Seed Dormancy Implications in Grain and Forage Production (1997)

Seed dormancy has been described for many species, and has promoted the curiosity of ecologists, who have recorded in detail the conditions and periods where seed dormancy is present. Physiologists have focused on understanding the mechanisms involved in seed dormancy induction and release. Agronomists have found seed dormancy to be problematic since it is related to the development of persistent weed seed banks and to the problems in predicting establishment of weed stands. If the crop seed has dormancy, its germination is restricted to a very narrow range of micro‐environmental conditions. In this paper, we discuss how seed dormancy and the knowledge of its nature could be used to improve agronomic practices for grain and forage production. Crops selected to have dormant seeds, or artificially generated dormant seeds, could be sown at any time. When seedling establishment is desired, seed dormancy level could be lowered to produce synchronic germination and seedling emergence by modifying the environment, (i.e., changing the light and thermal environment of the seeds) so that physiological changes could occur and trigger germination. Seed germination could be naturally induced by seasonal changes or artificially induced by sonication. Selecting crops with dormant seeds during ripening and storage could also prevent seeds from germinating in moist environments. Artificially induced seed dormancy could be achieved by spraying hormones or oils. With adequate knowledge about the seed dormancy characteristics of plant populations, environmental situations can be engineered to encourage release from seed dormancy, to promote or inhibit certain species in weed and range management. The Problem Seed dormancy has been viewed as problematic in grain and forage production for several reasons: (i) It is related to the development of persistent weed seed banks and failure in predicting establishment of weed stands. Both characteristics diminish the efficacy of weed control procedures; (ii) When agronomists domesticate wild species as crops, they find that if the crop seed has dormancy, its germination is restricted to a very narrow range of microenvironmental conditions. Instead, management of seed dormancy could be seen as a solution for many agronomic problems. Very few studies are focused on this idea. Literature Summary Seed dormancy refers to a physiological state in which seeds do not germinate under environmental conditions that would normally support germination of that species in the field. Physiologists have focused on understanding the mechanisms involved in seed dormancy induction and release. For example, in some summer species seed dormancy release is only attained if the seed population is subjected to some displacement within the soil profile, while others still require exposure to be stimulated with fluctuating temperatures to complete exit from dormancy. The thermal dependant seed dormancy mechanisms present in some weed species allow them to detect their spatial location. Similar thermal mechanisms are complemented by light sensitive mechanisms in other annual species. This requirement can be satisfied even after an exposure to light as brief as that resulting from soil tillage. Studies indicate that, under natural conditions, the photorreceptor phytochrome allows seeds of many grassland species subjected to grazing to detect the increased red/far red light ratio reaching the bottom of the canopy due to intense defoliation. On the other hand, agronomists have been searching for more conservationist cropping systems, since some methods currently used to grow crops have been associated with the deterioration of our soil and natural resource base. Applied Questions What cultural practices can be changed managing seed dormancy? Knowledge about different types of seed dormancy and their management can be used to modify both crop sowing and harvesting strategies. Range and weed management tactics could also be improved. What kind of improvements in sowing strategies can be attained? Seed bed preparation and sowing date are particularly critical when considering opportunistic, relay and intercropping systems. The goal in such systems is to have a population of ready‐to‐go seeds when an environmental window adequate for plant growth and establishment is available. Crops selected to have dormant seeds, or artificially generated dormant seeds, could be sown at any time. When seedling establishment is desired, seed dormancy level could be lowered to produce synchronic germination and seedling emergence by tilling, mowing, herbicide application, or grazing to change the light and thermal environment of the seeds. Germination could also be naturally induced by seasonal changes. How can seed dormancy be managed to reduce harvest and storage costs? Significant yield losses are incurred by preharvest sprouting and germination during storage. Selecting crops with dormant seeds during ripening and storage could prevent seeds from germinating in moist environments. Artificially induced seed dormancy could be achieved by spraying regulators of hormone synthesis or oils. What are the implications of seed dormancy for alternatives to traditional weed and range management? Both weed and range management practices are aimed at promoting desired species while reducing weeds and low quality forage species. With the adequate knowledge about the seed dormancy characteristics of plant populations, environmental situations can be engineered to encourage release from dormancy.