Effect of light on germination of seeds of Cactaceae from the Chihuahuan Desert, Mexico
In the Chihuahuan Desert, there are many cacti species considered to be at risk due to illegal extraction, land-use change and overgrazing. To reduce their illegal extraction, ex situ plant propa- gation has been suggested. However, the literature regarding seed germination biology of these species is scarce. We investigated the effect of light on germination percentages and germination rate (t50)in seeds of 28 cactus species from the Chihuahuan Desert. Seeds were incubated at a 14-h daily photoperiod (light) and in continuous darkness at 258C for 30 d, after which seeds failing to germinate in darkness were transferred to light for 30 d. Only 11 of the species had non-dormant seeds, germinating $70% in the light; thus an evaluation of the effect of light versus darkness on germination was confined to them. All species were positively photoblastic, and all of them had seeds weighing ,1mg. Ten species did not germinate in darkness, and one species had only 7% germination. From these 11 species, 8 did not germinate to a significantly higher percentage when the same set of seeds was transferred from dark to light, suggesting that darkness had triggered sec- ondary dormancy (skotodormancy). To our knowl- edge, these results are the first to show that darkness triggers secondary dormancy in cacti. Implications of having a light requirement for germination and having small seeds to accumulate a persistent soil seed bank are discussed. These results contribute to under- standing the germination biology of cactus species at risk, and could enhance the propagation of large numbers of cultivated individuals outside their habitats, promoting ex situ conservation.