Seed Germination Ecology of the Cold Desert Annual Isatis violascens (Brassicaceae): Two Levels of Physiological Dormancy and Role of the Pericarp

Aethionema arabicum dimorphic seed trait resetting during transition to seedlings

The transition from germinating seeds to emerging seedlings is one of the most vulnerable plant life cycle stages. Heteromorphic diaspores (seed and fruit dispersal units) are an adaptive bet-hedging strategy to cope with spatiotemporally variable environments. While the roles and mechanisms of seedling traits have been studied in monomorphic species, which produce one type of diaspore, very little is known about seedlings in heteromorphic species. Using the dimorphic diaspore model Aethionema arabicum (Brassicaceae), we identified contrasting mechanisms in the germination responses to different temperatures of the mucilaginous seeds (M+ seed morphs), the dispersed indehiscent fruits (IND fruit morphs), and the bare non-mucilaginous M- seeds obtained from IND fruits by pericarp (fruit coat) removal. What follows the completion of germination is the pre-emergence seedling growth phase, which we investigated by comparative growth assays of early seedlings derived from the M+ seeds, bare M- seeds, and IND fruits. The dimorphic seedlings derived from M+ and M- seeds did not differ in their responses to ambient temperature and water potential. The phenotype of seedlings derived from IND fruits differed in that they had bent hypocotyls and their shoot and root growth was slower, but the biomechanical hypocotyl properties of 15-day-old seedlings did not differ between seedlings derived from germinated M+ seeds, M- seeds, or IND fruits. Comparison of the transcriptomes of the natural dimorphic diaspores, M+ seeds and IND fruits, identified 2,682 differentially expressed genes (DEGs) during late germination. During the subsequent 3 days of seedling pre-emergence growth, the number of DEGs was reduced 10-fold to 277 root DEGs and 16-fold to 164 shoot DEGs. Among the DEGs in early seedlings were hormonal regulators, in particular for auxin, ethylene, and gibberellins. Furthermore, DEGs were identified for water and ion transporters, nitrate transporter and assimilation enzymes, and cell wall remodeling protein genes encoding enzymes targeting xyloglucan and pectin. We conclude that the transcriptomes of seedlings derived from the dimorphic diaspores, M+ seeds and IND fruits, undergo transcriptional resetting during the post-germination pre-emergence growth transition phase from germinated diaspores to growing seedlings.

Understanding the Impact of Sheep Digestion on Seed Germination in the Cold Desert Annual Diptychocarpus strictus with Emphasis on Fruit and Seed Heteromorphism

This study aimed to investigate the morphological characteristics of fruits and seeds from Diptychocarpus strictus, a plant species inhabiting the cold desert pastoral area of China. Furthermore, this study sought to evaluate the germination potential of these seeds following digestion by sheep. This study employed the sheep rumen fistula method to simulate rumen digestion at various time intervals. Subsequently, an in vitro simulation method was utilized to simulate true gastric and intestinal digestion after rumen digestion. Paper germination tests were then conducted to assess the impact of the digestive process on the heteromorphic seed morphology and germination. During rumen digestion, the seeds were protected by wide wings. The results revealed a highly significant negative correlation (p < 0.01) between seed wing length and digestion time. Post-rumen digestion, variations in the germination rate among seeds from fruits at different locations were observed. Indicators, such as germination rate, exhibited a highly significant negative correlation with rumen digestion time (p < 0.01). In vitro simulated digestion tests demonstrated that Diptychocarpus strictus seeds retained their ability to germinate even after complete digestion within the livestock's digestive tract. The polymorphic nature of Diptychocarpus strictus seeds, coupled with their capacity to survive and germinate through the digestive tract, facilitates the spread of these seeds. This finding has implications for mitigating desert grassland degradation and promoting sustainable ecological development.

