Factors affecting the induction and release of secondary dormancy in Kalanchoë seeds

Abstract. Several short daily R irradiations are required from the first day of incubation on water to induce germination of Kalanchoë seeds. When the same light treatment is given after a prolonged dark incubation period at 20°C, secondary dormancy prevents germination. Factors controlling the induction and breaking of secondary dormancy have been investigated. The induction of secondary dormancy is very temperature dependent. Locally puncturing the seed coat strongly delays it. Secondary dormancy is not induced in the presence of GA₃ during the first 10 d of dark incubation, although this growth substance cannot induce dark germination. Prolonged or cyclic daily R irradiations can relieve secondary dormancy of seeds kept on water, even after a dark period of 20 d. A 24 h treatment at 4°C restores responsiveness to short R exposures of slightly secondarily dormant seeds. The synergism between GA₃ and Pfr in non-dormant Kalanchoë seeds, leading to high effectiveness of even one short FR irradiation, still occurs in seeds made secondarily dormant before transfer to GA₃, but more R or FR irradiations, in combination with GA₃, are required for the release of secondary dormancy. A combination of red light and 6-benzyl-aminopurine is ineffective in removing dormancy.     

Indirect effect of abscisic acid on the induction of secondary dormancy in lettuce seeds

During temporary incubation at 25°C in buffered solutions (pH 4.0) of abscisic acid (ABA) seeds of lettuce ( Lactuca sativa L. cv. Olof) lost the red-light initiated ability to germinate in buffer. The development of secondary dormancy required an inhibitory ABA content in the seeds during a number of days. A temporary incubation in ABA during 24 h met these requirements only if the solution was about 100-fold more concentrated than during continuous incubation. Studies with 2-¹⁴C-ABA showed that the amount of ABA which had penetrated in 24 h was reduced by a factor 100 within 3 to 4 days during subsequent incubation in buffer. Both leaching and metabolic changes were involved in the reduction process. The nature of the metabolic products remained obscure. A shift to 2°C after incubation in ABA prevented the induction of secondary dormancy, but inhibited ABA metabolism. ABA did not interfere with the induction rate of secondary dormancy, and it was not required to maintain the state of dormancy. The sole function of ABA was the non-specific inhibition of germination, which indirectly facilitated the development of an ABA independent secondary dormancy. – The level of endogenous ABA was compared to the amount of ABA found in the embryo during and after incubation in ABA solutions marked with 2-¹⁴C-ABA. The level of endogenous ABA in air-dry seeds (0.11 ng/mg dry weight) corresponded to the minimal level at which penetrated ABA inhibited germination. This level had to be present at least during 4 to 5 days to inhibit the effect of red light. Since endogenous ABA was quickly reduced upon imbibition, a regulatory function of endogenous ABA in the inhibition of red light induced germination can be ruled out. A function in the temporary inhibition of dark germination and, consequently, in the development of secondary light irresponsiveness cannot be excluded, however.     

Conditions of induction of secondary dormancy in apple after breaking of primary dormancy by anaerobiosis and low temperature

Dormant embryos of Pyrus malus L. cv. Golden delicious, isolated from the fruits at harvest time or after a few months storage at 10 to 15°C, were kept under anaerobic conditions in order to eliminate primary dormancy. Germination tests were then carried out at different temperatures, using three modes of culture depending on the nature of the contact between the embryo and the medium. In CM the distal part of the two cotyledons was immersed in the medium. In RM only the embryonic axis was immersed. In C/2M the embryo was placed flat on the medium, the radicle and the external surface of one cotyledon being in contact with it.
Results showed that primary dormancy was released progressively depending on the duration of the anaerobic treatment. After a treatment of 11 or 13 days the last symptoms of primary dormancy were only apparent when germination tests were carried out at high temperatures (26–30°C) or in CM mode of culture.
When the embryos were kept at 4°C during 3 months inside the fruits, subsequent germination was inhibited at high temperature and in CM mode of culture. When the embryos were kept under anaerobic conditions (7 days) after the chilling treatmem inside the fruits, germination was no longer inhibited. It is concluded that the inhibition of germination at high temperature and in CM mode of culture is due to the persistence of traces of primary dormancy. Therefore, these conditions do not seem to induce secondary dormancy in apple embryos.
After elimination of primary dormancy by anaerobiosis. only application of (±) abscisic acid (3.8 and 19 μM) inhibited germination. These results support the idea that ABA is an important factor in the induction of dormancy. However, the question remains whether this secondary embryo dormancy has the same characteristics as the original primary dormancy.     

