No abstract

Determinations of the salt sensitivity of enzymes extracted from the halophilic alga Dunaliella viridis revealed that pentose phosphate isomerase, ribulose diphosphate carboxylase, glucose-6-phosphate dehydrogenase, and phosphohexose isomerase were inhibited by NaCl concentrations far lower than that in the growth medium (3.75 M). The inhibition was reversible and was not prevented by preparing the extracts in the presence of salt. Potassium, lithium, and cesium chlorides were equally inhibitory. In contrast, whole cells require rather high levels of NaCl for optimal growth, whereas growth is inhibited by low levels of the other cations. The results suggest a specific mechanism for the exclusion of sodium from the interior of the cell. The basis of the salt tolerance of the halophilic bacteria appears to be a distinctive form of enzymatic protein which is resistant to inactivation by high levels of salt and, in fact, requires substantial concentrations of salt for full enzymatic activity (6). The halophilic species of the green alga Dunaliella grow in concentrated salt solutions, but the basis of this salt tolerance is unknown. The present report describes the effects of sodium chloride and other salts on the growth of Dunaliella viridis and on the activity of several enzymes from this halophilic species.

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...

The effect of arsenate, arsenite, 2, 4-dinitrophenol, and anaerobiosis on early events in seed germination was investigated using both intact and punched seeds of lettuce (Lactuca sativa L.). It was found that punching the seed removes penetration barriers to the entrance of inhibitors without an undue loss of germination or light responses. The kinetics of the action of germination inhibitors were established by 2-hour pulse experiments. Arsenate and 2 , 4-dinitrophenol have very different kinetics. The inhibition of germination in punched seeds by arsenate given in conjunction with phosphate compared with the lack of inhibition of arsenate plus phosphate on the growing seedling, suggest a distinct metabolic change in the germinating embryo at some time between the onset of germination and subsequent seedling growth.

Lettuce seeds (Lactuca sativa var. Grand Rapids) were found to contain inhibitory substances, one of which is probably abscisic acid. Extracts from seeds were characterized by gas–liquid chromatography, and peaks coincident with abscisic acid were found.The germination water surrounding seeds made secondarily dormant was subjected to gas–liquid chromatography and was also found to contain peaks coincident with abscisic acid. It was also determined that the inhibitory substances are localized in the embryo but not in the endosperm or seed coat.