Semiannual dormancy cycling results in two seedling cohorts of annual species in the cold desert of Central Asia
Data files
Jan 22, 2025 version files 189.08 KB
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Guo_et_al._data.xlsx
184.79 KB
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README.md
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Abstract
Our aim was to quantify the life history strategies of the cold desert annual species Alyssum linifolium and Tetracme quadricornis (Brassicaceae), with particular emphasis on the seed stage. Freshly-matured seeds were tested for germination over a range of alternating temperature regimes in light and in dark, and the effects of seed coat scarification, cold stratification, dry after-ripening and GA3 on dormancy-break and germination were determined. Seeds were buried in the field at 0 (surface), 2 and 5 cm and germination of exhumed seeds tested at monthly intervals for 2 years. Seedling emergence of both species was monitored in the cold desert, and survival to maturity of plants from autumn- and spring-germinated cohorts was determined. Most fresh seeds were dormant, and dormancy was broken by all four treatments tested. In the early stages of dormancy-break, seeds germinated to low percentages over the range of temperatures, and with additional dormancy-break germination percentages increased. Thus, seeds have Type 6 nondeep physiological dormancy (PD). In the buried seed study, dormancy-break occurred in summer (June to August), and germination peaked in late summer. By late autumn (November), all non-germinated seeds had re-entered dormancy. During snowmelt in late winter (February to March), some dormancy-break occurred, and low percentages of seeds of both species germinated. In the field, seeds of both species germinate in autumn and in spring, with more seeds of A. linifolium germinating in autumn than in spring and more seeds of T. quadricornis germinating in spring than in autumn. A portion of plants from both seedling cohorts of both species survived to maturity (set seeds) with more spring- than autumn-germinating plants doing so for both species. Thus, both species behave as facultative winter annuals.
Synthesis: Seeds of A. linifolium and T. quadricornis have two dormancy cycles per year. In one cycle, dormancy is broken via warm temperatures in summer and in the other one via cold stratification in late winter. Semiannual dormancy cycling results in two germination cohorts in one year, and it may be a bet-hedging strategy in the rainfall-unpredictable cold desert environment.
README: Semiannual dormancy cycling results in two seedling cohorts of annual species in the cold desert of Central Asia
https://doi.org/10.5061/dryad.z34tmpgqp
Description of the data and file structure
This dataset contains original data related to the seed size, mass, viability, germination, dormancy-breaking treatments, and field emergence of A. linifolium and T. quadricornis. The data includes measurements of seed dimensions, viability assessments using TTC tests, germination percentages under various conditions, and the effects of different treatments on seed dormancy.
Files and variables
File: Guo_et_al._data.xlsx
Description: This dataset includes comprehensive data on seed characteristics (seed size, mass of 1000 seeds and seed viability), germination percentage, dormancy-breaking treatments, buried seeds, soil temperature and moisture conditions, and field emergence for two species: A. linifolium and T. quadricornis.
Variables
variables | units | description |
---|---|---|
seed_size | mm | Length, width and thickness of 20 randomly selected seeds of each species were measured |
mass_of_1000_seeds | g | Five replicates of 1000 seeds of each species were weighed |
seed_viability | % | Percentage of viable seeds |
initial_germination | % | Germination percentages across four temperature regimes and light conditions for the two species |
GA3 | % | Effects of varying GA3 concentrations and four temperature regimes on germination percentages |
seed_coat_scarification | % | Effect of seed coat scarification on germination percentages of fresh seeds at different temperature regimes in light. |
cold_stratification | % | Effect of cold stratification on germination percentages of freshly mature seeds at different temperature regimes in light. |
dry_storage | % | Effect of dry storage at different temperatures on germination percentages of seeds at different temperature regimes. |
buried_seeds | % | Germination percentage of seeds at different temperature regimes in light and continuous darkness following 0–24 months of burial in the natural habitat |
depletion_of_soil_seed_banks | % | The percentages of remaining viable seeds at different depths |
emergence_and_survival_of_AG | number | AG,autumn-germinated (AG) seedlings; the emergence/survival counts of the two species across different months in designated plots |
emergence_and_survival_of_SG | number | SG, spring-germinated (SG) seedlings; the emergence/survival counts of the two species across different months in designated plots |
- soil_temperature_and_moisture.xlsx
- Records soil moisture (%), maximum and minimum temperatures (°C) at different depths (0 cm, 2 cm, and 5 cm) over time from June 2021 to June 2023.
