A young prickly pear segment. From assimilate uptake to CAM photosynthesis

The nocturnal stage of carbon metabolism

In CAM metabolism, some CO₂ uptake occurs at night. Phosphoenolpyruvate carboxylase participates in its fixation, and the resulting organic acids accumulate in vacuoles. During the day, CO₂ released from them can supply the Calvin cycle. Separating these stages in time allows the plant to regulate carbon uptake while limiting water loss. [Niechayev et al. 2023]

Three measurements of a maturing plant

Niechayev, Mayer and Cushman followed development of seedlings and segments growing from adult Opuntia ficus-indica. They combined daily gas exchange with acidity of tissues collected at dawn and dusk, and measurements of carbon isotope ratios. Each method captures a different part of the process, current CO₂ flux, the acid storage cycle and the accumulated trace of the carbon fixation pathway. [Niechayev et al. 2023]

Cotyledons and first seedling segments were studied on days 30, 50, 75 and 100 after germination. A nocturnal acidity increase appeared early, and the difference between dawn and dusk became clearer in older segments. It was statistically significant in this tissue at the two latest measurement dates. [Niechayev et al. 2023]

CAM in an organ that still consumes resources

The youngest seedlings and developing growth initially showed a negative CO₂ exchange balance. Their respiration released more carbon than photosynthesis absorbed over the same period. A growing organ can therefore activate CAM while remaining a sink for resources stored in the seed or supplied by the older part of the plant. [Niechayev et al. 2023]

For a daughter segment, the parent segment is the assimilate source. The influx of these compounds supports new cell formation and tissue expansion. As surface area increases and the photosynthetic apparatus matures, atmospheric CO₂ uptake rises until the balance becomes positive. In the plants studied, a substantial contribution from nocturnal assimilation accompanied this transition. [Niechayev et al. 2023]

An isotopic trace of increasing function

In the first segments, mean δ¹³C shifted from −19.4‰ on day 30 to −14.5‰ on day 100. The less negative value agrees with an increasing contribution of carbon fixation through CAM. Cotyledons retained a different signature, corresponding to their different developmental stage and role in the young plant. [Niechayev et al. 2023]

Agreement with acidity and gas exchange measurements strengthens this interpretation. Isotopic composition alone integrates the history of carbon in the tissue, including compounds from earlier reserves. The daily record of CO₂ exchange directly reveals when a developing organ takes up gas from its surroundings. [Niechayev et al. 2023]

Growth, light and water

Seedlings were compared at two light intensities and under different hydration conditions. Growth rate and the magnitude of CO₂ uptake changed, but the daily record indicated development of CAM activity even in young plants. The result captures continuous functional maturation, from an organ using reserves to a segment supplying its own carbon for maintenance and further growth. [Niechayev et al. 2023]