The interplay of specific hormonal profile in fruit parts of sunflower inbred lines with contrasting dormancy levels during germination and dormancy breaking by exogenous application of plant growth regulators

Abstract

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 GA1. Exogenous treatment with GA3 and Ethephon overcame Xi3 seed dormancy through the changes in endogenous hormonal profiles of the embryonic axis. The exogenous application of GA3 induced JA, SA, and CKs accumulation and to modify the ABA/GA1 ratio, whereas Ethephon treatments mediated the ABA/GA1 ratio with the consequent Xi3 seed germination. Thus, the interplay of different phytohormones during early imbibition time seems to be the responsible for breaking sunflower seed dormancy.