CO2 uptake and water loss accompanying vernalization for detached cladodesof Opuntia ficus-indica

To help understand the environmental signals for flower-iud initiation, det ached unrooted cladodes of the CAM species Opuntia ficus-indica were used, a model system whose gas exchange characteristics were also examined. The n umber of buds initiated for cladodes detached in the late summer was highes t at day/night air temperatures of 15/5 degrees C. A long photoperiod of 14 h enhanced flower-bud initiation for 4 wk of vernalization at 15 degrees/5 degrees C compared with a photoperiod of 10 h, whereas both photoperiods h ad similar effects for 8 wk of vernalization. Flower-bud initiation did not depend on the net CO2 uptake capability of the cladode, although bud devel opment 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, an d 93% at 12 wk and transpiration by 43%, 62%, and 72%, respectively. The da ily net CO2 uptake during 12 wk of detachment averaged 92 mmol m(-2) d(-1) at 15 degrees/5 degrees C and 123 mmol m(-2) d(-1) at 25 degrees/15 degrees C. Daily net CO2 uptake predicted by an environmental productivity index w as in close agreement with the measured values, indicating that net CO2 upt ake for detached unrooted cladodes was similar to that of rooted plants und er drought conditions.