Study of In Situ Germination of Escontria chiotilla (Weber) Rose and Stenocereus griseus Haworth, Cacti from the Arid Tropical Scrub in Mexico

The vegetation of the municipality of Santiago Chazumba in Oaxaca, Mexico, includes the arid tropical scrub where the dominant species consists of the wild endemic cactus E. chiotilla together with small cultivated orchards of another cactus, S. griseus. Both are columnar cacti appreciated for their fruits. The first one is endemic and the second is an introduced cultivated species that is occasionally found in the wild where it could be displacing E. chiotilla. In order to promote the conservation of the later we are carrying out comparative studies on the physiology of their seeds. In this work we studied the in situ (in the wild and in the orchards) seed germinating capacity of both species, and compared it with seeds germinated under a range of temperatures in the laboratory. Methodology. Three replicates of 50 seeds of each species (collected in 2007 and 2008) were sown in Petri dishes with agar and incubated under a range of temperatures (15, 20, 25, 30 and 35 degrees C) and a 12/12 h photoperiod. In the field, 6 replicates of 50 seeds each, also sown in Petri dishes were placed at soil level in four locations in the scrub under direct sunlight and under the shade of a Bursera sp. trees. The seeds that did not germinate after 21 days were transferred to the laboratory and incubated at 25 degrees C. The data were analyzed by ANOVA. Results. In the laboratory, the germination capacity of the seeds of both species was above 90% at all temperatures tested, though the velocity was slower at both ends of the range. In the field, the seeds of E. chiotilla did not germinate at all but when transferred to 25 degrees C germinated in percentages above 70%. By contrast, the seeds of S. griseus germinated by 7% under direct sunlight and 29% under the shade. The extreme alternating temperatures in the scrub could be responsible for the poor germinating capacity of both species in the scrub, although both remain viable and germinate when transferred to suitable environmental conditions.