CO2 uptake of Opuntia ficus-indica (L.) Mill. whole trees and single cladodes, in relation to plant water status and cladode age

Most of net photosynthesis determinations in Opuntia ficus-indica come from measurements on individual cladodes. However, they have limitations when used to scale up to whole canopy gas exchange, because a large variability of carbon assimilation may occur within the canopy, due to, among others, differences in cladode age and intercepted radiation or individual cladode response to abiotic stresses. The aim of this work was to evaluate the application of open gas exchange chambers, simultaneously applied around the whole canopy, to measure net CO₂ uptake, continuously over a 24 h period, in single Opuntia ficus-indica (L.) Mill. potted trees and in relation with their water status. Net CO2 uptake was also measured for single cladodes differentiated by age. O. ficus-indica trees continued their photosynthetic activity 60 days after the irrigation was stopped, when soil water content was lower than 5%. At this stage, current-year and 1-year-old cladodes had become flaccid but still the daily net CO₂ uptake of non-irrigated trees kept the same rate than at the beginning of the experiment, while watered trees had doubled their net CO₂ uptake. The highest instantaneous rates and total daily net CO₂ uptake for both well-watered and non-irrigated trees occurred 60 days after the onset of the dry period, when maximal instantaneous rates were 11.1 in well-watered trees and 8.4 mol m^–2 s^–1 in non-irrigated trees. During the drought period, the chlorenchyma fresh weight decreased by 45% and 30%, in 1- and 2-yearold drought cladodes respectively, and marginally increased in currentyear ones (+20%). Net CO₂ uptake for 1-year-old and 2-year-old cladodes changed only at highest photosynthetic photon flux density and temperatures, and average seasonal net CO₂ uptake of 2-year-old cladodes was 15% lower than for 1-year-old ones. Whole-tree gas exchange measurements applied for the first time to O. ficus-indica indicated that whole cactus pear trees maintain similar net CO₂ uptake, contrary to previous evidence gathered on single leaves, even after 60 days of drought. A substantial water loss in the most photosynthetically active cladodes was not sufficient to alter this response. Differences in growth accrued by wet and non-irrigated trees were due to the fact that, as the season progressed, the former were able to increase their total daily CO₂ uptake and the latter were not.