Zephyranthes tubispatha is an ornamental species distributed along several countries of South America. Although it can be multiplied through bulbs or scales, seed germination is a simpler and more cost-effective process. Temperature plays a major role in the control of germination; however, its effect has been scarcely investigated in this species. In the present work, we characterized the germination responses of Z. tubispatha seeds to different temperatures and analyzed the role of key components of the antioxidant metabolism and phytohormones in their control. Seeds showed an optimal temperature range for germination between 14 and 20°C, with higher temperatures (HTs) being progressively inhibitory. While germination was almost nil above 28°C, it could be recovered after transferring the seeds to 20°C, suggesting that thermoinhibition was the underlying phenomenon. The duration of the HT incubation period affected both the time to germination onset and the germination rate at 20°C. Similarly, the activity of antioxidant enzymes, the production of reactive oxygen species in the embryo and the sensitivity to some germination promoters varied depending on the duration of the HT treatment. The addition of 20 μM fluridone was sufficient to recover germination dynamics as in the control treatment when given after a long-term incubation period (25 d) at HT. Ethephon supply was more effective than gibberellins to suppress thermoinhibition, suggesting that changes in the balance and/or sensitivity to ethylene and abscisic acid over time play an important role in the regulation of germination responses to thermal cues in this species.

Cistus species have seeds with hard coats which impose physical seed dormancy that can be released after seed scarification. In fire-prone habitats, the break of physical seed dormancy is usually related to the heat produced during fires. It is commonly accepted that most hard-seeded species, including those of the genus Cistus, are able to germinate under a wide range of temperatures in light as well as in darkness, once the seed becomes permeable. However, although many studies have focused on the release of physical dormancy only, a few have done so on the effect of environmental factors once dormancy is released. In this research, through a factorial experiment, we analysed the effects of light (light and darkness) and a range of temperatures (10, 15, 20, 25 and 30°C) on the seed germination of eight Cistus species after a heat shock. On average, almost 60% of the seeds did not germinate despite being viable, and this lack of germination increased with higher temperatures during the treatment. Although an idiosyncratic germination response emerged, temperature had a significant effect in all the species, reaching the highest levels of germination between 10 and 20°C. Light interacted with temperature in four cases by increasing the germination, especially under the least favourable temperatures. Environmental factors, such as temperature and light, appear to modulate the germination of the studied Cistus species after the release of physical seed dormancy.

Microphytobenthos (MPB) contributes up to half the primary production of estuaries. These microorganisms are sensitive to changes in sediment temperatures, particularly the extreme temperatures during exposure periods. This study investigates the physiological responses of MPB to freezing temperatures at two locations near Hobart, Tasmania during winter. Photosynthetic parameters were measured at 2 mm intervals to a depth of 10 mm. FV/FM values at three different distances from the shoreline at Kings Beach and Browns River in winter were between 0.584 and 0.617. rETRmax values were between 24.696 and 20.773. Maximum α values peaked in the subsurface rather than at the sediment surface. In vitro laboratory experiments (down to −5°C) showed little difference in response between the control and treatment groups, indicating no apparent effects of short term freezing on the MPB. Little change in photosynthetic parameters in response to freezing was probably associated with the resistance of light-harvesting reactions to freezing temperatures, recovery of the plasmalemma integrity or cryoprotection. Sediment composition and species composition were similar at both sampling sites. Therefore, responses of MPB were not due to species and grain size composition.

Temperature has a key influence over seed dormancy and germination, allowing wild plants to synchronize their life history with the seasons. In this review we discuss the signalling pathways through which temperature is integrated into seed physiology and the control of primary and secondary dormancy, with an emphasis on understanding maternal effects and responses dictated by the zygotic tissues. A key emerging paradigm is that temperature signalling in seeds must be understood in relation to whole plant genetics and physiology, as overlapping pleiotropic roles for temperature sensing and hormone signalling pathways are commonplace.

