Dormancy in Green Needlegrass Seed :Its Nature, Mode of Action, and Methods of Reproduction

Green needlegrass (Stipa viridula Trin.) in the past few years has received wide acceptance for use in range renovation throughout the Northern Great Plains. Maximum germination of newly harvested green needlegrass seed cannot be obtained due to a high incidence of seed dormancy. This study was undertaken to determine the nature of dormancy present in green needlegrass seeds, to study the mode of expression of factors that causes dormancy in the seeds, and to seek a method which will reduce the dormancy in order to obtain maximum germination readings in the laboratory and in field plantings. Seed dormancy was found to be the main cause of low germination of green needlegrass, both in the laboratory and in the field. Dormancy was present in the seeds of green needlegrass each year this study was conducted. Maturity studies conducted indicate dormancy is present in green needlegrass seeds in the immature stage of maturity as well as in the fully mature stage. Two types of dormancy were found to be present in green needlegrass: an external restriction and a chemical block within the seed. The special treatment for germination of green needlegrass seed prescribed by the Subcommittee on range Grasses of the Association of Official Seed Analysts did not entirely break this dormancy. Limited germination increases were obtained by removing the glumes, puncturing seed coats, and clipping the tip of the caryopsis, and also by mechanical and acid scarification

Temperature preconditioning of ryegrass (Lolium sp.) seed dormancy

The purposes of this study were to determine the effects of temperature, nutrients, growth regulators, and culm detachment during seed development on ryegrass seed dormancy and weight. The effects of storage temperature on seed dormancy were also studied. Seed dormancy of annual (Lolium multiflorum Lam.) and perennial (Lolium perenne L.) ryegrass was evaluated by observing germination in the dark at 30, 25, 20, 15, and 15-25C. Dormant ryegrass seeds failed to germinate at 30 and 25C, but as after-ripening occurred seeds became insensitive to germination temperature. Field-grown ryegrass varieties were found to differ in degree of seed dormancy when grown under the same environment. 'Gulf', 'Florida Rust Resistant', and 'Magnolia' annual and 'NK- 100', 'Manhattan', 'Atempo', 'Petra', and 'Pelo' perennial ryegrass varieties were considered dormant. 'Oobahikari' annual and 'Verna Pajbjerg' and 'Linn' perennial ryegrass varieties were nearly nondormant. Dormancy patterns of greenhouse-grown Manhattan perennial ryegrass seed differed from those of field grown Gulf annual ryegrass. Dormancy of Manhattan was reduced when seeds reached maximum dry weight; whereas, Gulf seeds were dormant at all stages of maturity. A detached culm technique was used in growth chamber and greenhouse studies to determine the effects of nutrients, growth regulators and temperature on seed weight and dormancy. The dormancy response of Gulf seed produced on detached culms was similar to that of seeds from intact plants; lending validity to the use of the detached culm technique in studying seed dormancy. Development of Gulf seed in solutions deficient in nitrogen, phosphorus, and potassium did not significantly reduce dormancy. Phosphorus deficiency was more detrimental to seed weight than deficiencies of nitrogen or potassium. Production of Gulf seed on detached culms in gibberellic acid, benzyladenine and sucrose reduced dormancy. Seed developed in sucrose on culms cut at the soil surface produced the largest seeds. However, these seeds were not equal in size to seeds from intact culms. Gibberellic acid had no effect on seed size, while benzyladenine solution significantly reduced seed weight. Ryegrass seed dormancy and weight were affected by temperature during seed development. Gulf seeds developed at low temperature were dormant; whereas, seeds developed at high temperature were nondormant. The duration of exposure to different temperatures and the stage of development at which the seeds were exposed to high or low temperature also influenced the degree of dormancy. Exposure to one week of low temperature during the ripening stage increased seed dormancy, while the same duration of exposure to high temperature immediately after anthesis reduced seed dormancy. Extended periods of low temperature during seed development increased seed weight, while seed weight was decreased if low temperature preconditioning was delayed until later stages of development. The greatest reduction in seed weight occurred when seeds were exposed to high temperatures during the second week of seed development. In storage studies with seven ryegrass varieties, dormancy was quickly overcome at storage temperatures of 30 and 20C, but storage at 5 and -18C increased dormancy