The inhibiting effect of light on germination of Phacelia tanacetifolia seed is overcome by removing the tip of the endosperm; however, immersion in solutions of high osmotic pressure reinstates the light sensitivity. Inhibition of germination by high temperature behaves similarly. Dormancy is ascribed to balance between mechanical constraint by the endosperm and "expansive force" of the embryo.
Measurement of Oxygen Uptake. The uptake of 02 was measured by means of a Gilson differential respirometer. Sterile precautions were taken as described previously (4). Each flask contained 10 seeds. Two flasks were included in each treatment.Incorporation of 1'C-Leucine into Protein. Lots of 10 seeds were husked, pricked at the dorsal side of the endosperm (a measure which facilitates entry of the applied compounds without affecting germination), sterilized in 1 % NaOCI for 2 min, washed, placed in a 25-mi Erlenmeyer flask with 2 ml of water, shaken at 21 -:-1 C for a period of time, then transferred to 1 ml of 1 mM leucine with 2.5 ,uc of uniformly labeled '4C-leucine (specific radioactivity 290 mc/mmole, New England Nuclear Corp.), and further shaken for 2 hr. The seeds were then washed in ice-cold, nonradioactive leucine solution, homogenized, extracted in 4 ml of 0.2 M NaCl plus 0.1 mm leucine, and centrifuged at 12,000g for 15 min. The pellet was suspended in 4 ml of the salt solution and recentrifuged. One twentieth of the pooled 12,000g supernatant fraction was precipitated with an equal volume of 15% trichloroacetic acid, and the precipitate (soluble protein) was collected on a Millipore filter. The filter was dried and the radioactivity was determined in a Nuclear Chicago gas flow detector with a counting efficiency of 40% for carbon-14. The pellet was suspended in water and precipitated in the same way as above. Approximately 80% of the radioactivity associated with the acid-insoluble material could be released by pronase, a potent proteolytic enzyme of bacterial source. Thus, this material is considered as insoluble protein.Microautoradiography of Seeds Exposed to 3H-Leucine. To locate the sites of protein synthesis, the seeds were exposed to L-leucine-4, 5-3H (specific radioactivity 6.0 c/mmole, Schwarz BioResearch), then washed in nonradioactive leucine, fixed, dehydrated and infiltrated with paraffin according to the technique of Feder and O'Brien (7). Sections 5 to 8 ,u thick were placed on a slide, dewaxed, coated with Kodak nuclear track emulsion type NTB 3, exposed, stained with toluidine blue-acid fuchsin, mounted, and examined under the microscope (10). RESULTSRespiratory Intensity. The rate of respiration (02 uptake) in dormant seeds was low as compared to that of nondormant seeds; the difference could be detected from the early hours of soaking (Fig. 1). In both kinds of seeds, the rate increased with time. However
Gibbereilic acid is known to break dormancy of several types of seeds: (a) light-promoted seeds, such as Grand Rapids lettuce seed (Lactuca sativa L. var. Grand Rapids); (b) lightinhibited seeds, such as the seed of the honey-bee plant (Phacelia tanacetifolia Benth; (c) seeds requiring stratification (storage at low temperatures in a moist condition), such as the hazel nut (Corylus avellana L.); (d) seeds requiring after-ripening (storage at room temperature in dry condition), such as the wild oat (Avena fatua L.).Recent findings indicate that when dormant hazel nut seeds are soaked at 5 C for 28 days, which is a natural means of breaking dormancy, the gibberellin-synthesizing mechanism is activated and that actual synthesis of gibberellins takes place when the seeds are transferred to a suitably higher temperature. Accumulation of GA results in germination (10).It has also been suggested that in the phytochrome-controlled germination of Grand Rapids lettuce seeds, the role of Pfr is to activate GA biosynthesis (8).One biochemical reaction known to be enhanced by GA is the synthesis of hydrolases (especially a-amylase) in the endosperm of cereal grains, such as barley (2,9,13). Since starch is a major food reserve in the cereal grains, its breakdown is generally assumed to be an essential process of germination. Many writers have implied that GA stimulates seed germination via amylase synthesis (1,7,12).Drennan and Berrie (5) compared the time course of amylase development in dormant and nondormant wild oats (Avena spp.). Their results indicated that both types of seeds contained traces of amylases. When dormant seeds were soaked, there was neither germination nor a change in the level of amylases. When nondormant seeds were soaked (in water), they germinated in 4 days and from the 6th day, amylase activity increased. Thus, they concluded that buildup of amylases is a postgermination phenomenon, and a lack of amylase synthesis must be eliminated as a possible cause of dormancy.We have investigated the time course relationship of GAinduced germination and amylase development in an attempt to find out if GA-induced germination is mediated by amylase production.
Abstract. Non-dormant and dormant seeds of Avena fatua metabolize 14IC-maltose in different ways: in non-dormant seeds, 14C-maltose administered to the endosperm is readily converted to sucrose in the scut6llum and translocated to the embryo; in dormant seeds, little sucrose is synthesized from 14C-maltose, and maltose and glucose tend to accumulate in the endosperm. It is suggested that biosynthesis of sucrose is essential for effective transport of the endosperm reserve to the embryonic axis in germinating seeds.
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