Combining ability, heterosis and genotype x environment interaction in tropical maize (Zea mays L.) under heat stress and optimal environments

Name : Pooja Devi Id. No. : 33978 Sem & Year of admission: I, 2008-09 Degree : Ph.D. Major : Genetics and Plant Breeding Department: Genetics and Plant Breeding Minor : Molecular biology and Biotechnology Advisor: Dr. Sitar Singh Verma Thesis Title: Combining ability, heterosis and genotype × environment interaction in tropical maize (Zea mays L.) under heat stress and optimal environments The present genetic study examined the tolerance of 9 tropical maize inbred lines under varying high temperature regimes to understand the genetic control of grain yield and various secondary traits associated with high temperature tolerance. Thirty-six crosses generated through half diallel mating among 9 inbred parents were evaluated along with 4 checks in 8 environments under optimal and heat stress conditions during Rabi 2010, Spring 2011 and Kharif 2011. The main objectives of the investigation were to (i) identify the gene action involved in the inheritance of grain yield and other secondary traits (ii) estimate the general and specific combining abilities, (iii) quantify the magnitude of heterosis for grain yield (iv) identify secondary traits associated with grain yield under heat stress (v) study genotype × environment interaction patterns and identify stable hybrids under optimal and heat stress environments. GGE biplot was used to divide the eight optimal and heat stress environments into three environmental groups based on the relationship between the testing environments and the best winning hybrids shared by the environments. These groups were: Group A (optimal), comprised environments HYDWWK, SAB, PNT; Group B (extreme heat stress), comprised HYDHS environment and Group C (moderate heat stress), comprised HYDWWR, KAR, LUD, ALB environments. This grouping was in accordance with the trial management and prevalent heat stress in these environments. Analysis of variance for combining ability revealed significant entry mean squares for most of the traits in each environmental group. Significant GCA and SCA mean squares were observed for most of the traits under extreme and moderate heat stress conditions, while SCA mean squares were non-significant for most of the traits under optimal condition. Additive genetic effects appeared to be more important for grain yield under optimal conditions, but both additive and non-additive genetic effects were more important under extreme and moderate heat stress conditions in this set of inbred lines. Inbred lines, L8 and L9 had consistently positive GCA effects for grain yield across optimal, extreme heat stress and moderate heat stress environments. These lines were also good combiners for anthesis-silking interval, tassel blast and leaf firing under both heat stress conditions. Specific combination (L8/L9) did not give a high SCA effects for grain yield, although the parents had high GCA effects. This implies that L8 and L9 have similar heterotic pattern. Therefore, a cycle of recurrent selection of this biparental cross would be helpful in accumulating favorable alleles before it is used as an inbred source population. GGE biplot results for GCA effects were in accordance with Griffing’s combining ability analysis. Biplot also identified crosses, L1/L8, L1/L9, L2/L8 and L4/L8 under optimal condition; L7/L9, L2/L9, L4/L7, L6/L7 and L1/L6 under extreme heat stress condition; and L2/L7, L2/L8 and L5/L6 under moderate heat stress condition as best mating partners for grain yield. On the basis of both high mean performance and high stability, stable hybrids were identified across each environmental condition. Anthesis-silking interval (ASI) can be used as an important secondary trait under extreme heat stress condition as it was under the control of additive gene action, had high repeatability and was negatively correlated with grain yield. Higher value of mid-parent and better-parent heterosis for grain yield was observed under both heat stress condition in comparison to optimal condition. Magnitude of heterosis for grain yield was higher under extreme and moderate heat stress environments in comparison to optimal environments due to the poor per se performance of inbred lines under these conditions. Therefore, development of high yielding and heat tolerant inbred lines was suggested to further improve the hybrid performance. (Sitar Singh Verma) (Pooja Devi) Advisor Authoress