Population histories of reproductive failure and low winter precipitation correlate with risk-averse seed germination in a Mediterranean-climate winter annual

Description

Year-to-year variation in successful reproduction favors risk-averse seed-germination traits, such as dormancy that spreads risk across years and/or germination stimulated by low-risk conditions. Studies of risk-averse germination often consider whether traits vary with suspected climate drivers of failure. Rarely are long-term demographic records of population failure also available. Supported by nearly two decades of demographic and climate monitoring, we investigated whether germination traits in the California-endemic winter-annual Clarkia xantiana ssp. xantiana can be predicted by populations’ records of reproductive failure and climate. We submitted seeds of 10 populations—4 with histories of failure—to factorial treatments of 12 levels of water potential and 3 of temperature, analyzing variation in the base water potential required for 20% germination, in the proportion of viable seeds remaining ungerminated after 14 d (a dormancy index), and in the time to germinate weighted by water potential above base (hydrotime). Populations with a history of reproductive failures had higher base water potential, greater dormancy, and marginally shorter hydrotime. With temperature base water potential increased and hydrotime declined. Dormancy was lowest at the intermediate temperature. A multi-trait index of risk-averse germination declined with mean winter rainfall and increased with a history of reproductive failure. Finding that populations from areas that receive low rainfall and that have experienced reproductive failure exhibit more risk-averse germination suggests local adaptation. Such intraspecific niche variation may contribute to this species’ geographic distribution. Rapid evolution of germination traits may help maintain adaptation during climate change, buffering populations from extinction.

Steps to reproduce

Each population’s seeds came from bulked collections from approximately 5 fruits from each of approximately 25 haphazardly chosen parent plants per population in June 2020. Seeds sat dry in the dark at room temperature for 3 months, before dry, dark storage at 4 °C for 8 months, until the start of the experiment. Annual demographic monitoring began in 2005 on discrete populations in this area, accompanied by climate monitoring via a network of automated weather stations. Reproductive failure history, germination fractions, and seasonal means and CVs of precipitation came from these records. We varied water potential and temperature in a factorial design, recording seed germination regularly, to estimate parameters of the hydrotime model of seed germination. We created 36 treatments in all combinations of three temperatures (5, 10, and 15°C) and 12 levels of water potential (-0.33 to 0 MPa, in intervals of 0.03 MPa). Deionized water was the 0 MPa solution; we made 33 custom solutions by adding polyethylene glycol (PEG 8000, Sigma-Aldrich), adjusting concentrations to match assigned water potentials at each experimental temperature. Experimental units for each treatment combination were 10 x 10 cm square Petri dishes. To set up each unit, we saturated two sterile cotton balls and a square of germination blotting paper with 100 indentations (Seedburo Equipment Company, Des Plaines, IL, USA) with 25 mL of solution and placed them in the dish. Then we placed 10 seeds from each of the 10 populations into a grid. After sealing plates with Parafilm (Bemis, Neehah, WI), we placed them into one of three temperature chambers with broad-spectrum LEDs set on a 12-hour cycle. Every 12 hours for 14 days, we counted seeds with emergent radicles. After each census, we shifted the position of each plate in the chamber. After 14 d, we tested the viability of ungerminated seeds by squeezing them with forceps to force embryos from seed coats. (Seeds lacking intact embryos totaled less than 3%.) We replicated the entire experiment in time, randomizing the assigned temperatures of the chambers in between. Following the accumulation of germinating seeds over time allows estimation of parameters that describe a population’s seed-germination water relations (the hydrotime model): chiefly: the base water potential in MPa above which specified percentages of seeds germinate; and hydrotime, the time to germination after experiencing water potentials above the base, weighted by the difference between water potential experienced and the base water potential (units are MPa-h). Linear regressions of the inverse of time to 20% germination against the water potential estimated model parameters. The X-intercept of the regression line is the base water potential required to achieve 20% germination. The inverse of slope is hydrotime. We also estimated the percentage of ungerminated, viable seeds that remained after 14 d.

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