The environmental conditions under which parental plants are reared can affect the seed characteristics of the progeny. The variation originating from such maternal effects has rarely been incorporated into models of seed germination. Here, using Palmer amaranth (Amaranthus palmeri S. Watson), we examined the effects of water stress during the growth of parental plants on the progeny seed characteristics, including weight, size, final germination, and parameters of a hydrotime germination model. We grew two populations (from California and Kansas) under continuous water-deficit or well-watered conditions. In both A. palmeri populations, progeny seeds originating from water-stressed plants were heavier and larger than those from well-watered plants. Plants exposed to water stress also produced seeds that were ~30% less dormant than seeds from control plants. To test whether the maternal environment affects the parameters of a hydrotime model, progeny seeds were subject to five water potentials (0, −0.2, −0.4, −0.6, and −0.8 MPa) and incubated at 20 and 30 C; germination was monitored daily. The estimated median base water potential (Ψb(50)), that is, the water potential at which 50% of seeds cannot germinate, was consistently lower for seeds from water-stressed plants than for seeds from well-watered plants. Our results showed that A. palmeri plants experiencing drought during their growth produce seeds that are less dormant and can germinate from drier conditions—a maternal response that seems to be adaptive. These findings also call for development of germination models that incorporate the environmental conditions of both the current and past seasons to better describe the variability in germination of weed seeds.

The repetitive use of ALS inhibitors for smallflower umbrella sedge (Cyperus difformis L.) control has selected for herbicide-resistant (R) populations that threaten the sustainability of rice (Oryza sativa L.) production and demand alternative control measures be developed. A better understanding of seedling recruitment patterns at the field level is required to optimize the timing and efficacy of control measures. Therefore, a population-based threshold model was developed for optimizing germination prediction in multiple acetolactate synthase (ALS)-R and ALS-susceptible (ALS-S) C. difformis biotypes and applied to field-level emergence predictions. Estimated base temperatures (Tb) ranged from 16.5 to 17.6 C with no clear pattern between biotypes; such values are higher than Tb values of other important rice weeds, as well as for rice. Germination rates increased linearly from 16 to 33.7 C. ALS-R seeds germinate faster due to smaller median thermal times to germination (θT(50)) while also displaying lower germination synchronicity across water potentials. Interestingly, ALS-R biotypes were capable of germinating under lower moisture availability, as indicated by their lower (more negative) base water potential values (Ψb(50)) for seed germination; Ψb(50) values ranged from −0.24 to −1.13 MPa. In-field soil germination measurements found thermal times to emergence varied across three water regimes (daily water, flooded, or saturated). Seedling emergence under the daily water treatment was fastest; however, total seedling density was lower than for the other water regimes. In order to optimize springtime C. difformis seedling emergence, soil moisture should be kept around field capacity, as germination is hindered at lower moisture contents. By predicting when most of the seed population germinates, the thermal-time model can address issues regarding the optimal timing for herbicide applications, thereby allowing for improved C. difformis management in rice fields.

Mangroves are highly adapted to extreme environmental conditions that occur at the interface of salt and fresh water. Adaptations to the saline environment during germination are a key to mangrove survival, and thereby, its distribution. The main objective of this research was to study the effect of salinity on seed germination of selected mangrove species and the application of a hydrotime model to explain the relationship between water potential of the medium and rate of seed germination. Germination of seeds was examined at 15, 25 and 35°C in light/dark over a NaCl gradient. Germination time courses were prepared, and germination data were used to investigate whether these species behave according to the principles of the hydrotime model. The model was fitted for the germination of Acanthus ilicifolius seeds at 25°C. Final germination percentage was significantly influenced by species, osmotic potential and their interaction at 25°C. Moreover, temperature had a clear effect on seed germination (Sonneratia caseolaris and Pemphis acidula) which interacted with osmotic potential. Only A. ilicifolius seeds behaved according to the hydrotime principles and thus its threshold water potential was –1.8 MPa. Optimum germination rates for seeds of the other species occurred at osmotic potentials other than 0 MPa. The descending order of salinity tolerance of the tested species was Aegiceras corniculatum > Sonneratia caseolaris > Acanthus ilicifolius > Pemphis acidula > Allophylus cobbe, suggesting that the viviparous species (A. corniculatum) is highly salt tolerant compared with the non-viviparous species. The results revealed that seeds of the study species exhibited facultative halophytic behaviour in which they can germinate over a broad range of saline environments. Use of a hydrotime model for mangroves was limited as germination of their seeds did not meet model criteria.

For seed banks showing seasonal changes in their dormancy level, the possibility of predicting temporal patterns of emergence depends on establishing a robust relationship between temperature and the rate of dormancy loss and induction. However, although the effect of temperature on dormancy loss has been extensively studied, less work has been advocated to the quantification of temperature effects on dormancy induction. In the present work, we quantified temperature regulation of dormancy induction in Polygonum aviculare seeds using the hydrotime model. To study induction into secondary dormancy, seeds previously released from primary dormancy through stratification at 5°C were stored at dormancy-inductive temperatures of 10, 15, 20 and 25°C for different periods. During storage, seeds were germinated at different temperatures and water potentials, and hydrotime model parameters were derived. Changes in hydrotime model parameters (mean base water potential for germination and its standard deviation, and the hydrotime required for germination) during dormancy induction were described by adjusting exponential equations. Obtained results indicated a minimum temperature for dormancy induction of 8.7°C and the existence of a bi-linear relationship between rate of induction into secondary dormancy and storage temperature, in which storage temperatures around 25°C showed a higher dormancy induction rate than those below 20°C. Developed model equations were then used to predict changes in germination behaviour during dormancy induction at different temperatures, showing a good agreement between simulated and observed values.