Code/software
This dataset can be analyzed using the following free and open-source software:
- R (version 3.3.2 or later)
- Perform an analysis of variance (ANOVA) followed by Duncan testing to analyze differences between groups.
Methods
Seed size, mass and viability
Length, width and thickness of 20 randomly selected seeds of each species were measured using an Olympus SZX16 microscope with a micrometer in one eye piece. Five replicates of 1000 seeds of each species were weighed with a Sartorius BP 221 S analytical balance (0.0001 g) and mean seed mass calculated. TTC (2, 3, 5- triphenyl tetrazolium chloride) tests were used to assess seed viability. Four replications of 100 fresh seeds were incubated in 5.5-cm-diameter Petri dishes containing 2 ml aqueous sterile solutions of 1% (w/v) TTC for 24 h at 20 °C in darkness. Seeds with an embryo that stained red were recorded as viable.
Germination of fresh seeds
Freshly matured seeds collected in 2021 were tested for germination. Four replicates of 50 seeds of each of the two study species were placed in 5.5-cm-diameter plastic Petri dishes on two layers of Whatman No. 1 filter paper moistened with 2 ml distilled water. Petri dishes were sealed with Parafilm and incubated at daily (12/12 h) temperature regimes of 5/20, 10/25, 15/30 and 20/35 °C in light (12 h daily photoperiod c.100 μmol m−2 s−1, cool white fluorescent light) or in continuous darkness by wrapping dishes of seeds with aluminum foil. The four temperature regimes approximate the mean daily minimum and maximum air temperatures for different months during the growing season (including the seed dispersal period in June) in the natural habitat: April and October, 5/20 °C; May and September, 10/25 °C; June and August, 15/30 °C; and July, 20/35 °C. Germination was recorded daily for 30 days, and treatments in darkness were checked using a green LED light with a spectral output between 515 - 530 nm and peaking at 520 nm (Vanq, Shenzhen, China); photon irradiance at seed level was <10 μmol m−2 s−1. Emergence of the radicle through the micropylar endosperm and seed coat was the criterion for germination. Following a test, each nongerminated seed was pinched with forceps to determine if it contained a firm, white embryo (indicating viability) or a soft, grey embryo (indicating non-viability) (Baskin & Baskin, 2014).
Dormancy-breaking treatments
The effects of gibberellic acid (GA3), seed coat scarification, cold stratification and dry storage on dormancy-break/ germination of fresh (10 days old) seeds collected in 2021 were tested.
GA3. Four replicates of 50 freshly matured seeds of each species were tested for germination in 0 (CK), 0.01, 0.1, 1 and 10 mmol L-1 GA3 solutions at 5/20, 10/25, 15/30 and 20/35 °C (12/12 h) in light/ darkness (12/12 h, hereafter light) or in continuous darkness. GA3 was dissolved in 2 ml of 75% ethanol (including the control), which was then diluted with distilled water to prepare a total solution volume of 100 ml at the final concentrations. The ethanol content of the solution was very low and thus it is unlikely to have had an effect on seed germination. Germination was recorded daily for 30 days, and viability of nongerminated seeds was tested at the end of the experiment.
Seed coat scarification. To test whether the (water-permeable) seed coat mechanically restricts embryo growth, the seed coat of four replicates of 50 freshly matured seeds of each species was scarified using fine sandpaper, being careful not to damage the embryo. Scarified and intact seeds (freshly matured seeds) were tested for germination in light at 5/20, 10/25, 15/30 and 20/35 °C.
Cold stratification. Freshly matured (10-day-old) seeds were placed on two layers of filter paper in a metal box (20 cm length × 10 cm width × 10 cm depth) containing wet sand (12% distilled water content, wet weight basis). The metal box was covered with a lid and kept in darkness in a refrigerator at 4°C. After 0 (CK), 1, 2, 4, 10 and 12 week(s) of cold stratification, four replicates of 50 seeds each were arbitrarily chosen to test germination in light at 5/20, 10/25, 15/30 and 20/35 °C.
