Summer Drought Delays Leaf Senescence and Shifts Radial Growth Towards the Autumn in Corylus Taxa

Background: Understanding the mechanisms by which woody perennials adapt to extreme water deficits is important in regions experiencing increasingly frequent and intense droughts. Methods: We investigated the effects of drought severity in the shrubs Corylus avellana L., C. maxima Mill., and their morphological intermediate forms, all from local Belgian origin, and C. avellana from a Spanish-Pyrenean origin. Potted saplings in a common garden were not receiving any water for a duration of 30 days in July 2021 and developed a range of visual stress symptoms. We assessed responses across the various symptom categories. Results: Droughted plants senesced later than the controls (up to 6 days). The most severely affected plants disproportionately displayed the longest delay (21 days). The delayed leaf senescence was reflected in the subsequent bud burst which was delayed for the droughted plants, with again the largest delay observed for the most severely affected plants. Interestingly, radial growth shifted towards the autumn among the drought-treated plants, suggesting compensation growth after growing conditions normalized. The Spanish-Pyrenean provenance, characterized by smaller plants with smaller leaves, developed visual drought symptoms later than the local provenance during the drought. Conclusions: The results indicate that severe early summer drought, followed by rewatering, not only diminishes radial growth but also prolongs the growth period, and delays leaf senescence. A prolonged time frame for radial growth and a delayed leaf senescence indicate a longer period in which carbon is incorporated in woody tissue or in non-structural carbohydrates. This can help the fine tuning of carbon sequestration modeling. The Pyrenean provenance, adapted to high altitude, holds an advantage under water-limited conditions.