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

Aims 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.Methods 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 Coryphanta clavata and Mammillaria compressa .Important findings All species were considered neutral photoblastic. Eleven species had similar seed germination in both light and dark conditions, and three taxa (M. compressa and the two varieties of Ferocactus latispinus) showed higher germination with light than without it. Agave salmiana, M. compressa and the two varieties of F. latispinus 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, M. compressa, and one adult-columnar species, C. clavata, 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.     

Induced climate change impairs photosynthetic performance in Echinocactus platyacanthus, an especially protected Mexican cactus species

The responses of desert plants to climate warming have been poorly assessed, perhaps due to the overall expectation that desert vegetation will expand as a consequence of this component of climate change. However, determining what plant species will tolerate the expected increase in temperature is a question that remains unanswered. The Chihuahuan Desert is the largest warm desert of North America, and predictive models of climate change indicate that summer temperatures in this desert will increase by 1–2 °C in the next decade. This study experimentally assessed the performance of an endangered cacti species from the Chihuahuan Desert under simulated warming conditions. Hexagonal open top-chambers (OTCs) were used to simulate the effects of global warming on five-years-old individuals of the specially protected species Echinocactus platyacanthus. Temperature was 1.9 °C higher in open top-chambers than in control plots. In contrast, relative humidity was 3.1% higher in control plots than in open top-chambers. E. platyacanthus showed 100% survival for 14 weeks in both OTC and control plots. However, induced warming negatively affected the photosynthetic performance of this species. Cacti located within OTCs displayed lower maximum quantum efficiency of photosystem II (F_v/F_m), effective quantum yield of photosystem II (Φ_PSII), and electron transport rate (ETR) values, but higher non-photochemical quenching (NPQ) values, than cacti from control plots. This is the first study focused on the potential impact of climate warming on survival and photosynthetic performance of young individuals of a succulent species from American deserts. Induced warming negatively affected the photosynthetic performance of young E. platyacanthus, but it also increased non-photochemical quenching, a mechanism for avoiding photoinhibition.