CO2 Uptake and Water Loss Accompanying Vernalization for Detached Cladodes of Opuntia ficus‐indica

To help understand the environmental signals for flower‐bud initiation, detached unrooted cladodes of the CAM species Opuntia ficus‐indica were used, a model system whose gas exchange characteristics were also examined. The number of buds initiated for cladodes detached in the late summer was highest at day/night air temperatures of 15°/5°C. A long photoperiod of 14 h enhanced flower‐bud initiation for 4 wk of vernalization at 15°/5°C compared with a photoperiod of 10 h, whereas both photoperiods had similar effects for 8 wk of vernalization. Flower‐bud initiation did not depend on the net CO2 uptake capability of the cladode, although bud development and survival did. During 12 wk of detachment, cladodes gained 20% in dry mass, reflecting a positive net CO2 uptake despite no water uptake by the unrooted cladodes. Both net CO2 uptake and water loss were maximal at 1 wk after detachment, CO2 uptake decreasing by 48% at 4 wk, 87% at 8 wk, and 93% at 12 wk and transpiration by 43%, 62%, and 72%, respectively. The daily net CO2 uptake during 12 wk of detachment averaged 92 mmol m−2 d−1 at 15°/5°C and 123 mmol m−2 d−1 at 25°/15°C. Daily net CO2 uptake predicted by an environmental productivity index was in close agreement with the measured values, indicating that net CO2 uptake for detached unrooted cladodes was similar to that of rooted plants under drought conditions.