Variation in the Respiration-to-Photosynthesis Ratio of Chinese Fir Saplings Is Associated with Foliar Non-Structural Carbohydrates

Foliar respiration-to-photosynthesis ratio (R/A) is a key indicator of carbon balance and has been widely used to infer plant productivity and carbon sequestration potential. Yet, the extent to which R/A varies among genotypes within a species remains unclear. Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is a dominant plantation conifer species in southern China. Here, we assessed genotypic variation in R/A and investigated its potential drivers in saplings of three Chinese fir genotypes (Gen3, Y020 and Y061) grown under uniform greenhouse conditions. R/A varied among genotypes, and the variation was driven mainly by differences in respiration, although photosynthesis also varied among genotypes. Compared to Gen3 and Y061, Y020 showed the highest R/A, indicating greater respiratory carbon loss relative to carbon gain. Across genotypes, respiration was positively associated with foliar nitrogen concentration (foliar N) and non-structural carbohydrate concentrations (NSC; including soluble sugars and starch), whereas photosynthesis was unrelated to foliar N and NSC. More importantly, R/A increased with NSC but not with foliar N. Overall, these results reveal substantial genotypic variation in R/A in Chinese fir saplings and suggest that NSC is an important factor associated with foliar carbon balance. Our findings have practical implications for selecting genotypes to enhance carbon sink capacity and stress resilience in plantation management.