Is the Life History Flexibility of Cold Desert Annuals Broad Enough to Cope with Predicted Climate Change? The Case of Erodium oxyrhinchum in Central Asia

Interannual seasonal variability in precipitation may strongly affect the life history and growth of desert annual plants. We compared the effects of dry and wet springs and dry and wet autumns on growth and F2 seed dormancy of plants from spring (SG)- and autumn (AG)-germinated seeds of the cold desert annual Erodium oxyrhinchum. Vegetative and reproductive growth and F2 seed dormancy and germination were monitored from September 2016 to November 2020 in the sandy Gurbantunggut Desert in NW China in Central Asia. Dry autumns decreased the density of AG plants, and dry springs decreased the density of SG plants and growth of SG and AG plants. In dry springs, SG plants were more sensitive to precipitation than AG plants, while in wet springs SG and AG plants had similar responses to precipitation. During growth in both dry and wet springs, most morphological characters of SG and AG plants initially increased rapidly in size/number and then plateaued or decreased, except for SG plants in dry springs. In dry springs, most morphological characters of AG plants were larger or more numerous than those of SG plants, and they were larger/more numerous for SG plants in wet than in dry springs. The percentage biomass allocated to reproduction in SG plants was slightly higher in a wet than in a dry spring. A much higher proportion of dormant seeds was produced by AG plants in a wet spring than in a dry spring. Projected changes in precipitation due to climate change in NW China are not likely to have much of an effect on the biology of this common desert annual plant.

Breaking Seed Dormancy during Dry Storage: A Useful Tool or Major Problem for Successful Restoration via Direct Seeding?

To facilitate the restoration of disturbed vegetation, seeds of wild species are collected and held in dry storage, but often there is a shortage of seeds for this purpose. Thus, much research effort is expended to maximize the use of the available seeds and to ensure that they are nondormant when sown. Sowing nondormant (versus dormant) seeds in the field should increase the success of the restoration. Of the various treatments available to break seed dormancy, afterripening, that is, dormancy break during dry storage, is the most cost-effective. Seeds that can undergo afterripening have nondeep physiological dormancy, and this includes members of common families such as Asteraceae and Poaceae. In this review, we consider differences between species in terms of seed moisture content, temperature and time required for afterripening and discuss the conditions in which afterripening is rapid but could lead to seed aging and death if storage is too long. Attention is given to the induction of secondary dormancy in seeds that have become nondormant via afterripening and to the biochemical and molecular changes occurring in seeds during dry storage. Some recommendations are made for managing afterripening so that seeds are nondormant at the time for sowing. The most important recommendation probably is that germination responses of the seeds need to be monitored for germinability/viability during the storage period.

Ecological significance of floristic composition and life forms of Riyadh region, Central Saudi Arabia

Riyadh region is one of the hardest habitats in Saudi Arabia with a hyper-arid climate. This study was conducted to investigate floristic composition and plant life forms of Riyadh region and their ecological significance. Work aimed to determine the prevailing plant families and biological spectrum of their components that reflects the phytoclimate and adaptation to hyper-arid conditions of the region. Work involved field surveys of different locations in Riyadh region where plant specimens were collected and identified. Collected plant species were then listed according to their families and data were used to assess the contribution of different plant families to the flora of the region. Study of life forms was conducted to classify recorded floristic elements into categories that reflect environmental conditions prevailing in the region. Prevailing plant families were Asteraceae (17.4%), Poaceae (11%), Brassicaceae (9.9%), and Fabaceae (7%). These percentages reflect wide ecological ranges especially for Asteraceae and Poaceae. High presence of species belonging to Astreaceae and Poaceae can be attributed to their adaptation to harsh conditions as well as to effective wind dispersal strategies of their diaspores. While the most frequent life form classes were therophytes with 52%, and chamaephytes with 30%, phanerophytes and hemicryptophytes represented 9% and 8%, respectively. Predominance of therophytes and chamaephytes over other life forms is a response to hyper arid climate with insufficient rainfall and the nature of region of few available microhabitats that can support high percentage of perennials.

