of a series of Indica chromosome segment substitution lines in Japonica background of rice, Rice Genetics Newsletter 16: 104-106.

Lanceras JC, Huang ZL, Naivukul O, Vanavichit A, Ruanjaichon V and Rragoonrung S, 2000. Mapping of genes for cooking and eating qualities in Thai jasmine rice (KDML105). DNA Research 28: 93-101.

NSPRC (National Standard of People's Republic of China), 2000, High quality paddy, GB/T17891-1999.

Tan YF, Li JX, Yu SB, Xing YZ and Zhang QF, 1999. The three important traits for cooking and eating quality of rice grains are controlled by a single locus in an elite rice hybrid, Shanyou63. Theor Appl Genet 99: 642-648.

Yamamoto R, Horisue N and Ikeda R, 1995. Rice Breeding Manual. Miscellaneous Publication of the National Agriculture Research Centre in Japan, No. 30, October.