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-indicacome from measurements on individual cladodes. However, they havelimitations when used to scale up to whole canopy gas exchange,because a large variability of carbon assimilation may occur within thecanopy, due to, among others, differences in cladode age and interceptedradiation or individual cladode response to abiotic stresses. The aimof this work was to evaluate the application of open gas exchangechambers, simultaneously applied around the whole canopy, to measurenet CO2 uptake, continuously over a 24 h period, in single Opuntiaficus-indica (L.) Mill. potted trees and in relation with their water status.Net CO2 uptake was also measured for single cladodes differentiatedby age. O. ficus-indica trees continued their photosynthetic activity60 days after the irrigation was stopped, when soil water content waslower than 5%. At this stage, current-year and 1-year-old cladodes hadbecome flaccid but still the daily net CO2 uptake of non-irrigated treeskept the same rate than at the beginning of the experiment, whilewatered trees had doubled their net CO2 uptake. The highest instantaneousrates and total daily net CO2 uptake for both well-watered andnon-irrigated trees occurred 60 days after the onset of the dry period,when maximal instantaneous rates were 11.1 in well-watered trees and8.4 mol m–2 s–1 in non-irrigated trees. During the drought period, thechlorenchyma fresh weight decreased by 45% and 30%, in 1- and 2-yearolddrought cladodes respectively, and marginally increased in currentyearones (+20%). Net CO2 uptake for 1-year-old and 2-year-old cladodeschanged only at highest photosynthetic photon flux density andtemperatures, and average seasonal net CO2 uptake of 2-year-old cladodeswas 15% lower than for 1-year-old ones. Whole-tree gas exchange measurements applied for the first time to O. ficus-indica indicated thatwhole cactus pear trees maintain similar net CO2 uptake, contrary toprevious evidence gathered on single leaves, even after 60 days ofdrought. A substantial water loss in the most photosynthetically activecladodes was not sufficient to alter this response. Differences in growthaccrued by wet and non-irrigated trees were due to the fact that, as theseason progressed, the former were able to increase their total dailyCO2 uptake and the latter were not.