Dew Yield and Its Influencing Factors at the Western Edge of Gurbantunggut Desert, China

Dew is a significant water resource in arid desert areas. However, information regarding dew is scarce because it is difficult to measure due to the harsh environment of locations such as Gurbantunggut Desert, China. In this study, a non-destructive field experiment was conducted from 2015 to 2018 at a desert test station located in the western edge of the Gurbantunggut Desert, using a calibrated leaf wetness sensor (LWS) to measure dew yield. The results are as follows: (1) Dew formed after sunset with the atmospheric temperature gradually dropping and evaporated after sunrise with the temperature increasing in the second morning. (2) Dew was featured as ‘high frequency and low yield’. The average daily dew yield during dew days was 0.10 mm with a daily maximum of 0.62 mm, while dew days accounted for 44% of the total monitoring days, with a monthly maximum of 25 days. Compared with rainfall, dew days were two times as frequent as rainy days, while the average annual dewfall (12.21 mm) was about 1/11th of the average annual rainfall (134.6 mm), which indicates the dew contribution to regional water balance is about 9%. (3) March–April and October–November are the main periods of dew occurrence in this region because accumulated snow begins to melt slowly in March–April, providing sufficient vapor for dew formation, and the air temperature difference between day and night in October–November is the highest in the year, meaning that the temperature drops rapidly at night, making it easier to reach the dewpoint for vapor condensation. (4) Daily dew yield (D) was positively correlated to relative humidity (RH) and the difference between soil temperature at 10 cm below the ground and surface soil temperature (Tss), and negatively correlated to wind speed (V), air temperature (Ta), surface soil temperature (Ts), cloud cover (N), dewpoint temperature (Td) and the difference between air temperature and dewpoint temperature (Tad). It should be noted that the measured values of all factors above were the average value of the overnight period. The multivariate regression equation, D = −0.705 + 0.011 ×RH− 0.006 ×N− 0.01 × V, can estimate the daily dew yield with the thresholds of the parameters, i.e., RH > 70%, N < 7 (oktas) and V < 6 m/s.

Effects of increased precipitation on the life history of spring- and autumn-germinated plants of the cold desert annual Erodium oxyrhynchum (Geraniaceae)

Future increased precipitation in cold desert ecosystems may impact annual/ephemeral plant species that germinate in both spring and autumn. Our primary aim was to compare the life history characteristics of plants from spring-germinating (SG) and autumn-germinating (AG) seeds of Erodium oxyrhynchum. Plants in field plots with simulated increases in precipitation of 0, 30 and 50 % in spring and summer were monitored to determine seedling survival, phenology, plant size, seed production and biomass accumulation and allocation. Germination characteristics were determined in the laboratory for seeds produced by plants in all increased precipitation treatments. Increased precipitation in spring significantly improved survival of seedlings from SG and AG seeds, but survival was less for AG than SG. An increase in precipitation increased the number of seeds per plant for both SG and AG, but AG produced more seeds per plant than SG. With increased precipitation, percentage of dormant seeds from SG increased significantly, while that of AG decreased slightly. Our study suggests that with increased spring and summer rainfall AG will produce an increased number of nondormant seeds that could germinate in autumn and SG an increased number of dormant seeds that become part of the soil seed bank. However, ability of some seeds to germinate in autumn and others in spring will be maintained as long as soil moisture is limited in autumn.

