Protein Synthesis in Dormant and Non-Dormant Cocklebur Seed Segments

Using the axial and cotyledonary segments of lower cocklebur (Xanthium pensylvanicum Wallr.) seeds, protein synthesis as shown by incorporation of radioactive leucine was examined in relation to their dormant status. During the first 9 h of water imbibition, the protein synthesis was higher in the dormant axes than in the non-dormant, after- ripened ones. When imbibed for more than 12 h non-dormant axes had a higher activity than dormant ones. This was also the case with the cotyledonary segments. Cyctoheximide, an inhibitor of protein synthesis, blocked protein synthesis in the axial tissue regardless of its dormant status, and thereby inhibited germination of the non-dormant seeds. In the dormant seeds, however, cycloheximide at 3 mM slightly stimulated germination without stimulating the C₂H₄ production. Based on these results, it is suggested that in cocklebur seeds there may be some proteinaceous system which is involved in the maintenance of dormancy.     

Dormancy and impotency of cocklebur seeds III. CO2- and C2H4-dependent growth of the embryonic axis and cotyledon segments

Growth of segments of embryonic axes and cotyledons excisedfrom dormant or nondormant cocklebur (Xanthium pennsylvanicumWallr.) seeds and CO₂ and C₂H₄ production in these segmentswere examined in relation to the effects of temperature, CO₂and C₂H₄. Both the nondormant axes and cotyledons grew evenat low temperatures below 23°C, but the dormant ones failedto grow. There was only little difference in the CO₂ evolutionbetween the nondormant and dormant ones, but both the axis andcotyledon segments from the dormant seeds exhibited little orno C₂H₄ productivity, unlike the nondormant ones, at low temperatures.However, a high temperature of 33°C caused rapid extensiongrowth and C₂2H₄ production even in dormant axes and cotyledons. The inability of dormant axes and cotyledons to grow disappearedcompletely in the presence of C₂H₄ at fairly low concentrations.Removal of endogenous CO₂ and C₂H₄ reduced the growth in bothaxes and cotyledons, while exogenous CO₂ mainly enhaced axialgrowth although exogenous C₂H₄ strongly stimulated the growthof both organs. Regardless of the dormant status, however, maximumgrowth of these organs occurred when C₂H₄ was given togetherwith CO₂. We suggest that dormancy in cocklebur seeds is dueto the lack of growing ability in both organs, caused by thelack of C₂H₄ productivity in both dormant axes and cotyledons,particularly in the former. (Received December 2, 1974; )     

Quantification of GA1, GA3, GA4, GA7, GA8, GA9, GA19 and GA20; and GA20 metabolism in dormant and non-dormant beechnuts

The endogenous levels of GA₁, GA₃, GA₄, GA₇, GA₈, GA₉, GA₁₉ and GA₂₀ were determined in beech seeds (Fagus sylvatica L.) treated with different dormancy breaking treatments. Gibberellins were analysed separately in cotyledons and embryo axes. After purification of the extracts, GAs were quantified by GC-MS-selected ion monitoring (GC-MS-SIM) with deuterated GAs as internal standards. The results showed that GAs corresponding to the 13-OH pathway seemed to be involved in dormancy breaking. Strong differences in GA₁, GA₃, GA₈, GA₁₉ and GA₂₀ levels between embryo axes and cotyledons of dormant and non-dormant beechnuts were detected with less pronounced differences for GA₄, GA₇ and GA₉ levels. Both the quantitative differences between dormant and non-dormant seeds in the analysed GAs corresponding to the 13-OH pathway, and the capacity of non-dormant seeds to carry out metabolic conversions when labelled GA₂₀ was injected into the seeds, reveal a dynamic role of GAs in dormancy release.     

