Physiology of Oil Seeds

To further elucidate the regulation of dormancy release, we followed the natural afterripening of Virginia-type peanut (Arachis hypogaea L.) seeds from about the 5th to 40th week after harvest. Seeds were kept at low temperature (3 ± 2 C) until just prior to testing for germination, ethylene production, and internal ethylene concentration. Germination tended to fluctuate but did not increase significantly during the first 30 weeks; internal ethylene concentrations and ethylene production remained comparatively low during this time. When the seeds were placed at room temperature during the 30th to 40th weeks after harvest, there was a large increase in germination, 49% and 47% for apical and basal seeds, respectively. The data confirm our previous suggestion that production rates of 2.0 to 3.0 nanoliters per gram fresh weight per hour are necessary to provide internal ethylene concentrations at activation levels which cause a substantial increase of germination. Activation levels internally must be more than 0.4 microliter per liter and 0.9 microliter per liter for some apical and basal seeds, respectively, since dormant-imbibed seeds containing these concentrations did not germinate. Abscisic acid inhibited germination and ethylene production of afterripened seeds. Kinetin reversed the effects of ABA and this was correlated with its ability to stimulate ethylene production by the seeds. Ethylene also reversed the effects of abscisic acid. Carbon dioxide did not compete with ethylene action in this system. The data indicate that ethylene and an inhibitor, possibly abscisic acid, interact to control dormant peanut seed germination. The inability of CO2 to inhibit competitively the action of ethylene on dormancy release, as it does other ethylene effects, suggests that the primary site of action of ethylene in peanut seeds is different from the site for other plant responses to ethylene. We have studied the regulation of dormancy release in Virginia-type NC-13 peanut seeds (8, 9). Seeds that germinate produce ethylene prior to hypocotyl-radicle emergence. All I Cooperative Investigations of the Plant Science Research Division, Agricultural Research Service, United States Department of Agriculture and the Texas Agricultural Experiment Station, Texas A&M University. 2 Mention of a trademark name or a proprietory product does not constitute endorsement by the United States Department of Agriculture or Texas A&M University and does not imply its approval to the exclusion of other products that also may be suitable. treatments of dormant seeds which promote germination also enhance ethylene production within 24 hr of the start of treatment. This enhancement of ethylene production is substantially above the low levels that naturally occur in dry or dormantimbibed seeds. All of the data are compatible with our hypothesis that ethylene produced by the seeds promotes germination. Although we recognize that this hypothesis is not absolutely proven at this time, we do not think it is only a coincidence that application of exogenous ethylene breaks dormancy of peanut seeds and other seeds as well. The purposes of the present paper are 3-fold. First, we present additional evidence bearing on the question of whether endogenous ethylene produced by peanut seeds escaping dormancy has a regulatory role or its presence is merely an indication that the seeds are no longer dormant and are, in fact, germinating. Secondly, we present data showing that exogenous ABA mimics natural dormancy and its action is competitively reversed by cytokinins, and most important, ethylene. Competitive action of ethylene with a natural growth inhibitor has not previously been reported. The interaction between substances that have opposing effects on plant growth has been found to be one means of dormancy regulation in both seeds and buds (1, 3, 6, 10, 1 1). Thus, our data indicate that such a promoter-inhibitor system may regulate germination of dormant peanut seeds. Our third purpose is to present results of an extensive study of ethylene physiology of afterripening peanut seeds with the view that additional data on this stage of development will be of general value to the understanding of afterripening of peanut seeds and perhaps other seeds as well. MATERIALS AND METHODS The basis for the experiments reported here were described in detail in previous communications (7-9). Briefly, dormancy of Virginia-type NC-13 peanut (Arachis hypogaea L.) seeds is a natural phenomenon which varies in intensity (degree or depth of dormancy) from year to year. Immediately after harvest and curing (drying of seeds to a safe storage moisture level of about 6%), afterripening begins. At 3 C, the storage temperature we use, afterripening will not be complete (90% germination) for several months (8, Fig. 5). But each lot of seeds is quantitatively different in depth of dormancy to some degree from previous lots, and dormancy is constantly changing as afterripening progresses. Thus, from a practical standpoint, it is impossible to do all of the desirable experiments with seeds that have the same quantitative degree of dormancy. Further, detailed dose-response curves have quantitative value only for a given seed lot at a given stage of afterripening. Within the limitations of such a system, we have conducted a sequential study on an annual basis in which we have built each series