Carbon enrichment enhances photosynthetic efficiency of yam bean (Pachyrhizus erosus L.) under low light conditions

Abstract Physiological responses and agricultural crop fecundity are significantly impacted due to contrasting sky conditions. Here we report the impact of the combined effect of varying photosynthetic photon flux density (PPFD) and elevated carbon dioxide on the photosynthetic responses of yam bean. We found that limited PPFD down-regulated the photosynthetic activity whereas the fertilization effect of CO2 enrichment alleviated the negative impact of limiting PPFD. The mean photosynthetic rates increased linearly and significantly across PPFD. Notably, the quantum of increment of mean photosynthetic rate across increasing PPFD was significantly higher at 600 and 800 ppm CO2 in comparison to 400 ppm and 1000 ppm due to the acclimation effect of carbon enrichment at 1000 ppm. Irrespective of the PPFD, the mean stomatal conductance (gs) rate remained high at 600, 800 and 1000 ppm in comparison to 400 ppm. As a general trend, at all CO2 levels, the mean gs increased significantly up to 50-1000 µmol m− 2 s− 1 PPFD followed by a slight down-regulation at 1200–1500 µmol m− 2 s− 1 PPFD. A similar trend was observed in the case of intracellular carbon dioxide concentration (Ci) and the ratio of intercellular to ambient CO2 concentration (Ci/Ca). As an effect of CO2 fertilization, the mean transpiration rate (E) increased linearly and significantly across all PPFD and CO2 concentrations. Our results suggest that the down-regulated photosynthesis of yam bean under cloudy conditions or limited PPFD will benefit from ECO2.