The Photosynthetic, Genetic And Agronomic Characteristics Of A Rice (Oryza Sativa L.) Xantha Mutant

A xantha mutant (yellow plant) IIB11 was selected in the progeny 300Gy ⁶0Co-γ-rays-irradiated seeds of a cytoplasmic male sterile (CMS) maintainer line, cv. II32 B. Through a series of backcrosses of IIB11 to CMS line II32 A, a new CMS line was developed and named as Huangyu A (B). The photosynthetic, genetic and agronomoic characteristics of the mutant were investigated, and the usefulness of the yellow leaf color as a visual marker in hybrid rice seed production and varietal purity testing was demonstrated using the new CMS line Huangyu A. The main results were as follows:1. The content of photosynthetic pigments in the leaves of the mutant Huangyu A (B) exhibited significantly different from that of the wild type II32 A (B). In comparison with those of II32 A and II32 B, during the 20th to 80th days after sowing, the contents of chlorophyll (Chl) in the leaves of Huangyu A, Huangyu B were reduced by 46.3%-53.2%, 43.3%-47.8%, and those of carotenoids (Car) by 21.5%-31.9%, 19.7%-30.9%, respectively. Consequently, the ratios of Chl a/Chl b and Car/Chl in Huangyu A (B) were increased from about 4.1 and 0.20 in II32 A (B) to more than 7.5 about 0.25. The contents of all photosynthetic pigments and the ratios of Chl a/Chl b, Car/Chl in hybrid II32 A/R3027 were similar to those in Huangyu A/R3027, and were close to those in the wild type parents.2. The gas exchange rates of the mutant leaves were higher than those in the wild type in all different daily time periods from 08:45 to 15:04 hrs. The net photosynthetic rate (P_N) reached 18.4 and 18.7μmol CO₂ m^-2s^-1 during 13:10-13:39 hrs in Huangyu A and Huangyu B, respectively, which were correspondingly 11.7% and 25.0% higher than those in II32A and II32B. When irradiance was above 500μmol m^-2 s^-1, the Pn of Huangyu A and Huangyu B was 19-28% and 11-17% higher than those of II32A and II32B, respectively. The apparent quantum yield (AQY) of Huangyu A (B) was similar to that of II32A (B). The PN and the AQY of the hybrid Huangyu A/R3027, however, were consistently higher than those of 1132 A/R3027, by 12-19% and 28%,respectively. Furthermore, the mutant showed lower non-photochemical quenching (qN) but higher maximal photochemical efficiency (FV/FM), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (FV7FM'), higher photochemical quenching (qp), higher effective PS2 quantum yield ($ps2), and higher non-cyclic electron transport rate (ETR) than its wild type.3. Jt was revealed that the xantha leaf mutation was controlled by a single recessive locus through segregation analysis of xantha leaf plants in F2 populations of crosses between the mutant and other normal green varieties. The mutation gene was tentatively named as xl(t). Through co-segregation analysis of the leaf color mutation and SSR markers in two F2 populations (Huangyu BXII32B and Huangyu AX Niqingzhan), the mutation was found to be linked with the SSR marker RM21 with the genetic distances of 3.74cM, and consequently the xl(t) gene was located on the chromosome 11.4. Huangyu A exhibited stabe male sterility with 99.4% sterile pollens, and showed similar rice blast and bacteria blight resistance to 1132 A. Although Huangyu A showed shorter plant, shorter growth period duration, lower rate of extruded stigma, less number of florets per panicle, the Fj hybrids of Huangyu A with restorer lines R3027, Minghui 63 and 99 Ya 162 showed higher heterosis than corresponding ones of crosses 1132 A, even Huangyu A hybrids were 9-11 days shorter growth duration from sowing to heading. The yields of Fi hybrids originated from Huangyu A were significantly higher than those originated from 1132 A.5. The varietal purity of Huangyu A, Huangyu A/R6547, Huangyu A/HX3, identified at seedling stage using the xantha leaf marker (100%, 96.68%, 96.71%, respectively), were highly consistent with the results evaluated in field plot testing at maturity period. The yield of Huangyu A seed multiplication was 1094.3kg ha'1. The yield of hybrid seed production of Huangyu A with R6547 and HX3 was 2363.3 and 2265.6 kg ha"1, respectively, outyielded by 7.9% and 20.9% over 1132 A with corresponding restorer lines, respectively.In summary, the xantha leaf rice mutant had higher photosynthetic rates than its parent. It is implied that the mutant is a unique material both for the basic research onphotosynthesis and for the development of high yielding rice cultivars. The mutation locus is located on the chromosome 11. The xl(t) mutation seemed without negative effects on hybrid heterosis, and could be integrated into CMS lines and used as a visual marker for rapidly testing and efficiently increasing varietal purity in seed and paddy production of hybrid rice.