SEED RESPONSE TO TEMPERATURE OF MEXICAN CACTI SPECIES FROM 2 LIFE FORMS - AN ECOPHYSIOLOGICAL INTERPRETATION

The effect of seven different constant temperatures and five ranges of alternating temperatures on seed germination of seven species of cact i from Puebla, Mexico was analyzed. Six cacti species germinated in a wide range of temperature. Columnar cacti were more tolerant to low te mperatures and germinated in a wider range of temperature than barrel cacti. One of the barrel cacti studied (Ferocactus recurvus) only reac hed full germination at 25 degrees C. Temperature fluctuations did not produce significant effects on germination compared to the results ob tained at constant temperatures. This may reveal differing ecophysiolo gical adaptations with respect to temperature requirements during the establishment conditions for each life form. Columnar cacti may become established mainly under the shade of desert shrubs, whereas barrel c acti maybe can also become established in open areas, beneath the shad e of small rocks or soil irregularities. In both cases, temperature fl uctuations are attenuated by the shade, but mean temperatures may be h igher in the second condition than beneath the shade of plants.