ROOT TRAIT RESPONSES TO PHOSPHORUS DEFICIENCY IN BAMBARA GROUNDNUT (VIGNA SUBTERRANEA L. VERDC)

Abstract

Bambara groundnut (Vigna subterranea L. Verdc.) is a legume whose growth is limited by moisture and phosphorus (P) deficiency. Limited research has focussed on belowground traits in Bambara groundnut, which is crucial for understanding tolerance to P deficiency. The objectives of this study were to (i) determine root morphological variation in Bambara groundnut cultivars under varying P levels and (ii) evaluate the root physiological mechanisms that improve Bambara groundnut’s performance in soils with varying P levels. Fifteen Bambara groundnut cultivars were planted in polyvinyl chloride pots containing 2 kg sandy soil for 55 days under green house conditions. The pots were supplied with three P levels: 10 mg P/kg for limiting P (low P) as iron phosphate, 10 mg P/kg for moderate P (MP), and 50 mg P/kg for high P (HP) as dihydrogen phosphate. The treatments were arranged in a completely randomized design. The root morphological traits, including root dry weight (RDW), shoot dry weight (SDW), root-to-shoot ratio (RSR), number of nodules (NN), total root length (TRL), number of first-order roots (NFOR), number of second-order roots (NSOR), second-order root length (SORL), root branching intensity (RBI), root diameter (RD), root surface area (RSA), and root volume (RV), exhibited significant (P ≤ 0.05) differences among Bambara groundnut cultivars at varying phosphorus levels. TRL, NSORL, and SORL decreased as soil P levels increased in all Bambara groundnut cultivars. BAM002 and BAM010 displayed the highest TRL, NFOR, NSOR, SORL, RSA, and RV at low P. Arbuscular mycorrhizal fungi (AMF) colonization and extracellular and intracellular acid phosphatase and phytase activities varied significantly among Bambara groundnut cultivars at different P levels, showing a decline as soil P levels increased. BAM002, BAM006, and BAM010 showed high AMF colonization and exhibited the highest amount of acid phosphatase and phytase activities in the rhizosheath at low P. Compared to root morphological traits, the variation in the effect size of AMF colonization (25-277.8%), rhizosheath acid phosphatase (30.2- 612.5%), and phytase (32.2-293.2%) was higher among the cultivars, except for SORL, which ranged from 23 to 1252.2%. Under P-limited conditions, BAM002, BAM006, , and BAM010 exhibited enhanced root growth, increased exudation of acid phosphatase and phytase, and high AMF colonization. These cultivars are suitable candidates for cultivation in P-limited environments. Incorporating their root phenotypes into elite Pinefficient Bambara groundnut could be key to optimizing P acquisition efficiency and developing varieties suited for P-stressed soils