A plant contains about 50% of it's mass absorbed from the air, from CO2 which it converts with 45% efficiency. 40-45% CO2 fixation/respiration seems to be optimal for all plants, it's the balance of how much their cells need aerobic respiration day and night, compared to photosynthesis. That 50% absorbed from the air is fixed with H2O which provides most of the other 50%, and the rest is ashes.
According to this research:
The CO2 captured by plants is the result of the differences between
atmospheric CO2 absorbed during the process of photosynthesis and the CO2
released by the atmosphere during respiration. This difference is converted into
biomass and tends to fluctuate between 45 and 50% of the plant’s dry weight.
(the research states dry weight C measurements, they don't seem to measure CO2 absorption directly)
So it's better to search for "photosynthesis respiration ratio" to find research that measures the efficiency of the CO2 absorption:
the ratio of respiration and photosynthesis (R : P) was relatively constant, 0·40–0·45, and independent of temperature. This was consistent with earlier work by Ryle et al. (1976) [...] Some mechanistic understanding of the constancy of R : P was provided by Atkin et al. (2005), who found that respiration and photosynthesis in most examined plant species acclimated similarly after temperature change. More recently, Atkin et al. (2007) confirmed homeostasis of R : P, in Plantago spp., but this was not maintained when plants were subjected to higher temperatures than usually experienced in their habitats. Differences between photosynthetic and respiratory acclimation to temperature have been reported by Campbell et al. (2007) and Hartley et al. (2006). Cheng et al. (2000) found that the R : P of sunflower was also insensitive to changes in atmospheric CO2 concentration. Overall, the research confirms Monteith's (1981) assessment that the ratio may vary over short time periods but is constant over periods of weeks and longer.
That document has lots of research references and technical words and concepts to follow up.
It's good also to see google images for R P ratio graphs and tests.
The conversion ratio is visible here, day is white, respiration is black.