Seed germination of Grand Rapids lettuce (Lactuca sativa L.) was inhibited at 38°C (thermoinhibition). Thermoinhibition was totally relieved by a combination of 100% oxygen and 10 mg I−1 kinetin, to a lesser extent by a combination of oxygen and ethylene but not at all by oxygen plus GA3. The combination of oxygen with kinetin or ethylene was more effective in preventing thermoinhibition than a combination of all three hormones and CO2 in air. The beneficial effect of oxygen plus kinetin was not related to either increased production of CO2 or ethylene by the seeds. In air, germination was partially inhibited by 2-aminoethoxyvinyl glycine (AVG) or 2,5-norbornadiene (NBD) and totally inhibited by AVG plus NBD. Oxygen plus kinetin almost completely alleviated their inhibitory effects. It was concluded that oxygen plus kinetin caused seeds to bypass an ethylene requirement for germination or increased the sensitivity of seeds to ethylene. In air at 38°C, seeds exhibited a high level of ethanolic fermentation, while in oxygen plus kinetin they respired aerobically. At 38°C in air, contents of ATP and total adenylates were reduced, while in seeds treated with oxygen plus kinetin these contents were similar or higher than in 25°C controls. A similar response was also observed for adenylate energy charge. It was concluded that part of the mechanism of thermoinhibition could be the failure of ATP content to reach a sufficient level to satisfy the requirement for germination at 38°C. It appeared possible that the prevention of thermoinhibition by oxygen plus kinetin could be related to increased ATP content in seeds caused by this treatment.

Lettuce (Lactuca sativa L.) seeds are inhibited from germinating above an upper temperature limit that is dependent upon cultivar, growing conditions and seed treatments. Thermoinhibition is accompanied by an increasing sensitivity of germination to reduced water potentials (ψ). We have employed a water relations analysis (hydrotime model) of seed germination rates to investigate the basis of thermoinhibition. Germination rates can be characterized by the distribution of base water potentials among seeds in the population (ψb(g)) and a hydrotime constant indicating the total accumulated hydrotime (MPa · h) above the base required for radicle emergence. The hydrotime model adequately described germination time courses across a range of ψ at both high and low temperatures. Increasing temperature caused the ψb(g) distributions to become more positive, accounting for the greater sensitivity to ψ and inhibition of germination. Increases in embryo osmotic potential and in the turgor yield thresholds of both the radicle and the endosperm/pericarp envelope contributed to this change. Seed priming (prehydration and drying) treatments speeded germination by reducing the hydrotime requirement. Priming also resulted in smaller increases in ψb(g) at high temperature, alleviating thermoinhibition by lowering the embryo yield threshold sufficiently to compensate for the increased endosperm resistance. The beneficial effects of priming in lettuce appear to occur primarily in the embryo, rather than in the surrounding envelope tissues.

The effect of temperature on the Pfr level required to induce germination in lettuce cv. Grand Rapids was investigated. Various Pfr levels were established in seeds, by irradiation with different fluences of red light, or by saturating irradiations with narrow bandwidth light. Both approaches indicated that increasing Pfr levels caused a progressive increase in the upper temperature limit for germination. The Pfr level required to induce 50% germination after a single light pulse increased from about 11% at 15° and 20°C to 86% at 30.5°C. Repeated red light irradiations raised the GT50 (temperature giving 50% germination) by about 2°C, suggesting that Pfr reversion limited germination at high temperatures. However, a large increase in Pfr level required to induce 50% germination was still apparent at high temperatures, even when relatively constant levels of Pfr were maintained by repeated irradiations with narrow bandwidth light. These results indicate that following a single light pulse, germination at high temperatures is limited solely by the availability of Pfr. The precise position of the upper temperature limit for germination is influenced by reversion; however, the underlying cause of thermoinhibition relates to an increasing requirement for Pfr at elevated temperatures.

