Multiple experiments and datasets from different projects were assembled to: (D1) select a temperature response function for thermal time estimation; (D2) estimate the thermal time duration between crop emergence and tassel initiation (Ttem-ti) for diverse maize lines; (D3) envirotype sensed photoperiod (DLs) in maize fields across the Northern Hemisphere; (D4) demonstrate methods for modeling a physiological reaction norm for flowering time plasticity (PRN-FTP); and (D5) compare the trait space of parameters estimated from the PRN-FTP in a diversity panel constituting separate breeding pools of maize. The current study focuses on days to anthesis (male flowering) as the final characteristic. (D1) Three temperature response functions (Methods S1, Eq. S1-S3; see Supplemental Information of the Main text) for measuring thermal time were compared using a dataset with 19 hybrids tested in at least 34 field environments, spanning latitudes from 30.5371°N to 51.5016°N (File S1; data sourced from the Genomes-To-Fields Initiative). For each environment, the sowing date, field coordinates, and best linear unbiased estimates (BLUEs) for calendar days from planting to anthesis were obtained from McFarland et al. 2020 and Rogers et al. 2021. Using temperature data, the genotype-specific BLUEs for calendar days to anthesis were converted to thermal time values. Three temperature response functions (Eq. S1-S3; see Supplemental Information of the Main text) for measuring thermal time were compared using a dataset with 19 hybrids tested in at least 34 field environments, spanning latitudes from 30.5371°N to 51.5016°N (File S1; data sourced from the Genomes-To-Fields Initiative). For each environment, the sowing date, field coordinates, and best linear unbiased estimates (BLUEs) for calendar days from planting to anthesis were obtained from McFarland et al. (2020) and Rogers et al. (2021). Using temperature data, the genotype-specific BLUEs for calendar days to anthesis were converted to thermal time values. (D2) The Ttem-ti was estimated for a collection of 239 maize inbred lines (panel 1) using a physiological formalism (Eq. 1 in Main text), based on data for crop emergence, phyllochron, and final leaf number recorded in two experiments under short-day (SD) conditions (File S2). The Ttem-ti measured in SD conditions is referred to as the basic vegetative phase to tassel initiation (BVP-ti; Kiniry et al., 1983a). Panel 1 primarily consisted of semi-tropical or tropical lines but included some temperate lines. Crop emergence and phyllochron were measured in a greenhouse phenotyping platform at the LEPSE of INRAE (PhenoArch, Montpellier, France; Cabrera-Bosquet et al., 2016) under well-watered conditions, while final leaf number was recorded in a field experiment in Puerto Vallarta, Mexico, also in well-watered conditions. For both experiments, daylengths throughout the growth cycle never exceeded 12.5 h, providing SD conditions in which photoperiod was not expected to suppress tassel initiation. Additional details for these experiments can be found in Supplemental Information of the Main text (Methods S2). (D3) Using the BVP-ti as a proxy for DLs, computational envirotyping of DLs was performed for fields across the Northern Hemisphere (latitudinal range of 13.7578 °N to 54.2900 °N) where maize was previously grown. For this, the geographical coordinates and planting dates of 680 field environments were assembled from past publications and ongoing projects (File S3). Crop emergence dates available from 15 field environments sourced from the Maize ATLAS project (Choquette et al., 2023) were used to compute a mean thermal time from sowing to emergence of 76 °Cd (estimated using Eq. S2; see Supplemental Information of the Main text), which was used for predicting times to emergence in the remaining environments where only the planting date was available. (D4) To test modeling procedures for the PRN-FTP, flowering time data from 37 field trials were combined into a multi-environment trial (MET) dataset (File S4). The dataset included observations that maximized observations for a common set of genotypes also present in dataset D2, including seven temperate inbred lines (B37, B73, M37W, Mo17, Oh43, LH123Ht, and 2369) and seven tropical inbred lines (CML10, CML258, CML277, CML341, CML373, Tzi8, and Tzi9). Still, representation of these lines across the 37 environments was not fully balanced. The temperate lines were present in a minimum of 13 and a maximum of 22 environments. The tropical lines were present in a minimum of 18 and a maximum of 37 environments. (D5) To examine the relationship between PRN-FTP parameters associated with flowering time per se and photoperiod sensitivity, MET data for a collection of 137 temperate, 66 tropical, and 62 admixed inbred lines constituting separate breeding pools (panel 2; Flint-Garcia et al. 2005) was assembled from previous experiments (File S5). These lines were observed across a maximum of three SD environments (maximum photoperiod 14.5 h) spanning 18.00 °N to 42.76 °N latitude.

Understanding how plant phenotypes are shaped by their environments is crucial for addressing questions about crop adaptation to new environments. This study investigated the interplay between developmental responses to temperature fluctuations and photoperiod perception in maize that contribute to genotype-by-environment variation in flowering time. We present a physiological reaction norm for flowering time plasticity (PRN-FTP) for studying large collections of genotypes tested in multi-environment trial (MET) networks. Using a new variable for computational envirotyping of sensed photoperiod, it was found that, at high latitudes, different genotypes in the same environment can experience hours-long differences in photoperiod. This emphasizes the importance of considering genotype-specific differences in the experienced environment when investigating plasticity. A statistical framework is introduced for modeling the PRN-FTP as a non-linear response function, with parameters putatively linked to different regulatory modules for flowering time. Applying the PRN-FTP to a sample of global breeding material for maize showed that tropical and temperate maize occupy distinct territories of the trait space for PRN-FTP parameters, supporting that the geographical spread and adaptation of maize was differentially mediated by exogenous and endogenous pathways for flowering time regulation. Our results have implications for understanding crop adaptation and for future crop improvement efforts.

Funding provided by: Agence Nationale de la RechercheROR ID: https://ror.org/00rbzpz17Award Number: ANR-16-IDEX-0006 Funding provided by: National Research Institute for Agriculture, Food and EnvironmentROR ID: https://ror.org/003vg9w96Award Number: PAF_18 Funding provided by: European UnionROR ID: https://ror.org/019w4f821Award Number: 817970 Funding provided by: Agence Nationale de la RechercheROR ID: https://ror.org/00rbzpz17Award Number: