Nothing is known about the effect of soil yeasts on the photosynthetic resource-use and carbon dynamics of plants. Here, we determined the effect of a plant growth-promoting isolate of Cryptococcus laurentii on the photosynthetic water and nutrient-use efficiencies, as well as the carbon economy of a Mediterranean sclerophyll, Agathosma betulina, grown under axenic conditions. The data showed that the higher photosynthetic water-use efficiency in yeast-inoculated plants was a consequence of higher maximum rates of CO2 assimilation, which was not related to foliar N and P content. We propose that photosynthetic stimulation in yeast-inoculated plants was a result of the increased demand for photosynthates of the yeast-root symbiosis.