No contribution of the pentose phosphate pathway in dormancy-breaking of cocklebur seeds

The role of the oxidative pentose phosphate (PP) pathway in the dormancy-breaking of cocklebur (Xanthium pennsylvanicum Wallr.) seeds was investigated. D-[1-¹⁴C]-glucose or D-[6-¹⁴C]-glucose was fed to dormant and non-dormant lower seeds or to their axial or cotyledonary segments which were imbibed for different durations, and C₆/C₁ ratios of respired ¹⁴CO₂ as an index of the PP pathway activity were calculated. Contrary to expectation, there was no significant difference in the C₆/C₁ ratios between the dormant and non-dormant seeds or segments during a water imbition period of 24 h, although the PP pathway actually operated already in an early stage of water imbibition. Also concerning the activities of G6PDH and 6PGDH, the key enzymes of this pathway, no difference between the dormant and non-dormant seeds was found. It was thus concluded that, unlike other seeds, there is no contribution of the PP pathway to the regulation of dormancy of the cocklebur seed.     

Physical forces in dormancy and germination of xanthium seeds

The germination of seeds of Xanthium pensylvanicum Wallr. occurs in 2 phases, an initial passive phase of water uptake followed by an active phase of growth. These 2 phases have been separated experimentally, and shown to occur similarly in isolated cotyledons and embryonic axes. Measurements of the physical thrust generated by the entire seed and its separate components of cotyledon and axis reveal that non-dormant Xanthium seeds develop more than twice the thrust of dormant seeds, and that this difference develops principally in the second phase of enlargement of the axis. Measurement of the forces required for piercing the testa of these seeds establishes that whereas the thrust developed by non-dormant seed is adequate to cause testa rupture, that developed by dormant seeds is not. It is concluded that the dormancy of Xanthium involves an inadequacy in the embryo for rupture of the testa.     

Tolerance of kikuyu grass to long term salt stress is associated with induction of antioxidant defences

Primary dormancy in A. retroflexus seeds wascompletely broken by dry storage or ethylene treatment and partially removedwith GA₃. Norbornadiene counteracted the dormancy breaking action ofethylene and GA₃. The GA₃ effect was lowered bycobaltous ions. ABA increased the ethylene requirement in primary dormant seeds.Dormant seeds had a similar or different ability to produce ethylene and ACCoxidase in vivo activity than did non-dormant seeds,depending on the period of incubation. Dormant seeds contained less endogenousACC than non-dormant seeds. Thus, ethylene seems to play an essential role inthe release of primary dormancy in A. retroflexus seeds.Ethylene also participates in the release of dormancy achieved by GA₃treatment. The results indicate that both ethylene biosynthesis and action isinvolved in the control of primary dormancy in Amaranthusretroflexus seeds.     

Nitric oxide,hydrogen cyanide and ethylene are required in the control of germination and undisturbed development of young apple seedlings

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are emerging as important regulators of plant development (germination, flowering, senescence), acting as secondary messengers in cooperation with classical phytohormones. Apple seeds are dormant, unless they undergo a 3 month long cold stratification. Deep dormancy of isolated apple embryos can also be broken by short pre-treatment with HCN or NO with the effect associated with enhanced ethylene synthesis. Non-dormant embryos germinate well and young seedlings grown from non-dormant embryos do not exhibit any morphological anomalies, such as asymmetric growth and greening of cotyledons. One of the aims of this work was to investigate the correlation between RNS- mediated (HCN- and NO-dependent) dormancy removal and ROS (H₂O₂ and O₂^−•) accumulation in the embryos. The beneficial effect of NO and HCN on germination of dormant apple embryos has been associated with marked increases in H₂O₂ and O₂^−• concentration in the embryos at early germination stages. We also analyzed growth of young seedlings developed from embryos pre-treatment with HCN or NO or exposed to ethylene (ethephone) and its precursor 1-aminocyclopropane-1-carboxylic acid (ACC). ACC and ethephone removed all morphological anomalies of the seedlings (asymmetric growth and greening of cotyledons) but the radicle growth was rather slight. We propose that accumulation of ROS provoked by HCN and NO pre-treatment is required for embryo germination “sensu stricto”, while ethylene is required for post-germination seedling growth.     

Metabolic Changes in After-Ripening Seed of Prunus cerasus

Metabolic changes were studied in embryonic axes and cotyledons isolated from after-ripening seeds of Prunus cerasus. During the seventh week of after-ripening, a striking increase in the respiration rate at 25° of embryonic axes occurred along with a sharp change from the dormant to non-dormant state of the seed. On the basis of C-6/C-1 ratio determinations this change may be related to an increased activity of the pentose phosphate cycle.     

