Environmental control of germination in semi-arid Mediterranean systems: the case of annuals on gypsum soils

Whether or not it is time for germination is a dilemma for annuals since they have only a single opportunity to reproduce successfully. This is critical for species living in stressful and unpredictable environments such as Mediterranean habitats. In order to clarify the environmental cues of germination, four annuals of different families, all of them occurring very frequently on gypsum soils, were selected and their germination observed under 14 climate scenarios, mimicking temperature and photoperiod conditions from autumn to spring, at two levels of water availability (continuous irrigation versus an initial single irrigation event followed by a progressive soil desiccation). In spring scenarios, two seed-storage conditions were compared: dry cold and room conditions. In the absence of water limitation, germination decreased from early autumn to late spring. Water scarcity always reduced germination, especially in early spring. Our results suggest a facultative winter germination behaviour and highlight the crucial role of dry cold storage in reducing spring germination. In conclusion, Mediterranean ephemerals showed a very plastic germination response that allows them to take advantage of favourable environmental conditions from autumn to spring. This environmental cueing is combined with the ability to dilute the risk through a variable rate of seed dormancy that, according to bet-hedging strategies, increases from secure autumn to riskier spring.