Thermal time models have been widely applied to predict temperature requirements for seed germination. Generally, a log-normal distribution for thermal time [θT(g)] is used in such models at suboptimal temperatures to examine the variation in time to germination arising from variation in θT(g) within a seed population. Recently, additional distribution functions have been used in thermal time models to predict seed germination dynamics. However, the most suitable kind of the distribution function to use in thermal time models, especially at suboptimal temperatures, has not been determined. Five distributions (log-normal, Gumbel, logistic, Weibull and log-logistic) were used in thermal time models over a range of temperatures to fit the germination data for 15 species. The results showed that a more flexible model with the log-logistic distribution, rather than the log-normal distribution, provided the best explanation of θT(g) variation in 13 species at suboptimal temperatures. Thus, at least at suboptimal temperatures, the log-logistic distribution is an appropriate candidate among the five distributions used in this study. Therefore, the distribution of parameters [θT(g)] should be considered when using thermal time models to prevent large deviations; furthermore, an appropriate equation should be selected before using such a model to make predictions.

Temperature may regulate seed dormancy and germination and determine the geographical distribution of species. The present study investigated the thermal limits for seed germination of Polygonum ferrugineum (Polygonaceae), an aquatic emergent herb distributed throughout tropical and subtropical America. Seed germination responses to light and temperature were evaluated both before (control) and after stratification at 10, 15 and 20°C for 7, 14 and 28 d. Germination of control seeds was ~50% at 10 and 15°C, and they did not germinate from 20 to 30°C. The best stratification treatment was 7 d at 10°C, where seed germination was >76% in the dark for all temperatures, except at 30°C, and < 60% in light conditions. A thermal time approach was applied to the seed germination results. Base temperature (Tb) was 6.3°C for non-dormant seeds and optimal temperature (To) was 20.6°C, ceiling temperature (Tc (<50)) was 32.8°C, and thermal time requirement for 50% germination was 44.4°Cd. We concluded that a fraction of P. ferrugineum seeds is dormant, has a narrow thermal niche to germinate (10 and 15°C) and that cold stratification (10°C) alleviated dormancy and amplified the thermal range permissive for germination of the species. Consequently, P. ferrugineum is expected to occur in colder environments, for example, at high altitudes. Higher temperatures decrease the probabilities of alleviate dormancy and the ability of their seeds to germinate.

Although the level of seed desiccation sensitivity (LSDS) may have an impact on plant species conservation, information is available for <10% of tropical angiosperms. A study was conducted to assess the LSDS of 28 tropical montane species in Sri Lanka. Seeds were extracted from freshly collected fruits. Initial weight was recorded, and thousand seed weight (TSW) was calculated. Seed moisture content (MC) was determined. LSDS was determined using seed desiccation experiments and predicted using the TSW–MC criterion. Seed storage behaviour was predicted using LSDS and storage data and using a model based on phylogenetic affiliation. The relationship between LSDS and seed dormancy, life form and forest strata was evaluated. Fresh seeds of only 12 species germinated to >80%. Although seeds of the other species had >80% viability, only 0–70% germinated due to dormancy. Seeds of five species had MC <15%, indicating desiccation tolerance (DT). Seeds of 12 species lost viability after desiccation, indicating desiccation sensitivity (DS). Seeds of Ardisia missionis, Psychotria gartneri and Psychotria nigra remained viable after desiccation, showing DT. Seeds of 17 species were DS and those of 11 species DT. The TSW of four species was >500 g. Thus, seeds of other species were predicted to be DT by the TSW–MC criterion. A relationship was identified between LSDS and the forest strata of the species. More canopy species produced DS than DT seeds. Since seeds of most of the studied species were DS, these species may be threatened due to prolonged droughts predicted for the region due to climate change.

The objective of this study was to determine how the current (10–16 weeks) and predicted future (2–8 weeks) length of cold stratification and current and predicted future post-stratification temperatures influence radicle and epicotyl emergence in acorns of Quercus robur. We tested radicle and epicotyl emergence at two temperatures corresponding to the current (15/6°C) and predicted future early autumn and spring temperatures (25/15°C) in Poland. We fitted models to describe and derive parameters for radicle and epicotyl emergences over time. The parameters included maximum percentage, rate of emergences, time to achieve the maximum emergence rate, emergence delay and time to 50% emergence. In most cases, the Gompertz model was the best fit, but in a few cases, the logistic model was the best. Richard's model for most of the cases did not converge. This model, according to both information criteria values, was the best fit for epicotyl emergence at 15/6°C following 8 weeks of cold stratification. Richard's model was also the best fit for epicotyl emergence at 25/15°C following 14 weeks of stratification.. Our results indicate that at temperatures typical for early autumn (15/6°C), the time necessary for radicle emergence from 50% of acorns was longer than that from acorns placed at 25/15°C. Four weeks of cold stratification extended 50% radicle emergence at 15/6°C to 70 d, whereas 12 weeks of stratification shortened the time to 11 d. When the acorns were incubated at 25/15°C, radicle emergence occurred faster than at 15/6°C and the time lag between radicle and epicotyl was shorter.

