The risk-takers and -avoiders: germination sensitivity to water stress in an arid zone with unpredictable rainfall

Abstract

Water availability is a critical driver of population dynamics in arid zones, and plant recruitment is typically episodic in response to rainfall. Understanding species’ germination thresholds is key for conservation and restoration initiatives. Thus, we investigated the role of water availability in the germination traits of keystone species in an arid ecosystem with stochastic rainfall. We measured seed germination responses of five arid species, along gradients of temperature and water potential under controlled laboratory conditions. We then identified the cardinal temperatures and base water potentials for seed germination, and applied the hydrotime model to assess germination responses to water stress. Optimum temperatures for germination ranged from 15 to 31 °C under saturated conditions (0 MPa), and three species had low minimum temperatures for germination (<3 °C). A small proportion of seeds of all species germinated under dry conditions (Ψ ≤ −1 MPa), although base water potential for germination (Ψ b50) ranged from −0.61 to −0.79 MPa. Species adhered to one of two germination traits: (i) the risk-takers which require less moisture availability for germination, and which can germinate over a wider range of temperatures irrespective of water availability (Casuarina pauper and Maireana pyramidata), and (ii) the risk-avoiders which have greater moisture requirements, a preference for cold climate germination, and narrower temperature ranges for germination when water availability is low (Atriplex rhagodioides, Maireana sedifolia and Hakea leucoptera). High seed longevity under physiological stress in H. leucoptera, combined with a risk-avoiding strategy, allows bet-hedging. The hydrotime model predicted lower base water potentials for germination than observed by the data, further supporting our assertion that these species have particular adaptations to avoid germination during drought. This study provides insights into the complex physiological responses of seeds to environmental stress, and relates seed germination traits to community dynamics and restoration in arid zones.

Physical Dormancy Release in Medicago truncatula Seeds Is Related to Environmental Variations

“…In addition, species that frequently and reliably produce seed can afford riskier germination under unfavorable conditions (e.g., small rainfall events) because the consequences of failure to establish are less dire than for species that do not reliably produce seed [17]. Desert annuals that do not frequently and reliably reproduce are model organisms for the study of the bet-hedging strategy [18].…”

Evolutionary divergence of potential drought adaptations between two subspecies of an annual plant: Are trait combinations facilitated, independent, or constrained?

“…Subspecies parviflora does not appear, however, to sacrifice other potential drought adaptations, as it has slower seed germination than xantiana on a saturated substrate, and it is more succulent than xantiana at the flowering stage. Longer time requirements for germination may confer drought avoidance, reducing the likelihood of germination in places and times where soil water is insufficient to complete a life cycle (Duncan et al, 2019a) and increasing the likelihood of establishing persistent seed banks that spread the risk of reproductive failure due to terminal drought (Saatkamp et al, 2011). Succulence may allow parviflora to delay, for a time, the effects of soil water deficit.…”

“…Where precipitation is ephemeral (e.g., some deserts) or seasonal (e.g., Mediterranean climates), several other strategies can provide drought adaptation (Kooyers, 2015). Such plants can escape approaching drought by reproducing early (Volis et al, 2002; Sherrard and Maherali, 2006) or avoid drought altogether by producing seeds that do not germinate without prolonged exposure to saturated soil, a cue to sustained water availability (Kos and Poschlod, 2010; Zeng et al, 2010; Duncan et al, 2019a, b). Seed dormancy can create persistent seed banks that spread the risk of encountering severe drought (and reproductive failure) among years that vary in precipitation (Cohen, 1966; Venable and Lawlor, 1980; Adondakis and Venable, 2004; Donohue et al, 2010; Saatkamp et al, 2011; Gremer and Venable, 2014).…”

Effect of shade and precipitation on germination and seedling establishment of dominant plant species in an Andean arid region, the Bolivian Prepuna

“…Noy-Meir [ 68 ], Schwining and Sala [ 69 ] indicate that the seeds of desert plants usually do not germinate after small rains, because they may be followed by drought. However, the rapid seed germination of species in arid regions such as those included in this study allows the species to take advantage of sporadic rains [ 70 ]. Thus, our results suggest that the minimum rainfall that triggers germination found for Acacia feddeana , Prosopis ferox , and Cercidium andicola are consistent, although higher, with the minimum rainfall found for other desert species of the genera Cercidium (19 mm) and Encelia (25 mm), among others [ 71 , 72 ].…”