C3 photosynthetic activity in ephemeral leaflets of the constitutive CAM species Opuntia ficus-indica

Abstract

Ephemeral leaflets are produced on emerging daughter cladodes of Opuntia ficus-indica, a constitutive crassulacean acid metabolism (CAM) species of major agronomic and ecological importance. Despite their ubiquity across Cactaceae, these transient organs have never been characterized as discrete photosynthetic units. Here we investigate whether leaflets operate via C3 or CAM photosynthesis and quantify their contribution to early cladode carbon (C) economy. Gas-exchange measurements, stomatal aperture analysis, titratable acidity assays, C isotope discrimination (δ13C), and 13CO2 pulse-labelling consistently indicate that leaflets operate via the C3 pathway: they exhibit daytime stomatal opening, minimal nocturnal acid accumulation, greater discrimination against 13CO2 (δ13C reaching -17‰ in mature leaflets compared with -10‰ to -11‰ in cladodes) and net daytime CO2 uptake at rates approximating 50 % of those in mature cladodes. Carbon-balance calculations indicate that leaflets supply ∼0.12 g C cladode-1 day-1, meeting approximately 20 % of the daily C demand of the expanding daughter cladode. Experimental leaflet removal and stomatal blockage reduced daughter cladode surface area by 28-34 % and dry weight by ∼38 % relative to controls at three weeks. These findings reveal an intra-plant co-ordination of C3 and CAM C fixation in spatially distinct organs and identify leaflets as adaptive source organs that accelerate biomass accumulation during pulsed periods of water availability in arid environments.