Hydrothermal sensitivities of seed populations underlie fluctuations of dormancy states in an annual plant community

Abstract

Plant germination ecology involves continuous interactions between changing environmental conditions and the sensitivity of seed populations to respond to those conditions at a given time. Ecologically meaningful parameters characterizing germination capacity (or dormancy) are needed to advance our understanding of the evolution of germination strategies within plant communities. The germination traits commonly examined (e.g., maximum germination percentage under optimal conditions) may not adequately reflect the critical ecological differences in germination behavior across species, communities, and seasons. In particular, most seeds exhibit primary dormancy at dispersal that is alleviated by exposure to dry after‐ripening or to hydrated chilling to enable germination in a subsequent favorable season. Population‐based threshold (PBT) models of seed germination enable quantification of patterns of germination timing using parameters based on mechanistic assumptions about the underlying germination physiology. We applied the hydrothermal time (HTT) model, a type of PBT model that integrates environmental temperature and water availability, to study germination physiology in a guild of coexisting desert annual species whose seeds were after‐ripened by dry storage under different conditions. We show that HTT assumptions are valid for describing germination physiology in these species, including loss of dormancy during after‐ripening. Key HTT parameters, the hydrothermal time constant (θHT) and base water potential distribution among seeds (Ψb(g)), were effective in describing changes in dormancy states and in clustering species exhibiting similar germination syndromes. θHT is an inherent species‐specific trait relating to timing of germination that correlates well with long‐term field germination fraction, while Ψb(g) shifts with depth of dormancy in response to after‐ripening and seasonal environmental variation. Predictions based on variation among coexisting species in θHT and Ψb(g) in laboratory germination tests matched well with 25‐yr observations of germination dates and fractions for the same species in natural field conditions. Seed dormancy and germination strategies, which are significant contributors to long‐term species demographics under natural conditions, can be represented by readily measurable functional traits underlying variation in germination phenologies.

Enhancing Conservation of a Globally Imperiled Rockland Herb (Linum arenicola) through Assessments of Seed Functional Traits and Multi-Dimensional Germination Niche Breadths

“…However, non-dormant, imbibed seeds exposed to warmer temperatures were not capable of germination because of thermo-inhibition. This type of post-after-ripening thermo-inhibition has been reported in other species and acts as a mechanism for non-dormant seeds to avoid germination during periods of elevated temperatures [57,59].…”

“…Similarly, conditional dormancy was reported in seeds of Linum catharticum [24]. Adjustments to the current storage treatments or application of other known dormancy-breaking treatments may be necessary to thoroughly alleviate dormancy in most L. arenicola non-deep physiological dormancy phenotypes [24,27,57,58] and permit more complete germination at permissive temperatures. It is also possible that the after-ripening treatment imposed on L. arenicola seeds in this study may have functioned in such a way as to alleviate dormancy and permit germination at cooler temperatures, which represent safe thermal conditions for germination of other Florida wildflowers [51,52].…”

Germination response of four Arabian desert grasses to low water potential

“…Limited rainfall results in low soil‐water availability, which affects seed germination (Liu et al 2020). Most desert species have specific mechanisms to ensure germination and seedling survival under such conditions (Wang et al 1998, Zeng et al 2010, Bateman et al 2016).…”

Ecological, (epi)genetic and physiological aspects of bet-hedging in angiosperms

“…For example, higher germination plasticity in a desert community can be achieved when the median value of base water potential, Ψ b (50), is lower and its standard deviation is higher. Indeed, larger water potential ranges allow larger differences in the germination fraction over different years or rain events within the same year, thus leading to plastic germination and bet-hedging strategies (Liu et al 2020). Moreover, high variance in year-to-year seed production per each seedling, hence higher variance in fitness, has been associated with small seeds and hydrothermal traits (Ψ and T) linked to slow and fractional germination (Huang et al 2016).…”