Life history responses of spring-and autumn-germinated ephemeral plants to increased nitrogen and precipitation in the Gurbantunggut Desert

Nitrogen deposition and precipitation change are not only hot topics of current global change but also the main environmental factors affecting plant growth. Thus, the effects of nitrogen and precipitation on the life history of spring-(SG) and autumn-germinated (AG) ephemeral plants of Erodium oxyrhynchum were researched in the Gurbantunggut Desert, northern China, and the aim was to understand the response of plants from different germination seasons to global change. SG and AG plants with increased nitrogen and precipitation plus nitrogen treatments were measured to determine seedling survival, phenology, plant traits, biomass accumulation and allocation and dormancy characteristics of offspring (seeds). The results showed that increased nitrogen and precipitation plus nitrogen treatments significantly improved the survival of SG and AG plants during the seedling stage, and precipitation plus nitrogen treatments also improved the growth and seed production of SG and AG plants, but increased nitrogen significantly inhibited their growth and seed production. Therefore, precipitation plays an important role in regulating nitrogen uptake by plants in arid and semiarid ecosystems. With increased nitrogen, SG and AG plants allocated more biomass into root and reproductive organs but allocated significantly less biomass into the leaf, with almost no change in biomass allocation to the stem. With nitrogen plus precipitation treatments, biomass allocation in all organs of SG and AG plants showed almost no change. Clearly, changes in soil moisture also affected biomass allocation of SG and AG plants. For offspring dormancy, SG and AG plants produced more nondormancy seeds with increased nitrogen but produced more dormancy seeds under precipitation plus nitrogen treatments. Hence, in a harsh environment, SG and AG plants produced more nondormancy offspring with low reproduction in order to occupy the habitat rapidly in the following year or produced more dormancy offspring with high reproduction in a suitable environment intended for spreading germination risk in time and conserve the population.

A role for fruit structure in seed survival and germination of Swartzia langsdorffii Raddi beyond dispersal

Diaspore structure has been hypothesised to play a role in seed viability and/or germination of recalcitrant seeds, especially for Swartzia langsdorffii. Thus, this work aims to (i) investigate the in situ contribution of pericarp and aril on seed viability and germination, and (ii) identify morphoanatomical traits of S. langsdorffii diaspores that allow its desiccation-sensitive seeds to remain viable. The role of the pericarp and aril in seed survival and germination was investigated by placing the whole fruit, whole seeds (arillate seed) and bare seeds (without aril) in soil in the forest understorey, assessing germination, emergence, dead, firm and predated seeds, and water content of pericarps, arils and seeds. Correlation analysis was performed between environmental variables and physiological parameters. Histochemical features of diaspores were also investigated. Pericarp water content fell after several months, while the aril maintained its water content. Seeds did not lose water even without the presence of the pericarp and aril. However, presence of the pericarp promoted seed water content, viability and germination long after dispersal. The embryo had a thickened outer periclinal cell wall. Pericarp and aril are not essential to prevent water loss in seeds, but do help to retain seed moisture, favouring viability maintenance and promoting germination during the rainy season. Morphoanatomical features of seeds are suggested as main factors that reduce water loss. Survival of these desiccation-sensitive seeds upon dispersal during the dry season appears to be facilitated by multiple diaspore features that prevent viability loss.

Fruit fracture biomechanics and the release of Lepidium didymum pericarp-imposed mechanical dormancy by fungi

The biomechanical and ecophysiological properties of plant seed/fruit structures are fundamental to survival in distinct environments. Dispersal of fruits with hard pericarps (fruit coats) encasing seeds has evolved many times independently within taxa that have seed dispersal as their default strategy. The mechanisms by which the constraint of a hard pericarp determines germination timing in response to the environment are currently unknown. Here, we show that the hard pericarp of Lepidium didymum controls germination solely by a biomechanical mechanism. Mechanical dormancy is conferred by preventing full phase-II water uptake of the encased non-dormant seed. The lignified endocarp has biomechanically and morphologically distinct regions that serve as predetermined breaking zones. This pericarp-imposed mechanical dormancy is released by the activity of common fungi, which weaken these zones by degrading non-lignified pericarp cells. We propose that the hard pericarp with this biomechanical mechanism contributed to the global distribution of this species in distinct environments.

