Heat stress effects on reproductive traits in cultivated and wild sunflower (<Emphasis Type="Italic">Helianthus annuus</Emphasis> L.): evidence for local adaptation within the wild germplasm

Heat stress (HS) is a major threat to current and future crop production. Crop improvement for HS tolerance is a major tool for dealing with HS and crop wild relatives (CWR) offer the greatest variability for such improvement. Here, we evaluated the HS tolerance on four reproductive traits in cultivated and wild sunflower and tested for local adaptation to HS within the wild germplasm. Three cultivars and 23 wild populations (from native and invasive ranges) were grown in field experiments for 2 years. Flowering heads were covered with white (control) and black (HS) paper bags during seven consecutive days. Additionally, biogeographic tools were used to test for local adaptation. HS increased air temperature on black bags compared to the white ones by 9.4 °C on average and strongly decreased seed number and yield with smaller effects on head diameter and seed weight. We found large variability for HS tolerance, mainly in seed number and yield. The invasive group outperformed the cultivated and native groups in both years. Biogeographic analysis reveals a clinal variation in HS tolerance, populations from wetter (but not from warmer) environments were more tolerant to HS. In addition, the positive correlation observed between reproductive traits under control conditions and HS tolerance helps to explain the better performance of the invasive populations. We proposed the use of invasive populations for future sunflower improvements in HS tolerance and the adoption of biogeographic tools in another CWR species to identify HS tolerant populations.