Changes in sensitivity of Amaranthus retrof/exus L. seeds to ethylene during preincubation. I. Constant temperatures

Abstract. Germination responses of redroot pigweed ( Amaranthus retroflexus L.) seeds to ethylene were determined at 25, 30, 35, or 40° C after preincubation at various temperatures (15–35° C) for different periods (0.5–32 d). After 7 d preincubation, seeds showed a log-linear germination response to ethylene concentration in most of the temperature treatments. Sensitivity to ethylene increased with longer preincubation; response thresholds of 0.03−0.09 cm³ m⁻³ were observed after 32 d, compared to 0.18−1.6 cm³ m⁻³ after 7 d of preincubation. Preincubation at 15 or 20° C generally enhanced germinability, whereas 25 or 30° C produced secondary dormancy, which was readily broken with ethylene. Temperature during preincubation also significantly influenced the slope of the dose-response curve. The responses of preincubated redroot pigweed seeds to ethylene suggested that, in the field, seeds would probably not lose their sensitivity to this gas during prolonged burial in soil.     

Influence of the Seed Envelope and Growth Regulators Upon Seed Dormancy in Witchweed (Striga lutea Lour.)

Seed of the angiospermous parasite, witchweed (Striga luteaLour.), normally germinate poorly or not at all unless adequatelypretreated and exposed to a germination stimulant which is obtainedfrom plant root exudates. Under certain conditions, scarificationof the seed envelope promoted germination in the absence ofthe stimulant. With adequately (2-week) pretreated seed, a cutor puncture through the aleurone at the radicular end inducedtypical germination. A puncture through the aleurone elsewhereon the seed induced little or no germination. A cut throughthe aleurone at the cotyledon end or centre of the seed inducedatypical germination. A puncture through the aleurone elsewhereon the seed induced little or no germination. A cut throughthe aleurone at the cotyledon end or centre of the seed inducedatypical germination in which the radicle elongated but didnot penetrate the intact envelope over the radicle. Incubationin oxygen did not promote germination regardless of the siteof seed scarification. Ten per cent carbon dioxide reduced thegermination of punctured seed. Seed germinated equally wellwhen scarified over the radicle and incubated in air, nitrogen,darkness, or light. Brief treatments with sulphuric acid alsoinduced germination. The softening effect of stimulatory acidtreatments upon the aleurone was first evident 6 h later andonly occurred at the radicular end of the seed. Indol-3yl-aceticacid inhibited and ethylene stimulated germination of scarifiedand non-sacrified seed. Gibberellic acid(GA₃) had no apparenteffect upon nonscarified seed but promoted germination of scarifiedseed. Inadequately (I-day) or excessively (12- to 16-week) pretreatedseed germinated poorly or not at all when treated with the germinationstimulant, ethylene, GA₃, or sulphuric acid. Some seed germinatedslowly when scarified over the radicle, but the germinationrates and totals were less than those of scarified seed pretreatedfor 2 weeks. Additions of stimulant, ethylene or GA₃, aceleratedthe germination rates of scarified but inadequately pretreatedseed. Of the treatments tested, only GA₃, increased the slowgermination of the excessively pretreated scarified seed. Results indicated that the aleurone restrained radicle elongation.Scarification over the radicle removed the restraint and permittedradicle emergence. However, the ability of the radicle to elongate,as influenced by time of seed pretreatment and exogenous stimulant,GA₃ or ethylene, determined whether or not the scarified seedgerminated.     

Ethylene, Nitrate and Nitrite Interactions in the Promotion of Dark Germination of Common Purslane Seeds

Ethylene (10 µ1^–1) caused about one-third of highlydark-dormant seeds of common purslane (Portulaca oleracea L.)to germinate in the dark. Attempts were made to increase germinationin the dark with nitrate and ethylene combinations. When applieddirectly to the seeds, KNO₃ did not stimulate germination andKNO₃ plus ethylene did not increase germination above that ofethylene alone. Pre-incubation of seeds in KNO₃ for 4 to 7 dbefore the ethylene applications significantly increased germination.The effects of the KNO₃ pre-incubation were additive at eachof four ethylene concentrations (0.1–100 µ11^–1).Potassium nitrate was effective only when ethylene followedthe KNO₃ pre-incubation period. Potassium nitrite stimulatedabout 25 per cent of the seeds to germinate without a pre-incubationperiod and without ethylene. Also, ethylene plus KNO₂ enhancedgermination above that achieved by either stimulus alone. Silvernitrate did not block the ethylene promotion of germination,but reversed the typical ethylene inhibition of seedling growthfollowing germination. The results support the views that nitrateexerted its effect via conversion to nitrite within the seedand that the rate of nitrate conversion may be a limiting factorin the dark germination of common purslane seeds. Ethylene mayfacilitate nitrite activity by increasing seed sensitivity tothe stimulus. Common purslane, Portulaca oleracea L., ethylene, nitrate, nitrite, germination, dormancy     

Effects of ethylene and some other environmental factors on different stages of germination in red root pigweed (Amaranthus retroflexus L.) seeds*

