Root carboxylate release of native plants in the Hengduan Mountains: Insights from X-ray fluorescence results on leaf manganese concentrations in herbarium specimens

Abstract

Root carboxylate release is a key strategy to mobilize poorly-available soil phosphorus (P). Chinaʼs Hengduan Mountains (HDM) are a global biodiversity hotspot covering various landscapes low in P availability, with the significance of root-released carboxylates in native species remaining unexplored due to challenges to assess exudates in natural habitats. Recently, leaf manganese (Mn) concentrations [Mn] as a proxy for root carboxylate exudation has been gaining acceptance. In the present study, using a handheld X-ray fluorescence (XRF) spectroscopy system, we analyzed leaf [Mn] rapidly and nondestructively, of herbarium material of 219 species of Cyperaceae, Asteraceae, Fabaceae, Brassicaceae, Polygonaceae, and Poaceae collected from HDM. We found more than 75% and 34% of analyzed species exhibited a mature leaf [Mn]> 100 and 300 mg Mn kg^‒1 respectively, which is similar to that of positive reference species of Cyperaceae, of which most species release large amounts of carboxylates. We observed major variation in leaf [Mn] in Fabaceae and grew six of these in low-P nutrient solution. Fabaceae with high leaf [Mn] tended to show rapid root carboxylate-exudation rates. Astragalus tongolensis showed low leaf [Mn] but also exhibited a rapid exudation rate of carboxylates, likely due to exudation of cations which may increase soil pH and reduce soil Mn availability. We show a rapid and nondestructive pipeline to analyze leaf [Mn] of herbarium material for an exploratory survey, and our validation within Fabaceae confirms the correlation between leaf [Mn] and carboxylate exudation. Moreover, because of the widespread occurrence of high leaf [Mn] across multiple families in the HDM, this proxy warrants further cross-family investigation. Many of the native plant species, including several putative mycorrhizal species in the HDM region, likely exhibit a P-mining carboxylate releasing strategy.

Graphical abstract

Keywords

belowground functional traits / biodiversity hotspot / root carboxylate exudation / leaf [Mn] / X-ray fluorescence spectroscopy

Highlight

	● Leaf [Mn] in herbarium specimens were analyzed using XRF easily and nondestructively.
	● Most of the native plants in the HDM region exhibited a high mature leaf [Mn].
	● High leaf [Mn] species tended to exudate more carboxylates under low-P conditions.
	● Plants in the HDM region likely use carboxylates to acquire P from low-P soils.

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