Extreme temperatures and thermal tolerances for seedlings of desert succulents

摘要: Extreme temperatures near the soil surface, which can reach 70°C at main study site in northwestern Sonoran Desert, markedly affect seedling survival. Computer simulations indicated that for rather spherical barrel cactus Ferocactus acanthodes (Lem.) Britt. & Rose maximum surface temperature decreased 8°C and minimum increased 3°C as height was from 1 mm up to 50 mm. Simulated changes shortwave longwave irradiation alone showed shading could decrease by about 5°C common desert agave, Agave deserti Engelm., raise 1°C. Actual field measurements on seedlings of both species, where would local air wind speeds addition irradiation, average 11°C summer raised winter. Seedlings grown day/iight 30°C/20°C tolerated low -7°C high 56°C, measured stain uptake chlorenchyma cells reduced 50%. Seedling tolerance slightly with age, F. more tolerant than A. deserti. Even taking acclimation into account (2.7°C increase per 10°C temperature), not be expected withstand exposed sites, consistent previous observations these occur only protected microhabitats. Based primarily greater (4.3°C 10°C), have a just barely survive sites. Wide ranges photoperiod had little effect thermal sensitivities either species. When drought osmotic pressure 0.5 MPa 1.3 MPa, species became 2°C less temperatures, nonadaptive environment, also occurs other In conclusion, tolerate tissue over 60°C when acclimated below -8°C temperatures. However, extreme environment adjacent requires sheltered microhabitats protect plants damage them their upper elevational limits.