Is root nutrient uptake a modular function? A test using Solidago canadensis

Abstract

Plant roots have been recognized to be modular, and a third order root cluster has been proposed to be the basic root module unit based upon the life cycle. This experiment examines root modularity of the nutrient-uptake function using stable isotope ¹⁵N. Solidago canadensis root clusters of second or third orders—from the same third or fourth order roots, respectively—were treated with ¹⁵NH ¹⁵₄ NO₃, NH₄NO₃, and de-ionized water for 15 and 180 min. The δ¹⁵N values of the root clusters were then analyzed and compared. The δ¹⁵N values of ¹⁵N-treated root clusters of both second and third orders were hundreds of times higher than that of the ¹⁵N untreated root clusters. However, the differences of the δ¹⁵N values among ¹⁵N untreated root clusters (though expressed some significant variations), did not indicate the ¹⁵N shared by the sister root clusters came from a common higher-order root. These results demonstrated functional modularity of root nutrient uptake, revealed a second order root, perhaps even a first order root to be a base module unit in terms of root nutrient uptake. The results also suggested that the concept of root modularity is function-specific. This experiment further revealed the importance of treatment timing in stabilizing the internal ¹⁵N/¹⁴N ratio in roots and avoiding top-down transportation of ¹⁵N back into roots to secure unbiased measurements.