Water availability regulates timing of germination, especially in those environments where it is a limiting factor. However, the water requirements for germination of most wild Mediterranean species are unknown. In this work we analysed the germination response to water stress of eight Cistaceae species with hard-coated seeds, which are typical for the Mediterranean shrublands. Seeds were exposed to a heat shock in order to overcome physical dormancy and then incubated under five water potentials (0, −0.2, −0.4, −0.6 and −0.8 MPa). Ungerminated seeds following these treatments were transferred to water in order to evaluate the recovery of germination. Additionally, at the end of the experiment, viability of still ungerminated seeds was examined. We analysed final germination percentage, time to reach 50% of the final germination (t 50), recovery of germination and seed viability in relation to water stress treatments. Furthermore, hydrotime analysis was performed; sensitivity to water stress, as characterized by Ψb(50), was checked in relation to seed size of species. Overall, water stress delayed and decreased germination but species showed different germination sensitivities, which were not related to seed size. Recovery of germination was high after disappearance of water stress but seed viability decreased in some species. Consequently, post-fire germination of Cistaceae must be strongly regulated by water availability and their seeds can recover germination capacity when sufficient water is available after long periods of incomplete hydration. However, some seeds die during the wait.

Although seed dormancy of temperate legumes is well understood, less is known about it in species that grow in subalpine/alpine areas. This study investigated dormancy and germination of four Vicia species from the Tibetan Plateau. Fresh seeds of V. sativa were permeable to water, whereas those of V. angustifolia, V. amoena and V. unijuga had physical dormancy (PY). One year of dry storage increased the proportion of impermeable seeds in V. angustifolia, but showed no effect on seed coat permeability in V. amoena or V. unijuga. Seeds of all four species also had non-deep physiological dormancy (PD), which was especially apparent in the two annuals at a high germination temperature (20°C). After 1 year of storage, PD had been lost. The hydrotime model showed that fresh seeds obtained a significantly higher median water potential [Ψb(50)] than stored seeds, implying that PD prevents germination in winter for seeds dispersed without PY when water availability is limited. After 6 months on the soil surface in the field, a high proportion of permeable seeds remained ungerminated, further suggesting that PD plays a key role in preventing germination after dispersal. Addition of fluridone, an inhibitor of abscisic acid (ABA) biosynthesis, evened-out the differences in germination between fresh and stored seeds, which points to the key role of ABA biosynthesis in maintaining dormancy. Further, fresh seeds were more sensitive to exogenous ABA than stored seeds, indicating that storage decreased embryo sensitivity to ABA. On the other hand, the gibberellic acid GA3 increased germination rate, which implies that embryo sensitivity to GA is also involved in seed dormancy regulation. This study showed that PY, PD or their combination (PY+PD) plays a key role in timing germination after dispersal, and that different intensities of dormancy occur among these four Vicia species from the Tibetan Plateau.

Seeds of Carduus acanthoides, Cynara cardunculus, Cirsium vulgare, Brassica campestris, and Sisymbrium altissimum were incubated at a range of decreasing osmotic potentials (Ψo) under fluctuating temperatures or the median temperature of the fluctuation cycle. Fluctuating temperatures promoted total seed germination in water and at reduced osmotic potential. Total germination was reduced as the Ψo decreased. However, this trend was smallest under fluctuating temperatures, signalling a higher tolerance of seeds to reduced osmotic potential. Effects of osmoticum and temperature were modelled with the hydrotime model. The parameters estimated from the model, the hydrotime constant (θH), the mean base water potential Ψb(50) and its standard deviation (σΨb) gave good descriptions of germination time courses. For all species, incubation under fluctuating temperatures shifted Ψb(50) values downwards without modifying their distribution substantially. This accounted for the greater tolerance of germination to reduced Ψo under fluctuating temperatures. To confirm that these effects were mediated by temperature fluctuations per se, the behaviour of C. acanthoides and C. cardunculus incubated at the minimum, the mean and the maximum temperature of the fluctuation cycle was also analysed. Constant maximum and minimum temperatures of the cycle did not stimulate germination, nor did they shift Ψb(50) towards more negative values. The hydrotime model provides a physiologically based quantitative description for germination promotion due to fluctuating temperature.

A model of seed germination is proposed which uses a variable with the units of an osmotic potential (virtual osmotic potential) to integrate the effect of a constant or a varying water potential. This differs from existing models that describe the effects of fixed water potentials on germination, or the effects of fixed priming water potentials on the subsequent germination at a fixed water potential. When a seed is sown, the virtual osmotic potential is assumed to fall at a rate that depends on the ambient water potential, and on the difference between its current and a minimum value. Radicle growth is assumed to initiate when the difference between the ambient water potential and the virtual osmotic potential exceeds a threshold. The germination of carrot and onion seeds at various fixed potentials below 0 MPa was well described by the virtual osmotic potential model. The model was also used to simulate the results of experiments in which seeds were given a single step change in water potential.