Root foraging is essential for plant growth and ecosystem functioning. In most plants, the furthest points of the root systems, known as absorptive roots, undertake this function. There has been a substantial amount of research into the variations among these absorptive roots.

Last week, Assistant Professor Junjian Wang from the School of Environmental Science and Engineering at Southern University of Science and Technology published a paper on root economics spectrum (RES), a common hypothesis about the relationship between the need to acquire resources and live a long life. The paper, titled “Nonlinearity of root trait relationships and the root economics spectrum,” was published in leading academic journal Nature Communications, in which it introduced his team’s latest research in plant ecology. Junjian Wang was one of the corresponding authors of the paper.

The RES hypothesizes that roots trade-off between acquiring resources and conserving life. It suggests that roots should follow a trend from fast foraging and short lifespan (an acquisitive strategy) to slow foraging and long lifespan (a conservative strategy). At a global scale, the RES gradient has been used to understand root tissue function and in explaining responses of ecosystem carbon and nutrient cycling to climate change. However, whether the RES exists was under debate as the previous root trait relationships in different studies are conflicting.

Junjian Wang and his colleagues opted for a much broader spectrum of plant species from across the globe, in order to assess the validity of RES and find new ways of measuring the relationships. With more than 800 species, the team conjectured that the relationships between root tissue density (RTD), root nitrogen concentration (RN) and root diameter would not be linear. They also hypothesized that non-woody plants would have weaker root trait relationships than woody plants, due to the harsher environments than non-woody plants grow in.

The team was able to show that there is an allometric relationship between the increase in the thickness of tissues outside stele (tToS) and the increase in stele radius with increasing root diameter for both woody and non-woody plants. Specifically, the thickness of the tToS increases at a faster rate than the stele radius does from thin to thick absorptive roots. This allometric relationship resulted in the non-linear relationships between the RTD, RN, and root diameter. These non-linear relationships suggest that traditional linearity-based methods can easily cause errors in understanding the root trait relationships. In addition, the study discussed how plant phylogeny, growth form, and mycorrhizal type altered the root trait relationships and the relevant ecological significance.

The team also explains why the RES may not exist. According to the RES, roots either have lighter mass (i.e., less investment), shorter life, and high foraging capacity, or have heavier mass (i.e., more investment), longer lifespan, and low foraging capacity. However, both the model based on the allometric relationship and the measured data indicate that the dry mass of the root increases monotonously with increasing diameter and does not decrease as the root tissue density decreases. Furthermore, the thick absorptive root can have a thick cortex, low tissue density and high fungal infection rate, and thus high nutrient absorption capacity and high nitrogen concentration. Therefore, the thick absorptive root may have both a longer life span and a higher nutrient absorption capacity, which contradicts the prediction of the RES.

The finding reconciles the debate on the relationship between RTD and root diameter for woody plant species. The studies reporting negative relationships included a higher proportion of species with thin roots and high RTD than studies reporting no relationships. This demonstrates that those studies reporting no relationships focus on the region of slow decrease of RTD with increasing root diameter. Therefore, nonlinearity of the root trait relationships could underpin how sampling bias from different parts of the nonlinear curves produces contradicting results as shown in recent studies.

Deliang Kong (Shenyang Agricultural University), Junjian Wang (SUSTech), Huifang Wu (Henan University), Oscar J. Valverde-Barrantes (Florida International University) and Ruili Wang (Northwest A&F University) were co-first authors. DeliangK ong, Junjian Wang and Yulong Feng (Shenyang Agricultural University) were corresponding authors of the study. Hui Zeng (Peking University), Paul Kardol (Swedish University of Agricultural Sciences) and Haiyan Zhang (Shenyang Agricultural University) participated in the study.

The research was supported by the National Key R&D Program of China, the National Natural Science Foundation, and the Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Prevention and Control.

Original paper: https://www.nature.com/articles/s41467-019-10245-6