ROS Accumulation as a Hallmark of Dehydration Stress in Primed and Overprimed Medicago truncatula Seeds

Seed priming protocols implement incomplete imbibition phases, as well as physical, chemical or biological treatments, to activate pre-germinative metabolism and stress response, thus improving germination performances, seedling establishment and stress tolerance according to agricultural productivity requirements. The dehydration phase following priming treatments represents a critical variable, since an excessively prolonged imbibition (overpriming) impairs desiccation tolerance, compromising seed viability and seedling establishment. Priming protocols generally optimize imbibition-dehydration timing empirically to avoid overpriming. Hence, a better understanding of the dynamics underlying the loss of desiccation tolerance represents a promising route to test and develop efficient and cost-effective priming techniques. In the present work, priming and overpriming conditions were defined to explore the role of desiccation tolerance in seed priming efficiency in the model legume Medicago truncatula. The positive effects of hydropriming and kinetinmediated hormopriming on germination parameters were screened in combination with conditions of short/prolonged priming and mild/severe overpriming. Biometric analyses highlighted contrasting responses in terms of germination performances and seedling development, while ROS (reactive xygen species) levels measured during dehydration positively correlate with the loss of desiccation tolerance in early seedlings, suggesting possible applications to monitor priming progression and predict overpriming occurrence.