Shifts in the optimal temperature for nocturnal CO² uptake caused by changes in growth temperature for cacti and agaves

When the day/night air temperatures were raised from 10°C/10°C to 30°C/30°C, the optimal tempearture for nocturnal CO 2 uptake by six species of cacti and three species of agave shifted from an average of 12°C to an average of 20°C. The maximum rate of CO 2 uptake was higher for Agave americana at the higher ambient temperature, lower for A. deserti , and much lower for A. utahensis , consistent with the relative mean temperatures of their native habitats. For the cactus Coryphantha vivipara , which had the greatest temperature shift observed (13°C), the halftime was 8 days for the upward shift and 4 days for the downward shift. The halftimes for the comparable shifts averaged 1.6 days for three other species of cacti and less than 1 day for two agave species. The shifts in the optimal temperature for nocturnal CO 2 uptake were in response to changes in nighttime temperature, at least for C. vivipara , and reflected temperature responses of both the stomates and the chlorenchyma.