A low-cost technique named ‘on-farm’ seed priming is increasingly being recognized as an effective approach to maximize crop establishment. It consists of anaerobically soaking seeds in water before sowing resulting in rapid and uniform germination, and enhanced seedling vigour. The extent of these benefits depends on the soaking time. The current determination of optimal soaking time by germination assays and mini-plot trials is resource-intensive, as it is species/genotype-specific. This study aimed to determine the potential of the seed respiration rate (an indicator of metabolic activity) and seed morphological changes during barley priming as predictors of the priming benefits and, thus, facilitate the determination of optimal soaking times. A series of germination tests revealed that the germination rate is mostly attributable to the rapid hydration of embryo tissues, as the highest gains in the germination rate occurred before the resumption of respiration. Germination uniformity, however, was not significantly improved until seeds were primed for at least 8 h, that is, after a first respiration burst was initiated. The maximum seedling vigour was attained when the priming was stopped just before the beginning of the differentiation of embryonic axes (20 h) after which vigour began to decrease (‘over-priming’). The onset of embryonic axis elongation was preceded by a second respiration burst, which can be used as a marker for priming optimization. Thus, monitoring of seed respiration provides a rapid and inexpensive alternative to the current practice. The method could be carried out by agricultural institutions to provide recommended optimal soaking times for the common barley varieties within a specific region.

Seed germination is regulated by temperature and can thus be quantified by thermal models, which can predict germination occurrence in biomes and plant survival under possible climate change scenarios. The objective of this study was to quantify germination based on thermal time and survival risk of 14 species in the Brazilian Cerrado in scenarios of future climate change. Seeds were collected in the warmer regions of the Cerrado, central Brazil, placed in incubators to germinate at constant temperatures of 10–50°C and evaluated every hour or day. Germination rate (R50), time for germination of 50% of the seeds (T50) and dent-like function were used to determine cardinal temperatures. Thermal time parameters were estimated using the Weibull model. Seed germination forecasts were made based on the International Panel on Climatic Change (IPCC) scenarios of global temperature increase. Base temperatures (Tb) ranged from 3.5 to 16.5°C, maximum temperatures (Tmax) from 35 to 50°C and optimum temperatures (To) from 30 to 35°C. Estimated thermal time varied from 484°C h to 400°C d at sub-optimal temperatures and 108°C h at 126°C d at supra-optimal temperatures. Species more distributed showed a higher thermal range of germination and are less susceptible to extinction in temperature increase scenarios. The results of this study suggest that seeds that are non-dormant after dispersal may be the most vulnerable in the future. In this context, our predictions contribute to understand how the survival of trees and shrubs will be affected in the Cerrado in the future.

Flavonoids are a group of phenolic secondary metabolites in plants that have important physiological, ecological and economic value. In this study, using the desert plant Artemisia sphaerocephala Krasch. as the sample material, the content and components of the total flavonoids in its seeds at seven different developmental stages were determined. In addition, the genes involved in flavonoid metabolism were identified by full-length transcriptome sequencing (third-generation sequencing technology based on PacBio RS II). Their expression levels were analysed by RNA-seq short reading sequencing, to reveal the patterns and regulation mechanisms of flavonoid accumulation during seed development. The key results were as follows: the content of total flavonoids in mature seeds was 15.05 mg g−1, including five subclasses: flavonols, chalcones, flavones, flavanones and proanthocyanidins, among which flavonols accounted for 45.78%. The period of rapid accumulation of flavonoids was 40–70 d following anthesis. The high expression of phenylalanine ammonia-lyase (PAL), 4-coumarate-CoA ligase (4CL) and UDP-glucose:flavonoids 3-o-glucosyltransferase (UF3GT) promoted the accumulation of total flavonoids, while the high expression of flavonoids 3′-hydroxylase (F3′H) and flavonols synthase (FLS) made flavanols the main component. Transcription factors such as the MYB-bHLH-WDR (MBW) complex and Selenium-binding protein (SBP) directly regulated the structural genes of flavonoid metabolism, while C2H2-type zinc finger (C2H2), Zinc-finger transcription factor (GATA), Dehydration-responsive element binding (DREB), Global Transcription factor Group E protein (GTE), Trihelix DNA-binding factors (Trihelix) and Phytochrome-interacting factor (PIF) indirectly promoted the synthesis of flavonoids through hormones such as brassinoidsteroids (BRs) and abscisic acid (ABA). These results provided valuable resources for the application of related genes in genetics and breeding.

Seeds of Papilionoideae (Leguminosae) species lose water after dispersal, increasing the proportion of water-impermeable seeds (physical dormancy, PY). However, changes in testa structure during this process remain unknown. The present study investigated how variation in relative humidity (RH) during the post-dispersion period affects the proportion of seeds of Erythrina speciosa with PY. Seeds from two populations were stored in drier (40% RH) and wetter (80% RH) environments and periodically subjected to physiological analysis (seed water content, imbibition and germination) and structural analysis of the testa (light and scanning electron microscopy). Drier storage resulted in seed dehydration, increasing the proportion of water-impermeable seeds and closure of cracks in the mucilaginous stratum. In contrast, wetter storage led to an increase in seed water content, a decrease in the proportion of seeds with PY, and the formation of cracks. As a result of variation in environmental humidity, we conclude that changes occur in the mucilaginous stratum of seeds, altering water loss and, consequently, the proportion of seeds with PY. Environments with low humidity cause a decrease in seed water content, the closing of cracks in the mucilaginous stratum, and, consequently, an increase in the proportion of water-impermeable seeds. On the other hand, a high RH environment increases the seed water content and the formation of superficial cracks, through which water enters during imbibition, causing a decrease in the proportion of seeds with PY.