Phenotypic differences between DH parental and control lines
In the 2018 field trials sown at Williams and Muresk (WA), low temperatures during heading and anthesis caused frost damage to all six DH populations (Fig. 1a). The most visible frost damage was on spikes (Fig. 1b). Since flowering times were differing not only between field trial locations but also between sowing areas within the same location, the representative anthesis data for the parental and control lines in the areas sown to Spitfire/Bethlehem (SpB) and Bethlehem/Westonia (BW) are presented in Supplementary Table 3 and Fig. 1c-d. On average, anthesis times for cultivars Gregory, Tungsten and Yitpi were significantly later than those for the other parental cultivars at both locations. The cultivar Suntop flowered two and five days later than the rest of cultivars in Williams and Muresk, respectively. The cultivar Bethlehem and its substitution lines B3B and B7A were the earliest flowering lines, while Westonia, Spitfire and Mace flowered two to three days later. The cultivar Wyalkatchem was between one and two days later than Mace and Westonia. In Muresk, the anthesis time of Suntop showed large differences (three days) between the SpB and BW sown areas (119 and 122), which led to significantly different levels of frost damage.
In Williams, the earlier-flowering lines of Bethlehem, B3B and B7A, were impacted the most by frost whereas the late-flowering lines Gregory, Tungsten and Yitpi were not affected (Fig. 2a-b, Supplementary Table 3). Among the cultivars with a similar flowering window, such as Spitfire, Mace and Wyalkatchem, Westonia was the most susceptible to frost, whereas Mace showed the highest level of tolerance. The same trend was observed in Muresk. Spitfire displayed frost tolerance in Williams but was susceptible in Muresk. Its anthesis window was very close to that of Mace in the SpB sown area and to Wyalkatchem in the BW area. Suntop showed high frost tolerance in Williams but was susceptible in Muresk, where its flowering window was shorter. Floret sterility also showed the same levels of frost impact in selected parental and control lines belonging to the three anthesis windows (Supplementary Fig. 1a-b).
According to the date of the frost event (14, 15 and 16 September 2018), the time of impact was between 9 to 18 days before the average anthesis time. The 2019 Muresk field trial data for Suntop at 16 days before anthesis (DBA) are presented in Supplementary Fig. 1c-g. The data show that wheat plants were at the flower development and anther differentiation stages at 16 DBA, according to Bonnett’s description (Bonnett, 1936), and some flowers were at meiosis stage. Based on Koonjul et al.(Koonjul et al., 2004), lines are most vulnerable to abiotic stress during spikelet development and meiosis stages. The 2018 frost events incidentally hit the most susceptible meiosis stages of our trial.