Abstract
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 kinetin-mediated 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 oxygen 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.
Highlights
Seed priming techniques are routinely employed to improve germination performances and stress tolerance of commercial seed lots [1,2]
Given the critical importance of dehydration phases in seed priming protocols and the risks connected to the loss of desiccation tolerance on seedling establishment, this work focused on the identification of stress signatures during dry-back
Based on the dehydration curves (Figure 2), the endpoint of dry-back was established at 4 h (DB4h) for 2 h-primed seeds (HP2h, KP2h), and at 6h for prolonged priming/overpriming conditions (HP24h, HOP1mm, HOP2mm, KP24h, KOP1mm, KOP2mm), when the relative water content (RWC) of DS was restored
Summary
Seed priming techniques are routinely employed to improve germination performances and stress tolerance of commercial seed lots [1,2]. For priming protocols, orthodox seeds can withstand significant desiccation events through their endowment in osmo-protective proteins and sugars stabilizing cellular components, while antioxidant compounds protect lipids, proteins and nucleic acids from the oxidative stress associated with dehydration [3,6,7,8,9,10,11,12,13]. The sum of these properties, known as ‘desiccation tolerance’, is acquired during seed maturation and progressively lost as prolonged imbibition and radicle development lead to the depletion of osmo-protectants [14]. The use of multiple biometrical indicators is useful to dissect the effectiveness of seed priming [17], highlighting differential effects on post-priming viability, germination synchronization and seedling development [18,19]
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