Caracterização da perda e reindução da tolerância à dessecação em sementes de girassol

Abstract

An obstacle faced in the study of recalcitrance is the fact that the seeds included in this group lose viability within a short period. However, during germination, orthodox seeds become desiccation intolerant. Although, they have been used as a model in studies that try to elucidate the alterations associated with desiccation sensitivity. Thus, this research aimed to characterize, through physiological and biochemical evaluations, the sunflower seeds behavior related to the loss of desiccation tolerance during the germination process and the reinduction of desiccation tolerance, for a better understanding of the desiccation sensitivity. Different imbibition times were chosen to evaluate the desiccation tolerance loss during the germination process, based on the seeds imbibition curve of the Hélio 251 sunflower hybrid, being: control, 0, 6, 12 and 20 hours of imbibition, 1 mm and 3 mm of radicle protrusion. After imbibition, the seeds were submitted to dehydration in silica gel, except the control treatment that was not submitted to imbibition and dehydration. In order to try to reinduce the desiccation tolerance, seeds with 1 and 3 mm of radicle were submitted to osmotic treatment with polyethylene glycol (PEG) in the following concentrations: PEG -1,4 MPa, PEG -1,8 MPa, and PEG -2,2 MPa. A treatment was only submitted to dehydration in silica gel, taken as a control of the desiccation tolerance reinduction. Seeds of these treatments were submitted to physiological evaluations by germination tests, first germination count, seedling emergence test, emergence speed index, and electrical conductivity test. It was also evaluated the abundance of heat-stable proteins and the expression of isoenzymes: superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), esterase (EST), isocitrate lyase (ICL), and glutamate dehydrogenase (GDH), through electrophoresis. Sunflower seeds lost the desiccation tolerance after 1 mm of radicle protrusion. The treatments of desiccation tolerance reinduction, with PEG applied to the sunflower seeds with 1 mm radicle, were able to reestablish germination in low percentages, especially for the treatment with PEG -1.8 MPa. For the seeds with 3 mm radicle protrusion, the treatments of desiccation tolerance reinduction were not efficient for germination re-establishment. There is reduction in the abundance of heat-stable proteins and in the expression of the CAT and POX isoenzymes with the loss of desiccation tolerance in sunflower seeds. In addition, there was a reduction in the esterase isoenzyme expression, related to the low physiological quality of seeds with the loss of desiccation tolerance. The expressions of the reserves mobilization isoenzymes, ICL and GDH were not affected by the loss of desiccation tolerance. The treatments of desiccation tolerance reinduction did not change the expressionof the evaluated isoenzymes (SOD, CAT, POX, EST, ICL, and GDH).