Dual action of respiratory inhibitors: inhibition of germination and prevention of dormancy induction in lettuce seeds

`Grand Rapids' lettuce Lactuca sativa L. seeds germinate readily at 15°C but poorly at 25°C in darkness. When held in dark at 25°C for an extended period, the ungerminated seeds become dormant as shown by their inability to germinate or transfer to 15°C in darkness. Induction of dormancy at 25°C was prevented by exposure to CN<sup>−</sup>, azide, salicylhydroxamic acid (SHAM), dinitrophenol, and pure N<sub>2</sub> as determined by subsequent germination at 15°C on removal of inhibitors. The effectiveness of inhibitors to break dormancy declined as dormancy intensified. At relatively low levels, CN<sup>−</sup>, SHAM, and azide promoted dark germination at 25°C while at high levels they were inhibitory. Uptake of O<sub>2</sub> by seeds held at 25°C for 4 days in 1.0 millimolar KCN was inhibited by 67% but was promoted 61% when KCN was removed. Correspondingly greater inhibition (79%) and promotion (148%) occurred when 1.0 millimolar SHAM was added to KCN solution. When applied alone, SHAM had little effect on O<sub>2</sub> uptake. These data indicate that Cyt pathway of respiration plays a dominant role in the control of both dormancy induction and germination of lettuce seeds, and `alternative pathway' is effectively engaged in presence of CN⁻. The channeling of respiratory energy use for processes governing germination or dormancy is subject to control by physical and chemical factors.

A scheme is proposed that illustrates compensatory use of energy for processes controlling dormancy induction and germination. A block of germination, e.g. by low water potential polyethylene glycol solution or a supraoptimal temperature spares energy to be utilized for dormancy induction while a block of dormancy induction by low levels of CN⁻ (similar to GA and light effects) drives germination. Blocking both processes by inhibitors (e.g. CN⁻, CN⁻ + SHAM) presumably leads to accumulation of `reducing power' with consequent improvement in O₂ uptake and oxidation rates of processes controlling germination or dormancy induction upon removal of the inhibitors.

     

A quantitative model for loss of primary dormancy and induction of secondary dormancy in imbibed seeds of Orobanche spp

Seeds of three species of Orobanche were conditioned (e.e. stored fully imbibed in darkness) for periods up to 210 d in order to model relief of primary dormancy and induction of secondary dormancy. The data were consistent with the hypothesis that periods for loss and induction of formancy and loss of viability are normally distributed in populations of imbibed sees and that these three processes are independent. There was a positive, linear relationship between the rate of loss of primary dormancy and temperature from 10-30C in O. aegyptiaca and O. cernua and 10-25C in O. crenata. In all three species, the rate of induction of secondary dormancy was highest at 10°C and decreased with increase of temperature up to about 20°C, above which there was little further change in the rate with temperature. The resulting model explained over 90% of the variation in germination after conditioning in both O. aegyptiaca and O. cernua. In O. crenata, however, this model was only satisfactory at 10 and 15°C. At higher temperatures, dormancy was relatively stable for periods of conditioning from 70 to about 154 d. Possible explanations for this are discussed. Applications of these models for estimating the time required to reduce Orobanche infestations in the field are also briefly discussed.Key words: Orobanche spp., broomrape, seed dormancy, temperature, dormancy model      

Factors affecting secondary sex ratio in Iranian Holsteins

The objective of this study was to evaluate the factors affecting secondary sex ratio (SSR) in Iranian Holsteins. Data of 942,941 Holstein calving events from the Animal Breeding Center of Iran, recorded between January 1996 and December 2007, were used in the analysis. A multivariable logistic regression model was used to model the logit of the probability of a male calf being born. Male births accounted for 49.6% of the total observations. The ratio of males to females varied from 52.5:47.5 in calving year 1996-1999 (odds ratio (OR) = 1.18; P < 0.0001), to 48.5:51.5 in calving year 2004-2007. The greatest occurrence of male births was observed in spring (OR = 1.02; P < 0.0001), and the lowest incidence of male births was for summer or fall calvings. Also, the frequency of male births decreased from parity 1 to parity 4 and beyond (P < 0.0001; OR = 1.11). The greatest number of sires had the SSR equal to 0.5 with a minimum SSR of 32% while the maximum was 97%. Among cows that had a male birth, the chance of delivering a male calf again was 25.5% when cows had delivered a male once (OR = 1.14; P < 0.0001), and 12.7% if a male calf was delivered twice by a cow. This indicated that characteristics peculiar to the dam influence the sex of her offspring and suggests some degree of repeatability of calf sex within cows.     