Growth,sugar accumulation,and dark respiration of suspension cell cultures derived from contrasting alfalfa cultivars

Fall dormancy results in decumbent, slow shoot growth of alfalfa (Medicago sativa L.) in autumn and reduced shoot regrowth rates after herbage removal in summer. Although fall dormancy is used to predict alfalfa adaptation, we possess a poor understanding of the biological mechanisms underlying fall dormancy. Our objective was to examine growth and carbohydrate metabolism of suspension cell cultures derived from contrasting alfalfa cultivars that genetically differed in fall dormancy. Suspension cells were grown in B5h media containing 2% sucrose. Cells derived from fall non-dormant plants accumulated sugars more rapidly after transfer to fresh media and to higher concentrations than did cells derived from fall dormant alfalfa cultivars. Dark respiration rates of cells derived from non-dormant plants were similar to those derived from fall dormant plants when growth was limited at low cell sugar concentrations. However, both cell growth and dark respiration rates increased in cells derived from non-dormant cultivars in response to greater cell sugar concentrations. High growth rates of cells derived from rapid growing, fall non-dormant alfalfa cultivars were associated with rapid sugar uptake and higher cell respiration rates when compared to cells derived from dormant alfalfa cultivars.     

Hormonal and environmental regulation of seed germination in flixweed (<Emphasis Type="Italic">Descurainia sophia</Emphasis>)

Flixweed is one of the most abundant weeds in North America and China, and causes a reduction in crop yields. Dormancy of flixweed seeds is deep at maturity and is maintained in soil for several months. To identify regulators of seed dormancy and germination of flixweed, the effect of environmental and hormonal signals were examined using dormant and non-dormant seeds. The level of dormancy was decreased during after-ripening and stratification, but long imbibition (over 5 days) at 4 °C in the dark resulted in the introduction of secondary dormancy. The strict requirement of duration of cold treatment for the break of dormancy may play a role in the seasonal regulation of germination. The germination of non-dormant flixweed seeds was critically regulated by red (R) and far-red (FR) light in a photoreversible manner. Sodium nitroprusside, a donor of nitric oxide (NO), promoted germination of half-dormant seeds, suggesting that NO reduced the level of seed dormancy. As has been shown in other related species, light elevated sensitivity to GA₄ in dark-imbibied flixweed seeds, but cold treatment did not affect GA₄-sensitivity unlike in Arabidopsis. Taken together, our results indicate that seed germination in flixweed and its close relative Arabidopsis is controlled by similar as well as distinct mechanisms in response to various endogenous and environmental signals.     

Control of seed dormancy in Nicotiana plumbaginifolia: post-imbibition abscisic acid synthesis imposes dormancy maintenance

The physiological characteristics of seed dormancy in Nicotiana plumbaginifolia Viv. are described. The level of seed dormancy is defined by the delay in seed germination (i.e the time required prior to germination) under favourable environmental conditions. A wild-type line shows a clear primary dormancy, which is suppressed by afterripening, whereas an abscisic acid (ABA)-deficient mutant shows a non-dormant phenotype. We have investigated the role of ABA and gibberellic acid (GA₃) in the control of dormancy maintenance or breakage during imbibition in suitable conditions. It was found that fluridone, a carotenoid biosynthesis inhibitor, is almost as efficient as GA₃ in breaking dormancy. Dry dormant seeds contained more ABA than dry afterripened seeds and, during early imbibition, there was an accumulation of ABA in dormant seeds, but not in afterripened seeds. In addition, fluridone and exogenous GA₃ inhibited the accumulation of ABA in imbibed dormant seeds. This reveals an important role for ABA synthesis in dormancy maintenance in imbibed seeds. Received: 31 December 1998 / Accepted: 9 July 1999     

