Phytohormones and reactive oxygen species mediate processes such as germination and dormancy. The elucidation of the physiological and biochemical events implicated in the transition from dormancy to germination in different plant species such as sunflower becomes a topic of interest. In this study, we investigated the spatiotemporal variation of salicylic acid (SA), hydrogen peroxide (H 2 O 2 ) and the activity of two antioxidant enzymes (catalase, CAT -EC 1.11.1.6 and ascorbate peroxidase -EC 1.11.1.11) in embryonic axis and cotyledons of dry and imbibed seeds of dormant (B123) and non-dormant (B91) sunflower lines. The results showed that embryonic axis had higher level of SA and H 2 O 2 than cotyledons in both lines. In dry seeds, B123 embryo (embryonic axis + cotyledons) showed a higher SA content than B91. After dry storage at room temperature, SA decreased in B123 embryos to a value close to that registered in B91 embryos. B123 embryonic axis of dry seeds presented higher H 2 O 2 levels than B91. Dry storage led to an increase of H 2 O 2 levels and a decrease of CAT activity in B123 embryonic axis. During imbibition, B123 seeds stored for 33 days displayed an increase in SA level in the embryonic axis (3 h of imbibition) and this lower level correlated with a decrease in H 2 O 2 (6 h of imbibition). Thus, the embryo-imposed dormancy in B123 dry seeds was associated with high levels of SA and low H 2 O 2 , whereas the dormancy release was linked with SA decrease and increase of H 2 O 2 as a consequence of lower CAT activity.

The seed coat is a multifunctional tissue that plays an important role in dormancy and germination of sun ower seeds. In addition, abscisic acid (ABA) and gibberellins (GAs) are the primary phytohormones that antagonistically regulate seed dormancy, although other phytohormones such as auxins (AUX) and Jasmonates (JAs) also play a critical role in the modulation of these processes. Here, we investigated the role of the seed coat in imposition and release of sun ower seed dormancy. We evaluated the endogenous level of ABA, jasmonic acid (JA) and indol-3-acetic acid (IAA) in dry and imbibed seed coats of two inbred Helianthus annuus lines (B123 and B91), and analysed the seed coat anatomy of their dry cypselas. The results showed that the dormancy of B123 seeds is associated with a low content of ABA, JA and IAA in seed coat, and a high content of these phytohormones in the embryo. A dry storage period increased ABA and JA contents in the seed coat, although it decreased them in the embryo. Both the embryo and seed coat showed a reduction in IAA after the storage period. No relevant differences were registered in the histological analysis of the seed coat. Altogether, our results remark the relevance of the interaction between the seed coat and the embryo in the release of dormancy in sun ower seeds.

Sunflower (Helianthus annuus L.) seed dormancy at harvest is an undesirable characteristic regarding productive systems. From a commercial point of view, the dormancy generates production and marketing problem with a negative impact on crop yields. To overcome this problem, we studied different treatments, including the application of growth regulators. In addition, as the interaction of various hormones in different sunflower fruit parts is not fully understood regarding regulation of dormancy and germination processes, we also (1) characterized the dormancy level of sunflower seeds of Xi3 and X274 inbred lines; (2) assessed the effect of exogenous growth regulators on dormancy release; (3) evaluated the endogenous content of abscisic acid (ABA), gibberellins (GAs), 12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), salicylic acid (SA), auxins, and cytokinins (CKs) in embryonic axis, cotyledons, pericarp, and seed coat-endosperm; and (4) analyzed the kinetics of the accumulation of these phytohormones in Xi3 embryonic axis during early imbibition (3 to 12 h) of cypselas treated, or not, with growth regulators effective in the dormancy release. We were able to classify the two sunflower lines studied according to dormancy level: The Xi3 line was dormant and X274 line was non-dormant at harvest. The results showed that the dormancy of Xi3 seeds is associated with a high level of ABA, OPDA, and JA, while germination of X274 dry seeds at harvest time is related to high levels of GA 1 . Exogenous treatment with GA 3 and Ethephon overcame Xi3 seed dormancy through the changes in endogenous hormonal profiles of the embryonic axis. The exogenous application of GA 3 induced JA, SA, and CKs accumulation and to modify the ABA/GA 1 ratio, whereas Ethephon treatments mediated

The seed coat is a multifunctional tissue that plays an important role in dormancy and germination of sunflower seeds. In addition, abscisic acid (ABA) and gibberellins (GAs) are the primary phytohormones that antagonistically regulate seed dormancy, although other phytohormones such as auxins (AUX) and Jasmonates (JAs) also play a critical role in the modulation of these processes. Here, we investigated the role of the seed coat in imposition and release of sunflower seed dormancy. We evaluated the endogenous level of ABA, jasmonic acid (JA) and indol-3-acetic acid (IAA) in dry and imbibed seed coats of two inbred Helianthus annuus lines (B123 and B91), and analysed the seed coat anatomy of their dry cypselas. The results showed that the dormancy of B123 seeds is associated with a low content of ABA, JA and IAA in seed coat, and a high content of these phytohormones in the embryo. A dry storage period increased ABA and JA contents in the seed coat, although it decreased them in the embryo. Both the embryo and seed coat showed a reduction in IAA after the storage period. No relevant differences were registered in the histological analysis of the seed coat. Altogether, our results remark the relevance of the interaction between the seed coat and the embryo in the release of dormancy in sunflower seeds.