Gene expression during seed maturation in Brassica napus in relation to the induction of secondary dormancy

Gene expression in two cultivars of Brassica napus (AC Excel and DH12075) has been compared at the full-size embryo, desiccation, and mature stages of seed development. Seed of these cultivars differ in their potential to exhibit secondary dormancy following environmental stress; Excel has high potential and DH12075 has low potential. A majority of genes were down-regulated during maturation in both cultivars but a significant number of differences in gene expression between the cultivars were apparent in the transition from full-size embryo to mature seed. However, most differences were apparent in the desiccation stage and some of the differences were in genes related to signaling processes and protein biosynthesis. We suggest that the propensity of Brassica seeds to manifest secondary dormancy may be determined by changes in gene expression that occur during late seed development.     

Differences in the nature of thermodormancy and far-red dormancy in lettuce seeds

Lettuce seeds cv. Noran germinate at 23°C in light as well as in darkness. However dormancy can be induced either by a long exposure (24 h) to far-red radiation or by an exposure of 48–72 h to a temperature of 37°C. The difference in response of these two types of dormant seeds to conditions inducing germination indicate that in both types P_fr is inactivated, but that a dark process required for immediate action of P_fr does not proceed at 37°C as it does during far-red radiation.     

Transition from primary dormancy to secondary dormancy in cocklebur seeds

Abstract The transition from primary dormancy to secondary dormancy was examined using upper cocklebur (Xanthium pennsylvanicum Wallr.) seeds. The non-after-ripened seeds with primary dormancy responded to chilling, anoxia, KCN, and NaN₃ with an increase in germination. However, their maximal responses to these treatments only occurred after a period of water imbibition, probably a reflection of the increasing growth potential of the axial tissue which was accompanied by the increase in the capacities of respiration and ethylene production. On the other hand, the establishment of secondary dormancy was accompanied by a decrease in respiration and ethylene production of seeds, and in the growth potential of both axial and cotyledonary tissues. The decrease in growth potential of these tissues occurred regardless of whether they were excised from after-ripened seeds or non-after-ripened seeds. It is inferred that the primary dormancy of cocklebur seeds is a state maintained in un-germinated seeds for a long time through a spontaneous transition to secondary dormancy.     

Involvement of ABA in induction of secondary dormancy in barley (Hordeum vulgare L.) seeds

At harvest, barley seeds are dormant because their germination is difficult above 20 degrees C. Incubation of primary dormant seeds at 30 degrees C, a temperature at which they do not germinate, results in a loss of their ability to germinate at 20 degrees C. This phenomenon which corresponds to an induction of a secondary dormancy is already observed after a pre-treatment at 30 degrees C as short as 4-6 h, and is optimal after 24-48 h. It is associated with maintenance of a high level of embryo ABA content during seed incubation at 30 degrees C, and after seed transfer at 20 degrees C, while ABA content decreases rapidly in embryos of primary dormant seeds placed directly at 20 degrees C. Induction of secondary dormancy also results in an increase in embryo responsiveness to ABA at 20 degrees C. Application of ABA during seed treatment at 30 degrees C has no significant additive effect on the further germination at 20 degrees C. In contrast, incubation of primary dormant seeds at 20 degrees C for 48 and 72 h in the presence of ABA inhibits further germination on water similarly to 24-48 h incubation at 30 degrees C. However fluridone, an inhibitor of ABA synthesis, applied during incubation of the grains at 30 degrees C has only a slight effect on ABA content and secondary dormancy. Expression of genes involved in ABA metabolism (HvABA8'OH-1, HvNCED1 and HvNCED2) was studied in relation to the expression of primary and secondary dormancies. The results presented suggest a specific role for HvNCED1 and HvNCED2 in regulation of ABA synthesis in secondary seed dormancy.     

Factors affecting the premature induction of phosphopyruvate carboxylase in neonatal rat liver

1. Phosphopyruvate carboxylase activity rapidly appears in the liver of prematurely delivered rats and development of activity is prevented by injection of actinomycin D just before delivery. 2. The activity is considerably decreased by puromycin and amino acid analogues and thus appears to be due to enzyme synthesis. 3. Newborn or premature animals show a transient intense phase of hypoglycaemia after delivery. 4. When the hypoglycaemic phase is prevented by glucose injection little phosphopyruvate carboxylase activity appears in the liver, but galactose, mannose and fructose, which have no effect on the blood glucose concentration, also repress enzyme development. 5. Lactate, pyruvate and glycerol injections repress the premature development of phosphopyruvate carboxylase. 6. Injections of glucagon, adrenalin and noradrenalin into the rat foetus in utero result in development of phosphopyruvate carboxylase activity. 7. These findings are discussed in relation to the mechanism of initiation of enzyme synthesis in neonatal rat liver.     