The control of bud dormancy in potato tubers

Potato (Solanum tuberosum L.) tuber buds normally remain dormant through the growing season until several weeks after harvest. In the cultivar Majestic, this innate dormancy persisted for 9 to 12 weeks in storage at 10° C, but only 3 to 4 weeks when the tubers were stored at 2° C. At certain stages, supplying cytokinins to tubers with innately dormant buds induced sprout growth within 2 d. The growth rate was comparable to that of buds whose innate dormancy had been lost naturally. Cytokinin-treatment did not accelerate the rates of cell division and cell expansion in buds whose innate dormancy had already broken naturally. Gibberellic acid did not induce sprout growth in buds with innate dormancy. We conclude that cytokinins may well be the primary factor in the switch from innate dormancy to the non-dormant state in potato tuber buds, but probably do not control the subsequent sprout growth.Abbreviations tio ⁶ade 6-(4-hydroxy-3-methylbut-trans-2-enyl amino)purine, zeatin - tio⁶ado 6-(4-hydroxy-3-methylbut-trans-2-enyl amino)-9--D-ribofuranosyl purine, zeatin riboside     

Responsiveness of Amaranthus retroflexus seeds to ethephon, 1-aminocyclopropane 1-carboxylic acid and gibberellic acid in relation to temperature and dormancy

Dormant Amaranthus retroflexus seeds do not germinate in the dark at temperatures below 35°C. Fully dormant seeds germinate only at 35–40°C whereas non-dormant ones germinate within a wider range of temperatures (15 to 40°C). Germination of non-dormant seeds requires at least 10% oxygen, but the sensitivity of seeds to oxygen deprivation increases with increasing depth of dormancy. 10^–6 to 10^–4 M ethephon, 10^–3 M 1-aminocyclopropane 1-carboxylic acid (ACC) and 10^–3 M gibberellic acid (GA₃) break this dormancy. In the presence of 10^–3 M GA₃ dormant seeds are able to germinate in the same range of temperatures as non-dormant seeds. The stimulatory effect of GA₃ is less dependent on temperature than that of ethephon, while ACC stimulates germination only at relatively high temperatures (25–30°C). The results obtained are discussed in relation to the possible involvement of endogenous ethylene in the regulation of germination of A. retroflexus seeds.Abbreviations ACC 1-aminocyclopropane 1-carboxylic acid - GA₃ gibberellic acid - SD standard deviation     

The oxygen consumption of non-dormant and dormant larvae of Chironomus plumosus (Diptera)

The physiological expression of a short-day-induced dormancy (Oligopause) of 4th-instar Chironomus plumosus larvae was investigated by measuring the oxygen consumption. By means of a polarographic continuous-flow respirometer the oxygen consumption of single larvae or groups of larvae (up to 5) could be measured continuously over a period of several hours. The oxygen consumption of dormant larvae is lower than in non-dormant larvae: in relation to the individual it is reduced by about $\text{1}\text{3}$, in relation to 1 g of dry weight by about $\text{1}\text{2}$. Dormant larvae are heavier than non-dormant larvae. In the dormant larvae, the proportion of dry weight to total weight is higher than in the non-dormant larvae. The increased proportion of dry weight is composed of a size-specific (dormancy-independent) and a dormancy-specific component. Independent of the sex and dormancy state, the larger larvae have a lower oxygen consumption per weight unit than the smaller larvae. A sex-specific difference can be observed: when animals with identical oxygen consumption per weight unit are compared, the females are considerably heavier than the males.     

Glycolytic activity in embryos of Pisum sativum and of non-dormant or dormant seeds of Avena sativa L. expressed through activities of PFK and PPi-PFK

