The composition of non-structural carbohydrates affects the respiration and morphology of tree fine roots in Japan Alps

Abstract

Non-structural carbohydrates (NSC), such as soluble sugars and starch, are important drivers of physiological function and survival in woody plants. The soluble sugar and starch in the organs of woody plants might affect their resource acquisition, but little is known about their direct association with the fine root physiological function. Here, we aimed to quantify the storage of soluble sugar and starch in the fine roots of two woody species, found under the alpine treeline, namely, Maries' fir (Abies mariesii) and Erman's birch (Betula ermanii). We also set out to identify the relationship between each NSC and both fine root respiration and morphology of the plants. To this end, the concentrations of soluble sugar and starch, root respiration rate, and specific root length (SRL) were measured in three root diameter classes: <0.5, 0.5–1.0, and 1.0–2.0 mm. In both species, we found that the sugar concentration of <0.5 mm roots was lower than that in both 0.5–1.0 and 1.0–2.0 mm roots, whereas there were no clear differences in starch concentration among the three classes. Additionally, our results showed that the sugar concentration was negatively correlated with the root respiration rate and SRL, whereas starch concentration did not show any correlation with these factors. Altogether, these findings suggest that the soluble sugar in fine roots can act as a direct source of energy for the roots to aid soil resource acquisition, whereas the starch can serve as an indirect, longer-term storage reservoir. Such distinct differences in the storage and function of soluble sugar and starch in fine roots are likely to improve our understanding of the sink-size of each process in terms of carbon consumption for root respiration and root morphology for soil resource acquisition.