Dry storage. To determine the effect of after-ripening on seed dormancy-break, freshly matured (10-day-old) seeds were dry stored in paper bags at (1) room temperature (15-25 °C, 30-50 % relative humidity) for 0, 1, 2.5, 4 and 8 months, (2) high temperature (20-35 °C, 30-50 % relative humidity) for 0, 1, 2.5 and 4.5 months and (3) a greenhouse in summer (30-45 °C, 30-55 % relative humidity) for 0, 0.5, 1 and 2 months. After each period and condition of storage, four replications of 50 seeds of each species were arbitrarily chosen and tested for germination in light at 5/20, 10/25, 15/30 and 20/35 °C.
Effect of seed burial depth in field on dormancy-break and germination
Following collection in 2021, about 1500 fresh matured (10-day-old) seeds each of the two species were put into nylon-mesh bags (5 cm length × 3 cm width). These bags were placed at soil depths of 0 (soil surface), 2 and 5 cm on 9 June 2021, in a stable sandy area (rather than on mobile sand dunes) to prevent wind accumulation of sediments, thereby altering soil thickness above the seeds. In total, we prepared 144 seed bags: 24 months × 3 depths × 2 species = 144 bags. Soil temperature and moisture at depths of 0, 2 and 5 cm were recorded hourly by a data collector (Em50 METER Co. Ltd, USA). One seed bag from each depth of each species was exhumed at monthly intervals from July 2021 to June 2023. At the time of sampling each month, we inspected all the seed bags on the soil surface and recorded the number of seeds that had germinated in them during that month. In subsequent monthly samplings, we recorded the number of viable and non-germinated seeds and seeds that germinated in the current month. The sum of these data (previously recorded germinated seeds, viable non-germinated seeds and the current month’s germinated seeds) equaled the initial count of 1500 buried seeds. This accounting method ensured that we could confidently rule out the possibility of seed decay being counted as germination.
From the seed bags excavated each month, the viable non-germinated seeds buried in the soil were separated from soil (sand) particles and tested in light and in dark at 5/20, 10/25, 15/30 and 20/35 °C to simulate the response of seeds to seasonal temperature conditions, thereby accurately reflecting their dormancy status. Following the methods of Baskin and Baskin (1983, 1986, 2014) and Cao et al. (2012, 2014) for soil-buried seed dormancy cycling experiments, the seeds from each bag were divided into replicates. Specifically, four replicates of 25 seeds (or one-fourth of the remaining seeds if fewer than 200) were used in each test. The percentages of A. linifolium and T. quadricornis seeds remaining in the soil seedbank over time during a 2-year period were calculated.
Field emergence and survival of plants to maturity
In October 2021, ten 1 m × 1 m plots were arbitrarily established in the southern edge of the Gurbantunggut Desert where A. linifolium and T. quadricornis were widely distributed. Newly emerged seedlings of A. linifolium and T. quadricornis were labelled, and the date was recorded when they were first found. Number of surviving seedlings/plants was recorded in each of the 10 plots in October 2021 and March and May 2022, which represent emergence of the autumn-germinated (AG) seedlings, emergence of the spring-germinated (SG) seedlings and completion of the life history of adult plants, respectively. Using number of AG and SG seedlings and number of adult plants that had completed their life cycle, we calculated the survival percentages of AG and SG plants.
Statistical analysis
All data analyses were carried out in R software (v3.3.2, R Core Team 2016). The response variables were expressed as the mean ± SE (non-transformed data appear in all figures and tables). We calculated the SE using the formula , where SD is the standard deviation of the germination percentages and n is the number of replicates. Germination percentages were calculated as (NG/NT) × 100, where NG is the number of seeds that germinated and NT is the total number of viable seeds tested. A general linear model (GLM) with binomial family (‘logit’, var (y)=log (y/ [1- y])) model (Warton & Hui, 2011) was used to test the effects of different experimental treatments, including temperature, time and light condition, on seed germination, followed by multiple comparison with Tukey’s HSD tests using the ‘glht’ function of the ‘multcomp’ package. Level of significance was set at P < 0.05.