Summary A quantative cytochemical assay for PP_i-PFK activity in the presence of Fru-2,6-P₂ is described along with its application to determine levels of activity in embryos of Pisum sativum and Avena sativa. The activity of ATP-PFK has also been studied in parallel as have PFK activities during the switch from dormant to non-dormant embryos in Avena sativa. PP_i-PFK activity, has been demonstrated in all tissues of Pisum sativum embryos and of Avena sativa embryos including the scutellum and the aleurone layers. The PP_i-PFK activity was greater than that of ATP-PFK in both dormant and non-dormant seeds though with only marginally more activity in the dormant as opposed to the non-dormant state.Abbreviations AMP adenosine monophosphate - ATP adenosine triphosphate - Fru-1,6-P₂ fructose 1,6-bisphosphate - Fru-2,6-P₂ fructose 2,6-bisphosphate - Fru-6-P fructose 6-phosphate - FB Pase 2 fructose 2,6-bisphosphatase (EC 3.1.3.46) - Gl-3-PD glyceraldehyde-3-phosphate dehydrogenase - NAD nicotinamide adenine dinucleotide - NBT nitroblue tetrazolium - PEP phosphoenolpyruvate - PFK 6-phosphofructokinase (EC 2.7.1.11) - PFK₂ 6-phosphofructo-2-kinase (EC 2.7.1.105) - PP_i pyrophosphate - PP_i-PFK pyrophosphate: fructose 6-phosphate 1-phosphotransferase (EC 2.7.1.90) - PVA polyvinyl alcohol (G04/140 Wacke Chemical Company)     

The compartmentation of ethylene in developing cotyledons of Phaseolus vulgaris L.

Isolated cotyledons of Phaseolus vulgaris L. cv. Canadian Wonder accumulated ¹⁴C₂H₄ (0.7–1 l l^-1) from air to give partition coefficients of 1 to 4, which greatly exceeded the value obtained with steam killed cotyledons (0.05) and with water (0.11). After ¹⁴C₂H₄ treatment, 98% of the ¹⁴C in the tissue remained as ¹⁴C₂H₄. The labelled ethylene accumulated by cotyledons was released only slowly (1–10% h^-1) either in an air stream or into toluene. Heating to 60°C for 2 h, but not freezing and thawing, caused the immediate release of ¹⁴C₂H₄ from the tissue. Propylene and vinyl chloride competitively inhibited the accumulation of ¹⁴C₂H₄.Cotyledons emanated endogenous ethylene at a very low rate but after heating (although not freezing and thawing) 13 nl of ethylene per g fresh mass were released within minutes. It was concluded that french bean cotyledons hold ethylene in a compartmented form in sufficient amount to account for at least 200 h of emanation.Abbreviation PPO diphenyloxazole     

Differentially expressed genes associated with dormancy or germination of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> seeds

Differential display analysis using dormant and non-dormant Arabidopsis thaliana (L.) Heynh seeds resulted in a set of genes that were associated with either dormancy or germination. Expression of the germination-associated genes AtRPL36B and AtRPL27B, encoding two ribosomal proteins, was undetectable in the dry seed, low in dormant seed, and high under conditions that allowed completion of germination. Expression of these genes was also found to be light-regulated and to correlate with germination speed. Expression of the dormancy-associated genes ATS2 and ATS4, encoding a caleosin-like protein and a protein similar to a low-temperature-induced protein respectively, was high in the dry seed and decreased during germination. Expression of ATS2 and ATS4 was high in primary and secondary dormant seed but low in after-ripened or chilled seed. The expression of both genes was also light-regulated, but no relationship with temperature-dependent germination speed was found.     

Involvement of phytohormones in germination of dormant and non-dormant oat (Avena sativa L.) seeds

Oat seeds are susceptible to high temperature dormancy. Dormant grainsdo not germinate at 30 °C unless afterripened, dry, for severalweeks. Isolated embryos of dormant grains do germinate, especially ifGA₃ is added to the germination medium. ABA inhibits germinationproportionally to the concentration applied and GA₃ can overcome theABA inhibitory effect. Measurements of endogenous ABA and several GAs revealedthat the initial levels of ABA in dormant and non-dormant grains were quitesimilar. But, endogenous ABA in non-dormant seeds almost disappeared within thefirst 16 h of imbibition, while the amount in dormant grains haddecreased by less than 24%. The level of GA₁₉ in non-dormant seedswas higher, and GA₁₉ appears to be converted to GA₂₀ within the first 16h. The GA₂₀ was converted to GA₁ at leastduring the first 48 h of the germination process. Bothphytohormones thus appear to be involved in the germination process ofnon-dormant seeds. ABA first declines, while GA₁ is producedduring the first 16 h of imbibition to allow proper germination.Indormant grains the level of ABA remained high enough to prevent germinationduring at least a week and precursor GAs were not converted to GA₁.