Photoperiod associated late flowering reaction norm: Dissecting loci and genomic-enviromic associated prediction in maize

Context: Tropical maize lines are known to have extended flowering times when grown in northern temperate environments, a response largely due to the prolonged day length. This adaptation challenge is referred to as Photoperiod Associated Late Flowering (PALF) and poses a significant barrier to using tropical maize genetics in temperate agricultural settings. Objectives: This study aims to (i) investigate the genomic and environmental factors that influence PALF in recombinant inbred lines (RILs) derived from a tropical parent crossed with three temperate-adapted maize lines. (ii) Evaluate the performance of these RILs under different environmental conditions in Texas, Iowa, and Wisconsin over the years 2018-2021. (iii) Explore the efficacy of combining enviromic and genomic data to predict flowering times more accurately. Methods: We analyzed 540 RILs across three populations in field trials conducted in Texas, Iowa, and Wisconsin. Flowering times were recorded alongside measurements of local day lengths and solar radiation levels. A genome-wide association study (GWAS) was performed to identify loci linked to PALF. The predictive power of combining enviromic and genomic data was assessed by calculating the cross-validation error rates of flowering time predictions. Results: PALF was prominently observed in the year 2019, heavily influenced by the planting dates that year. Some RILs in Iowa and Wisconsin flowered exceptionally late due to longer day lengths during a key period (reaching up to similar to 15 hours) and greater solar radiation compared to Texas. The combination of enviromic and genomic data provided more accurate predictions of flowering times, particularly between 59 and 68 days after planting for day length, and 41 - 50 days for solar radiation. The GWAS identified several key loci, with the top hits explaining between 20% and 29% of the variation in PALF. Predictive accuracy was significantly higher in Iowa and Wisconsin than in Texas, attributable to loci responsive to the extended photoperiods experienced in these states. Conclusions: The study demonstrated that tropical maize lines can moderately adapt to the extended photoperiods typical of northern temperate climates with a photoperiod day length between similar to 13.5 and similar to 15 hours. By integrating enviromic and genomic data, the accuracy of flowering time predictions is significantly enhanced, aiding in the selection and breeding of tropical maize lines suitable for cultivation at higher latitudes. The identification of critical genomic regions through GWAS provides a valuable foundation for further genetic improvements, potentially improving tropical maize for temperate regions.