Effect of Seed Position on Parental Plant on Proportion of Seeds Produced with Nondeep and Intermediate Physiological Dormancy

The position in which seeds develop on the parental plant can have an effect on dormancy-break and germination. We tested the hypothesis that the proportion of seeds with intermediate physiological dormancy (PD) produced in the proximal position on a raceme of Isatis violascens plants is higher than that produced in the distal position, and further that this difference is related to temperature during seed development. Plants were watered at 3-day intervals, and silicles and seeds from the proximal (early) and distal (late) positions of racemes on the same plants were collected separately and tested for germination. After 0 and 6 months dry storage at room temperature (afterripening), silicles and seeds were cold stratified for 0-16 weeks and tested for germination. Mean daily maximum and minimum temperatures during development/maturation of the two groups of seeds did not differ. A higher proportion of seeds with the intermediate level than with the nondeep level of PD was produced by silicles in the proximal position than by those in the distal position, while the proportion of seeds with nondeep PD was higher in the distal than in the proximal position of the raceme. The differences were not due only to seed mass. Since temperature and soil moisture conditions were the same during development of the seeds in the raceme, differences in proportion of seeds with intermediate and nondeep PD are attributed to position on parental plant. The ecological consequence of this phenomenon is that it ensures diversity in dormancy-breaking and germination characteristics within a seed cohort, a probable bet-hedging strategy. This is the first demonstration of position effects on level of PD in the offspring.

Delayed dehiscence of the pericarp: role in germination and retention of viability of seeds of two cold desert annual Brassicaceae species

Considerable variation occurs in post-maturity timing of dehiscence in fruits of Brassicaceae species, and several studies have shown that the pericarp plays an important role in seed germination and retention of viability in species with indehiscent fruits. However, little is known about the significance to seed biology of delay in pericarp dehiscence for <1 year in the field. Thus, we determined the role of the pericarps of Leptaleum filifolium and Neotorularia korolkovii, which open in <1 year after fruit maturity and dispersal, in seed germination and retention of seed viability. We compared dormancy-break via after-ripening in the laboratory and germination phenology and retention of seed viability in intact siliques and isolated seeds buried in an experimental garden. Seeds of both species have Type 6 non-deep physiological dormancy, which is enhanced by the pericarp. Seeds of both species after-ripened during summer 2013, and some of them germinated in autumn and some in the following spring in watered and non-watered soil. Germination percentages of seeds in siliques increased in soil in spring 2014, after the pericarps had opened. Most isolated seeds of L. filifolium and N. korolkovii had germinated or were dead by spring 2014 and summer 2015, respectively, whereas 60% of the seeds of both species in the (opened) pericarps were viable after 24 months. Thus, although the pericarp opened 9-10 months after burial, its presence had a significant effect on seed dormancy, germination phenology and retention of viability of seeds of L. filifolium and N. korolkovii.

Effects of germination season on life history traits and on transgenerational plasticity in seed dormancy in a cold desert annual

The maternal environment can influence the intensity of seed dormancy and thus seasonal germination timing and post-germination life history traits. We tested the hypotheses that germination season influences phenotypic expression of post-germination life history traits in the cold desert annual Isatis violascens and that plants from autumn- and spring-germinating seeds produce different proportions of seeds with nondeep and intermediate physiological dormancy (PD). Seeds were sown in summer and flexibility in various life history traits determined for plants that germinated in autumn and in spring. A higher percentage of spring- than of autumn-germinating plants survived the seedling stage, and all surviving plants reproduced. Number of silicles increased with plant size (autumn- > spring-germinating plants), whereas percent dry mass allocated to reproduction was higher in spring- than in autumn-germinating plants. Autumn-germinating plants produced proportionally more seeds with intermediate PD than spring-germinating plants, while spring-germinating plants produced proportionally more seeds with nondeep PD than autumn-germinating plants. Flexibility throughout the life history and transgenerational plasticity in seed dormancy are adaptations of I. violascens to its desert habitat. Our study is the first to demonstrate that autumn- and spring-germinating plants in a species population differ in proportion of seeds produced with different levels of PD.