Nocturnal versus diurnal CO(2) uptake: how flexible is Agave angustifolia?

Agaves exhibit the water-conserving crassulacean acid metabolism (CAM) photosynthetic pathway. Some species are potential biofuel feedstocks because they are highly productive in seasonally dry landscapes. In plants with CAM, high growth rates are often believed to be associated with a significant contribution of C(3) photosynthesis to total carbon gain when conditions are favourable. There has even been a report of a shift from CAM to C(3) in response to overwatering a species of Agave. We investigated whether C(3) photosynthesis can contribute substantially to carbon uptake and growth in young and mature Agave angustifolia collected from its natural habitat in Panama. In well-watered plants, CO(2) uptake in the dark contributed about 75% of daily carbon gain. This day/night pattern of CO(2) exchange was highly conserved under a range of environmental conditions and was insensitive to intensive watering. Elevated CO(2) (800 ppm) stimulated CO(2) fixation predominantly in the light. Exposure to CO(2)-free air at night markedly enhanced CO(2) uptake during the following light period, but CO(2) exchange rapidly reverted to its standard pattern when CO(2) was supplied during the subsequent 24h. Although A. angustifolia consistently engages in CAM as its principal photosynthetic pathway, its relatively limited photosynthetic plasticity does not preclude it from occupying a range of habitats, from relatively mesic tropical environments in Panama to drier habitats in Mexico.