Factors affecting the premature induction of tyrosine aminotransferase in foetal rat liver

1. Premature delivery of foetal rats by uterine section results in the rapid appearance of tyrosine aminotransferase activity in foetal liver, after an initial lag period of 3-6hr. 2. The premature induction of activity is completely repressible by actinomycin D given soon after delivery and partially repressible by puromycin and amino acid analogues. 3. Glucagon injections into foetal rats in utero lead to production of tyrosine aminotransferase in the foetal liver, but adrenalin and nor-adrenalin are without effect. 4. Injections of glucose, galactose, fructose and mannose into prematurely delivered rats repress the development of tyrosine aminotransferase activity about 50% when they are given 2hr. after delivery, but glucose has no significant effect when injected at delivery. 5. The results are discussed in relation to current hypotheses on the role of hormones in enzyme induction in foetal development.     

Factors affecting the induction of malolactic fermentation in red wines with Leuconostoc oenos

Malolactic fermentation was induced in red wines by inoculation with several strains of Leuconostoc oenos . The progress of Malolactic Fermentation was monitored by following the kinetics of bacterial growth and degradation of malic acid. These kinetics varied significantly depending on the strain of Leuc. oenos inoculated, the strain of Saccharomyces cerevisiae used to conduct the alcoholic fermentation, and the wine properties of pH and concentrations of ethanol and sulphur dioxide. Rapid, predictable malolactic fermentation was achieved by inoculating a high density (> 10⁶ cfu/ml) of Leuc. oenos , whereby malic acid degradation was not connected to the growth of the bacterial cells. Wines after malolactic fermentation were not bacteriologically stable and supported the growth of Leuc. oenos inoculated into the wines.     

Priming relieves dormancy in lettuce seeds independently of changes in osmotic constituents

The relationship was studied between germination and dormancy of lettuce seeds ( Lactuca sativa L. cv. Musette) and both soluble amino nitrogen metabolism and osmotic potential. Germination at 15°C in darkness coincided with a rise in the levels of free amino acids and total soluble amino nitrogen compounds and in the activity of glutamine synthetase (GS, EC nr. 6.3.1.2). In further experiments GS activity was used as indicator of soluble amino nitrogen metabolism. GS activity increased after the start of growth indicated by an increasing intolerance to desiccation. At 30°C seeds did not germinate, unless dormancy was broken beforehand during incubation at 2° or 15°C (priming). The alleviation of dormancy occurred much earlier than the rise in the activity of GS. Priming at 15°C in polyethylene glycol instead of water retarded the breaking of dormancy and at –1.28 MPa even stimulated the induction of secondary dormancy, but did not prevent a continued rise in the activity of GS. GS activity was also not reduced during induction of secondary dormancy by dehydration of primed seeds, which antagonized the beneficial effect of priming. Psychrometric measurements showed that osmotic potential (Ψ_π) of the seeds remained constant during prolonged priming in polyethylene glycol at 15°C. During incubation in water, Ψ_π increased both prior to and after the moment of germination to less negative values. It is concluded that changes in the level of dormancy in lettuce seeds occur independently of soluble amino nitrogen metabolism and of changes in Ψ_π.     

Factors affecting induction and development of in vitro rooting in apple rootstocks

Shoots of apple rootstocks raised in vitro were transferred to various rooting media to study the effect of different factors on root initiation and development. Various concentrations of indole-3-butyric acid (IBA) initiated rooting but maximum rooting percentage was found with 2.0 and 2.5 mg l(-1) of IBA in M7 and with 1.0 mg l(-1) of IBA in MM106. The drawback was that the roots were thick, short and with profuse callus. The presence of activated charcoal (AC) in the rooting medium improved the rooting quality but reduced the rooting percentage in both the rootstocks. In high auxin dip of 70, 80 and 90 mg l(-1) IBA for 2, 2 and 1 hr showed 75-85 per cent rooting in M7, but lacked reproducibility of the results. Whereas in MM106, 66 - 70 % rooting was achieved with 70 mg l(-1) of IBA dip for 3 h. Root induction in shoots in IBA containing liquid medium (LM) in dark for few days and root elongation in IBA--free medium in light proved most effective. On the other hand, continuous light treatment showed reduced rooting. Reduction of MS salts and sucrose in root elongation medium showed decreased rooting. Plantlets from two--stage rooting procedure showed more rapid growth and satisfactory survival during hardening of plants and on transfer to field.