A water relations (hydrotime) model was employed to characterize the responses of lettuce (Lactuca sativa L.) seed germination to temperature, water potential (ψ), endosperm disruption, and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). The base water potentials (ψb), standard deviations of base water potential and hydrotime constants (θH) estimated by the model were used to predict the germination rates at reduced ψ and supraoptimal temperatures in the presence or absence of ACC. The distribution of ψb values among seeds in the population (ψb(g)) increased (became more positive) as the imbibition temperature increased, indicating a greater sensitivity of seeds to reduced ψ at thermoinhibitory temperatures. Slitting the seeds at the cotyledonary end to disrupt the integrity of the endosperm extended the high temperature range for germination and maintained lower ψb(g) distributions compared with intact seeds as temperature increased. ACC (10 mm) in the imbibition solution raised the upper temperature limit for germination by about 2°C, and also lowered ψb(g) values by 0.1 to 0.2 MPa. The effect of ACC on ψb(g) was more pronounced at higher imbibition temperatures and in slit seeds. ACC (via conversion to ethylene) extends the high temperature limit for lettuce seed germination by acting in the embryo to maintain a lower ψ threshold for the initiation of growth as temperature increases. Evidence from other species as well suggests that this may be a general mechanism for the action of ethylene in promoting seed germination.

The effects of seed priming on germination and endo-β-mannanase activity at inhibitory and non-inhibitory temperatures in thermosensitive ‘Dark Green Boston’ (DGB) and in thermotolerant ‘Everglades’ (EVE) lettuce were investigated. A single-seed endo-β-mannanase assay was used to follow the enzyme activity during priming. Seeds were primed at 15°C in aerated solutions of polyethylene glycol (PEG) with constant light, then redried. Primed and non-primed seeds germinated 100% at 20°C. At 35°C, non-primed and primed EVE seeds germinated 100%, whereas non-primed seeds of DGB germinated only 4%. During priming, endo-β-mannanase activity increased between 24 and 48 h in EVE and between 24 and 72 h in DGB after the beginning of osmotic imbibition. Endo-β-mannanase activity persisted in primed seeds following seed drying, was detected before radicle protrusion and was present in the micropylar region in front of the radicle tip. Higher enzyme activity was observed in primed seeds and EVE compared with non-primed and DGB seeds. The results suggest that priming may overcome the inhibitory effect of high temperature in thermosensitive lettuce seeds due to increased endo-β-mannanase activity, possibly leading to a weakening of endosperm, thus overcoming thermodormancy.

Abstract In seed formation the role of ethylene has received little attention. The data available on zygotic embryogenesis suggest an association of the ethylene biosynthetic pathway and seed maturation. Over the course of dicot embryogenesis, ACC-oxidase mRNA can be expressed in the cotyledons and embryonic axis. However, as maturation proceeds, cotyledonary ACC-oxidase expression disappears. In some seeds that develop primary dormancy, ethylene synthesis can be among the prerequisites for breaking dormancy. Moreover, the persistence of dormancy may be related to the difficulty of the embryonic axis to produce the necessary ethylene levels or to low tissue sensitivity. The use of inhibitors of ethylene biosynthesis or its action has provided data implicating an ethylene requirement for seed dormancy or germination in some species. However, the role of ethylene in germination remains controversial. Some authors hold that gas production is a consequence of the germination process, while others contend that ethylene production is a requirement for germination. Furthermore, among seeds that require ethylene, some are extremely sensitive to the gas, while others require relatively high levels to trigger germination. Recent studies with Xanthium pennsylvanicum seeds suggest that β-cyanoalanine-synthase is involved in ethylene-dependent germination. In addition, regulation of the partitioning of S-adenosyl-L-methionine (AdoMet) between the ethylene vs polyamine biosynthetic pathways may be a way of controlling germination in some seeds. Such regulation may also apply to the reversal of seed thermoinhibition, which can occur when polyamine synthesis is inhibited, thereby strongly channelling AdoMet towards ethylene. The biological models and approaches that may shed additional light on the role of ethylene during seed germination are presented.