ABSTRACT

Cotton (Gossypium hirsutum L.), now grown as an annual crop, continues to show some characteristics of a perennial; therefore, there is a need to focus on early maturity. Early maturing cotton cultivars are befitting for the farming community and textile industry. These cultivars could increase cotton production by mitigating risks associated with pests and unfavourable weather conditions towards the end of the season. The present study thus, aimed at identifying stable early maturing high yielding compact genotypes, deciphering the genetics underlying earliness, and comparative transcriptomic studies related to early square development in upland cotton. The studies related to early maturity and morphology traits in the various generations (P1, P2, F1, F2, BC1P1 and BC1P2 or F2:3) of the three crosses ESS-20 × FLT-25, S-32 × FLT-25, and NNDC-30 × NNDC-47 showed more than one significant scale indicating epistasis, except for vertical flowering interval in S-32 × FLT-25. The additive-dominance model in most characters proved inadequate either due to epistasis or genotype-environment interaction. A preponderance of partial or over-dominance towards the early maturing parents and either duplicate or complementary epistasis detected in the three crosses indicated that selection for early maturity would be effective in later generations. Among the set of 20 cotton genotypes that were evaluated in three planting densities across two years, AMMI analysis revealed highly significant variations for environment, genotype, and GEI effects for a few early maturity and yield-related traits suggesting that the genotypes responded differently to various environments. Genotype selection index revealed that S-32 was stable for days to first flowering, days to first boll opening, per cent crop harvest at first pick, production rate index, and seed cotton yield. AMMI I and AMMI II biplot revealed that genotype A-2 with low IPCA scores was stable for Bartlett’s index, earliness percentage, and seed cotton yield. Based on the multi-trait stability index (MTSI), genotypes A-2, NNDC-30, and S-32 were selected as early maturing and high-yielding with wide adaptation to all three spacings. Multi-trait genotype ideotype distance index (MGIDI) revealed that NNDC-30 was suitable for high-density planting (60 cm × 15 cm); ESS-3 and ESS-20 were suitable for recommended spacing (60 cm × 30 cm) and Sahana was suitable for sparse density sowing (60 cm × 60 cm). Genotype ESS-22 was found to be robust in all six test environments based on plant-type characterization. Comparative transcriptomic analysis in both leaf (L) and square (S) of early maturing (NNDC-30) and late-maturing (NNDC-47) genotypes identified 1,281 differentially expressed genes (DEGs) between NNDC30-L and NNDC47-L and 1,242 genes between NNDC30-S and NNDC47-S with a majority of DEGs on chromosome A05. The results of qRT-PCR showed that GhGA20OX1 (Ghir_A07G015480) encoding Gibberellin 20 oxidase 1 showed an upregulation of 5.11-fold in NNDC30-S. Thus, by improving plant type with compact canopy, particularly by increasing plant density and compacting canopy, GhGA2OX1 could contribute to early flowering and higher cotton yields in NNDC-30 under high-density planting system.