Effects of Germination-promoting Substances Given in Conjunction with Red Light on the Phytochrome-mediated Germination of Dormant Lettuce Seeds (Lactuca sativa L.)

Abstract

Ethylene or thiourea can substitute for gibberellic acid but not for red light in breaking the secondary dormancy induced by extended dark storage of fully hydrated lettuce seeds (Lactuca sativa cv. Grand Rapids). After 10 days of storage, ethylene, thiourea, or gibberellic acid applied either separately or in any combination in conjunction with red light induced near maximal germination. When applied separately without red light, none of the substances promoted germination of seeds stored 10 days. Combinations of any two or all three of the substances in the absence of red light induced some germination but no combination was as effective as any single substance given with red light. two discs of Whatman No. 1 filter paper and 1.5 ml of liquid. Seeds were Lactuca sativa cv. Grand Rapids, 1970 harvest, purchased from Buckerfields Ltd., Vancouver, B.C., Canada. These seeds were stored at -20 C and were highly light-sensitive.Light treatments, when given, were 5 min of red (R, A 645 nm, one-half band width 50 nm, 4.7 X 10 ergs cm sec-) or far red (FR, X >700 nm, 4.4 X 103 ergs cm-2 sec1) according to the procedures of Speer (5). All seeds received FR irradiation 0.5 hr after the onset of imbibition in order to suppress initial germination. Seeds were irradiated in some cases with R either immediately after FR or after some multiple of 48 hr following the onset of imbibition.Ethylene was used at concentrations of 5 to 50 ftl 1, achieved by placing Petri dishes containing the seeds, filter papers, and distilled water or growth promoters in glass desiccators. The desiccators were then evacuated, at which time the appropriate volume of ethylene was injected into the tubing leading to the desiccator stopcock. which was then opened to the atmosphere.This procedure was to ensure infiltration of the Petri dishes with the ethylene-air mixture. Burdett and Vidaver (1) have shown this range of ethylene concentration to be optimal for the promotion of germination at 20 C of seeds of this type, which is confirmed by data in Table I. Where added, GA:, (Sigma Chemicals, St. Louis, Mo.) concentration was 0.5 mm and thiourea was 50 mm. both of which were optimal for pronmotion of germination under the conditions wLVed: addition of GA, or thiourea following DS was done by transfering seeds to new dishes containing these substances.Time intervals from the beginning of imbibition (day 0) until final treatments with any, all, or none of the growth promoters. with or without R irradiation, are referred to as dark storage. After a DS period, any seeds which germinated were discarded and subsequent germination percentages are those of the remaining seeds. Very few seeds germinated after the first 48 hr of DS unless given treatment in addition to FR (Table II). The 48-hr post-treatment interval, at the end of which germination was scored, is called the germination test.All operations, except where noted above, and the final counting of germinations were carried oLut either in darkness or under dim green safelights known to be nonp...

Induction of Secondary Dormancy in Chenopodium bonus-henricus L. Seeds by Osmotic and High Temperature Treatments and Its Prevention by Light and Growth Regulators

“…5 (29). However, as KIN + ETH + GA4+7 is more active than GA4+7 in light or in darkness (Table III), the effect of KIN + ETH component might be separated from those of GA4+7 and/or Pfr.…”

“…Previous studies have shown that secondary dormancy is imposed in seeds of lettuce and several weed species by prolonged dark imbibition in water (16,29,32,35), in an osmoticum (16,15), at supraoptimal temperatures (12,13), in anaerobic conditions (8) or in the soil (8,31,36). In several of these studies exposure to light of various durations or intensities broke the secondary dormancy.…”

“…However, as KIN + ETH + GA4+7 is more active than GA4+7 in light or in darkness (Table III), the effect of KIN + ETH component might be separated from those of GA4+7 and/or Pfr. Because the action of KIN and/or ETH and not of GA or light is related more directly to stress and/or thermodormancy alleviation (4,19,29), the use of KIN + ETH along with GA4+7 seems to allow the removal not only of the secondary dormancy but also the removal of adverse cellular changes. Probably a prolonged dark imbibition induces certain changes, more rapidly at 29 C than at 15 C in the PEG solution, which in turn diminishes the Pfr and/or GA4+7 effects but not, or more slowly, the effect of KIN + ETH + GA4+7 (Table III) Fig.…”

“…Although a number of studies have been conducted on the breaking of secondary dormancy in seeds by light and growth regulators under favorable and unfavorable germination conditions (9,19,22,29,33), we are not aware of any comparative study on the effects of irradiation and growth regulators to prevent the induction of secondary dormancy.…”

“…These seeds now failed to germinate under the condition (darkness) which previously allowed their germination. Continuous Ught or daily brief red Ught irradiations during prolonged imbibition in polyethylene glycol solution at 15 C or in water at 29 C prevented the establishment of the secondary dormancy and caused an advancement of subsequent germination. Far red irradiations inmediately following red irradiation reestabUshed the secondary dormancy indicating phytochrome participation in "pregerminative" processes.…”