Abstract. Ethylene was found to promote two distinct processes during germination of redroot pigweed (Amarantus retroflexus L.) seeds: embryo expansion that splits the seed coat (incomplete germination), and radicle penetration through the more elastic endosperm (complete germination). The two events can be separated in time by subjecting seeds to low water potential or low CO₂ levels, which arrest germination of some seeds at the incomplete stage. Ethylene applications to incompletely germinated seeds promote complete germination, with a response threshold near 0.02 cm³ m^?3 and saturation near 0.5 cm³ m^?3. Higher ethylene concentrations (0.5 to 50 cm³ m^?3) given during the first day of seed imbibition also increase the percentage of seeds which initiate embryo expansion and split the seed coat. Light and elevated CO₂ also promote radicle penetration of the endosperm in seeds incubated under water stress. The results support the view that the germination pause at the incomplete stage is an adaptation to environmental stresses that can be overcome with exogenous ethylene or certain other stimuli.     

Peroxidase involvement in lignification in water-impermeable seed coats of weedy leguminous and malvaceous species

Abstract. The development of water impermeable seed coats of two members each of the leguminoseae family [ Crotalaria spectabilis Roth, Sesbania exaltata (Raf) Cory] and the malvaceae family [Anoda cristata (L.) Schlecht, Abutilon theophrasti Medic.] was investigated. Highest peroxidase (POD) activity of Anoda and Abutilon seed coat extracts was highly correlated with the developmental stages when soluble phenolics were maximally converted into lignin. Although extensive lignification occurred during seed coat development in both legumes, the patterns of POD activity, soluble phenolic levels and time of lignification were different from those of the malvaceous species. POD activity levels in developing coats of the malvaceous seeds increased as phenolics decreased. Both POD activity and phenolic levels decreased during seed coat development of the legumes. POD was immunocytochemically and immunochemically detected in seed coats of all four species; however, results for polyphenol oxidase were negative. The results confirmed POD involvement in lignification of leguminous and malvaecous species and support and extend our earlier view that POD is involved in lignin formation during development of impermeable seed coats.     

Changes in sensitivity of Amaranthus retroflexus L seeds to ethylene during preincubation. II. Effects of alternating temperature and burial in soil

Abstract. The effects of diurnally alternating temperatures and of prolonged burial in the soil on germination response of redroot pigweed ( Amaranthus retroflexus L.) seeds to ethylene were investigated. Percentage germination in a 12 h/12 h, 23° C/35° C temperature regime roughly equalled that observed at constant 35° C, and greatly exceeded that observed at 30°C. Preincubation for 61 d in alternating temperatures, which were gradually increased to simulate soil warming in spring, caused little germination in the absence of ethylene, but considerably enhanced sensitivity to ethylene. Seeds kept in soil in the same temperature regime failed to show the response to ethylene, and the soil itself removed ethylene from the soil atmosphere.
After burial in a field plot either over winter or during the summer, seeds had a very low ethylene response threshold (0.01−0.05 cm³ m⁻³) and strong response to ethylene (70–95% germination at 51 cm³ m⁻³ compared to 1–20% without ethylene). Germinability of seeds buried overwinter declined between 10 May (85%) and 24 May (7%), and 90% of those recovered on or after 24 May had a visible rupture in the seed coat. Apparently, germination had begun during burial, but was arrested by unknown causes in an early phase and was followed by seed deterioration.
Although the role of ethylene in germination of buried seeds remains uncertain, the greatly enhanced sensitivity to ethylene observed in pigweed seeds after burial deserves further investigation.     

Some effects of nitrate-treated soil upon the sensitivity of buried redroot pigweed (Amaranthus retroflexus L.) seeds to ethylene, temperature, light and carbon dioxide

Abstract Fresh dormant redroot pigweed (Amaranthus retroflexus L.) seeds were buried 5 cm deep in the field at Stoneville, MS in November 1981. Potassium nitrate (200 kg ha ¹) or nothing was applied to the soil in the fall of 1981 and the late winter of 1982. Seeds were recovered at intervals under darkness during the following 2 years and tested for responses to ethylene, temperature, light and carbon dioxide. During the first overwintering, nitrate enhanced loss of primary dormancy and increases seed sensitivity to temperature, light and ethylene. The loss of dormancy reached a maximum at 25 to 30 weeks (early summer) after burial. Examination of the recovered seeds indicated that about 80% of the non-treated seeds and 98% of the nitrate-treated seeds germinated in situ during the period of maximum loss of dormancy. Thus, after one overwintering period, about 20% of the original buried seed population remained dormant in nontreted soil and 2% remained dormant in the nitratetreated soil. After the second overwintering, the percentages of dormant seeds remaining in nontreated or treated soil were both only 1–2%. Nitrate reduced dormancy and enhanced germination in early summer following the first overwintering. Regardless of treatment, the remaining 1 2% of seeds in soil after the second year were of low sensitivity to the germination stimuli (ethylene, temperature, light) and constituted the long-lived portion of the original seed population.