Heterosis and combining ability for callus growth rate in rice

To explain heterosis in rice (Oryza sativa L.) at the cellular level, we studied the inheritance of callus growth rate in seed-derived calli from a 14-by-14 diallel cross that used four japonica, four javanica (the tropical type of japonica), and six indica cultivars. Callus growth rates were calculated from gains in fresh callus weight that were measured after 28 d of culture at 20, 28, 35, and 40 degrees C in the dark. All F-1 crosses between subspecies had significantly (P < 0.05) higher callus growth rates than the parental lines. The F-1 crosses showed greater tolerance for low temperature than their parents. Analysis of variance of diallel crosses showed that additive and dominance effects on the callus growth rate were both significant (P < 0.01). Variances for general (GCA) and specific combining ability (SCA) in callus growth rate were also highly significant. Among crosses between subspecies,japonica x indica showed the most positive SCA effects for callus growth rate at 28 and 35 degrees C, while japonica x japonica and indica x indica showed negative SCA effects at 20 to 35 degrees C. The magnitude of the SCA effects on hybrids at 28 degrees C correlated significantly with that of plant growth in the field from a previous study (r = 0.867, 0.01 < P < 0.05, df = 4). Midparent heterosis for the callus growth rate was clearly positive and significant and was greatest in japonica x indica crosses in the combination groups at 28 degrees C. Heterosis and SCA effects for this trait at 28 degrees C were related to the genetic distance between parental subspecific groups.