Photosynthesis, a complex physiological and biochemical process, is regulated by a number of elementary factors in rice (Oryza sativa L.). However, it is likely that the factors are not equally effective in determining the process. Some studies suggested that three factors are importance of photosynthetic function (Mae 1997, Yang et al 2003, Kouril1 et al, 1999). The first is total leaf nitrogen content (TLN), because to some extent photosynthesis correlates positively with TLN (Mae, 1997). The Second is chlorophyll content of leaf (CC), as a major photosynthetic pigment and crop breeding (Yang et al. 2003). The third is chlorophyll a/b ratio (CR), which is often used to characterize the developmental state of the photosynthetic apparatus, the isolated pigment-protein complexes and is also used for ecological purposes (Kouril1 et al. 1999). Therefore, it is meaningful to detect the QTLs of the traits associated with photosynthesis such as TLN, CC and CR for high photosynthetic efficiency in molecular breeding in rice.

A mapping population of 81 F₁₁ lines (Recombinant Inbred Lines: RILs), derived from a cross between a japonica variety Kinmaze and an indica variety DV85 by single-seed descent method, was used to detect quantitative trait loci (QTL) for traits associated with photosynthetic function. To control the environmental influences on phenotypic evaluation, RILs, along with their parents were divided into four groups according to the heading data of two previous years and sown for TLN, CC and CR, which measured in leaves of rice at seven days after heading, on 14, 21, 28 May and 4 June in 2002 at Nanjing Agricultural University, P.R. China.

Composite interval mapping was implemented using QTL Cartographer software (Basten et al. 1999), with a threshold of LOD>2.5. A total of six putative QTLs were detected with percentage of variance explained (PVE) running between 11.2 ~ 29.6%, and LOD of QTLs 2.66 ~ 4.81 (Table 1, Fig. 1). Of those putative QTLs, three for TLN were detected on chromosomes 1, 2 and 11, with PVE of 17.3%, 15.3% and 13.7%, respectively, two controlling CR on chromosomes 3 and 4, PVE 13.8% and 29.6%, one controlling CC on chromosome 1, PVE 11.2%.

So far as, among three QTLs detected for TLN, the QTL on chromosome 2 was also detected previously linking to the marker G365 (Ishimaru et al. 2001), and other two QTLs on

chromosomes 1 and 11 were newly reported in this study. Interestingly, the QTL controlling CC, namely qCC-1 reported here, was detected in the region of the RFLP marker C122 on chromosome 1, where harbored NADH-glutamate synthase structure gene according to the previous study (Sasaki et al. 1994). Because the biosynthesis of chlorophyll begins with glutamate (Samuel, 1999), qCC-1 would play a vital role in photosynthetic function. However, no QTL controlling CC were detected 30 days after heading, suggesting that the effect of the QTL controlling CC decreased during leaf senescence.

Acknowledgment

Thanks to Dr. H. Yasui and A.Yoshimura (Kyushu University, Japan) for their kindly providing as materials.

References

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