Screening for Spikelet Fertility and Validation of Heat Tolerance in a Large Rice Mutant Population

A total of 10 000 M4 individuals in Jao Hom Nil (JHN) mutant population was treated with high temperature (40 ℃ to 45 ℃) during the day time (6 h) from the booting to the harvesting stages, and ambient temperature (33 ℃ to 35℃) was used as the control. The results of screening and yield trials found that the mutant line M9962 had a high spikelet fertility of 78% under heat stress. In addition, the other mutant lines, including M3181 and M7988, had a spikelet fertility of approximately 70%. However, the JHN wild type, Sin Lek, RD15 and RD33 had very low spikelet fertility of 34%, 14%, 9% and 4%, respectively. The lower spikelet fertility at an elevated temperature resulted in a dramatic decrease of filled grain and contributed to a loss in 100-grain weight. M9962 is a potential genetic stock for use in a heat tolerance breeding programme. In addition, spikelet fertility at high temperature was representative of heat tolerance and can be used as a screening trait for heat tolerance during the reproductive phase on a large scale.

Effects of Heat Stress at Vegetative and Reproductive Stages on Spikelet Fertility

Seed-setting rate, yield components and grain quality traits of 169 accessions from an exotic rice germplasm were tested under high temperatures from 40 oC to 45 oC for 6 h during the daytime at the vegetative and reproductive stages, respectively. The results showed that heat stress significantly decreased the seed-setting rate of all the accessions, but the heat stress effects varied among accessions. Based on the decreases in seed-setting rate at high temperatures, N22 was the most tolerant, followed by AUS17, M9962, SONALEE and AUS16. Moreover, the reductions in seed-setting rate and yield under heat stress were more serious at the vegetative stage （45 d before heading） than at the booting stage （15 d before heading）. In addition, heat stress also affected grain quality, especially by conferring chalkiness to most of the accessions, but SONALEE did not change much. The heat-tolerant accessions identified here and the phenotype protocols developed could be used in future genetic studies and breeding programmes focused on heat tolerance.

Breeding Novel Short Grain Rice for Tropical Region to Combine Important Agronomical Traits, Biotic Stress Resistance and Cooking Quality in Koshihikari Background

Breeding program strategies to develop novel short grain white rice varieties such as japonica(short grain)that introgress biotic stress resistance and high grain quality have been developed usingindica rice(Pin Kaset+4 and Riceberry)for applications in japonica rice(Kashihikari)improvement.Fourbreeding lines showing promising agronomic performance with short grain and low amylose content(<20%)were obtained.In addition,sensory testing of these breeding lines showed high scores thatsimilar to Kashihikari.Two promising lines,KP48-1-5and KP48-1-9,which possessed a combination offour genes resistance to different biotic stresses(Bph3+TPS+Xa21+Pi-ta)and four genes for grainquality(G83+SSlla+w+badh2),were deveiaped using marker-assisted selection(MAS)with thepedigree method.The current study clearly illustrated the successful use of MAS in combining resistanceto multiple biotic stresses while maintaining a high yield potential and preferred grain quality.Moreover,the results indicated that this breeding program,which includes crossing temperate japonica with indica,can create novel short grain rice varieties adapted to a trapical environment like the japonica type.