Investigating Seed Dormancy In Switchgrass (Panicum virgatum L.)

Switchgrass (Panicum virgatum L.) is a perennial warm-season grass native to North America established through seed. As in many other perennial grasses, freshly harvested seeds exhibit dormancy, resulting in delayed and sporadic germination and emergence, jeopardizing the stand establishment success. Dormancy can be imposed by morphological, physical, and physiological properties of the seed, or by a combination of them. Dormancy in switchgrass was alleviated by mechanical or chemical scarification and by stratification, suggesting physical and/or physiological dormancy. Furthermore, success of the treatments depend on the age of the seeds, after-ripening period, cultivar, temperature, cultural practices, storage conditions, seed weight, and seed size, among others. The causes, mechanisms, and physiology of switchgrass dormancy are not well understood. Understanding these factors will provide important and valuable information for future breeding programs focused on the development of low-dormant cultivars, and for the improvement of dormancy-breaking treatments. We tested the hormonal response of dormant switchgrass seeds to inhibitory and stimulatory temperatures, and investigated the contribution of the lemma, palea, embryo, and endosperm to dormancy in switchgrass seeds with high and low dormancy. Glumes, lemma, palea, and endosperm contribute to the dormancy in the seed lots analyzed. Germination was 50% higher in seeds in which glumes, lemma, and palea were removed. As the imbibition time prior to removal of covering layers was extended from 12 to 24 hours, removal of the palea did not improve the germination percentage. Removal of the endosperm resulted in faster germination and higher germination percentage. In excised embryos (no glumes, lemma, palea, endosperm) germination was 100% suggesting a lack of morphological dormancy, characterized by underdeveloped embryos, but rather a “physiological dormancy”. In a four year-old seed lot, with low dormancy, the germination percentage was not affected by the removal of the different layers. However, intact seeds germinated slower than seeds with covering removed. Germination was independently affected by temperature, abscisic acid, and gibberellins. Sensitivity of the seed to different levels of ABA and GA was affected by temperature, translated into a significant effect of the temperature x hormone interaction.