Effect of light on seed germination and seedling shape of succulent species from Mexico

<st>Aims</st> Light requirements for cactus seed germination have been considered to be associated with their adult plant height and seed mass, but this has not been thoroughly studied for other succulent species. In order to understand seed photosensitivity from desert species belonging to Asparagaceae (subfamily Agavoideae) and Cactaceae, we performed a germination experiment with and without light for 12 species and 2 varieties from 1 species from the Southern Chihuahuan Desert. We also determined if adult growth is totally determined by seedling ‘growth form’ in cacti. <st>Methods</st> We performed a germination experiment using light and darkness for 13 species from Southern Chihuahuan Desert: 10 rosette species (Asparagaceae), as well as 1 globose, 1 columnar and 2 varieties from 1 depressed-globose species (Cactaceae). The response variables were seed germination percentage and relative light germination (RLG). In addition, in order to determine if adult-globose cacti could have cylindrical seedlings, we calculated the shape index (height/width ratio) for <it>Coryphanta clavata</it> and <it>Mammillaria compressa</it>. <st>Important Findings</st> All species were considered neutral photoblastic. Eleven species had similar seed germination in both light and dark conditions, and three taxa (<it>M. compressa</it> and the two varieties of <it>Ferocactus latispinus</it>) showed higher germination with light than without it. <it>Agave salmiana</it>, <it>M. compressa</it> and the two varieties of <it>F. latispinus</it> had higher RLG than the other species. Seed mass was an important factor because with higher seed mass there was lower dependence to light. These findings support the hypothesis that small seed mass and light requirements have coevolved as an adaptation to ensure germination. One adult-globose cactus species, <it>M. compressa</it>, and one adult-columnar species, <it>C. clavata</it>, had small seeds and neutral fotoblasticism. Seedlings from these two species exposed to light were cylindrical and those under darkness conditions were columnar. Perhaps seeds from this species are able to germinate in the dark because they produce columnar seedlings with the ability to emerge from greater soil depths where sunlight cannot penetrate.