Effect of temperature and osmotic potential on the germination of two Asteraceae growing in a lava field in central Mexico

Background/Question/Methods

Temperature and water availability are the two main requirements for seed germination, particularly in arid environments as deserts and lava fields with shallow soils. These factors affect seed imbibition and thus germination percentage, germination rate (velocity) and lag time. Dahlia coccinea and Senecio praecox grow in a xerophilous shrubland developed in a lava field in the South of the Basin of Mexico. In order to know the optimal temperature for germination and the minimal osmotic potential (OP) for germination at optimal temperature, we evaluated seed germination in both a temperature and an OP gradient. Experiments were performed in laboratory conditions. First we measured the fresh and dry mass of seeds, and calculated seed water content as a percentage of its dry (WC_db) or fresh mass (WC_fb). To evaluate temperature effect, seeds were sown 1) at five constant temperatures from 15 to 35°C, at intervals of 5°C, and 2) at three fluctuating temperature (25–30, 20–30 and 25–35°C). To evaluate the OP effect, seeds were sown in different Polyethylene Glycol (PEG8000) solutions (0, –0.3, –0.5, –0.8, –1 and –1.5 MPa) and at 25°C. Final germination percentage, rate and lag time were analyzed.

Results/Conclusions

In both species, seed fresh and dry masses were highly related; both WCdb and WCfb were around 7%. Temperature and OP had significant effects on germination responses in both species. Optimal temperature for germination was 25°C, and above or below of it, the germination rate decreases. In D. coccinea lag time was long only at 15°C, and in S. praecox germination was delayed as temperature increased or decreased from 25°C; it did not occur at 35°C. In D. coccinea, germination was >50% at 0, –0.3, –0.5 and –0.8 MPa whereas in S. praecox this occurred only at 0, –0.3 and –0.5 MPa. In both species, germination rate was higher at 0 MPa, and decreased as the potential did. Lag time increased as OP decreased. The ability to germinate at high temperatures (30°C) and low OP may be related to early germination during sporadic rains that precede of the rainy season. Dahlia coccinea germinates and establishes earlier than other species in the same habitat, as Buddleia cordata and Wigandia urens which germinate at –0.3 and –0.5 MPa, respectively. Nevertheless, S. praecox is a succulent species whose seedlings tolerate more water stress than those of the above mentioned species.