Root architectural adaptations of finger millet genotypes to phosphorus deficiency in acidic soils

Introduction

Nearly 4 billion hectares of soil worldwide register pH values below 5.5, and phosphorus availability in these soils drops sharply because aluminium and iron oxides lock P into inaccessible forms [1] . Finger millet (Eleusine coracana (L.) Gaertn.) is grown widely across sub-Saharan Africa and South Asia on marginal acidic soils where P fertilizer use stays low [2] . Root architecture shapes P uptake because phosphate ions diffuse through soil over very short distances typically less than 1 mm [3] . Plants that produce longer roots, denser root hairs, and more lateral branches explore a larger soil volume [4] . Some genotypes also secrete acid phosphatases and organic acids, releasing P from soil complexes [5] . Whether morphological and biochemical responses operate independently or in coordination across finger millet genotypes remains unclear [6] . This research compared root architectural and biochemical adaptations in five finger millet genotypes differing in P efficiency, grown in acidic soil with contrasting P levels, aiming to identify which root traits best explain genotypic differences in P acquisition [7] .

Research area description

The experiment was conducted in the greenhouse complex of the University of Córdoba (37.88°N, 4.77°W), Córdoba, Spain. The region has a Mediterranean climate with hot summers and mild winters. Acidic soil (pH 4.8) was collected from an experimental acidic plot maintained by the university for nutrient efficiency trials [9] .

Materials and Methods

Materials

Five finger millet genotypes GPU-28, GPU-67, VL-149, PR-202, and HR-374 were chosen based on prior field screening for P-use efficiency [2] . Seeds were sourced from the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) germplasm bank.

Acidic soil (Ultisol, pH 4.8, available P 3.2 mg kg⁻¹, exchangeable Al 2.7 cmol kg⁻¹) was collected from the 0-20 cm horizon, air-dried, and sieved through a 2 mm mesh. The experiment ran from March to June 2022. Ethics approval for plant-based research was granted under the university's agricultural experimentation guidelines (Protocol UC-AGR/2022-014, approved 20 February 2022).

Methods

Plants were grown in rhizotrons (30 × 40 × 3 cm) filled with 4 kg acidic soil amended to two P levels: sufficient (40 mg P kg⁻¹ as KH₂PO₄) and deficient (5 mg P kg⁻¹). A completely randomized design with four replications was used. At 45 days, roots were harvested and washed. Root length was measured using WinRHIZO software. Root hair density was counted under a stereomicroscope at 40× magnification. Acid phosphatase activity was determined using p-nitrophenyl phosphate at pH 5.0 and measured at 405 nm [6] . Shoot P was determined by the vanadomolybdate method after dry-ashing at 550 °C [10] .

Experimental design

The trial used a 5 × 2 factorial arrangement (five genotypes × two P levels) in a completely randomized design with four replications, giving 40 experimental units. Rhizotrons were placed at 45° angle on steel racks inside the greenhouse, with daily watering to 70% water-holding capacity. Greenhouse temperature was maintained at 28/22 °C (day/night) with natural photoperiod. Positions were re-randomized weekly to reduce edge effects [11] .

Results

Comprehensive interpretation

Root trait responses to P deficiency followed a clear genotypic gradient (Table 1, Figure 2). GPU-28 showed the largest increases across all measured traits, extending root length by 42.7% and root hair density by 68.3% under low P. HR-374, by contrast, adjusted root length by only 9.3% and hair density by 12.8%. The pie chart comparison (Figure 1) confirmed that tolerant GPU-28 relied on root length and root hair proliferation for P acquisition, while susceptible HR-374 depended on root diameter and surface area traits that are less effective for intercepting immobile P ions. Acid phosphatase activity mirrored morphological plasticity (Table 2), with GPU-28 showing an 83.6% increase versus 21.7% in HR-374. Shoot P concentration under deficiency was highest in GPU-28 (2.14 mg g⁻¹) and lowest in HR-374 (1.12 mg g⁻¹), confirming that root architectural and enzymatic adjustments translated into actual P uptake differences (Figure 3).

Discussion

The strong link between root hair density increase and shoot P aligns with work in common bean, where root hair length explained up to 80% of P uptake variation on acid soils [5] . The 68.3% increase in GPU-28 exceeds the 40-50% range reported for tolerant rice genotypes under similar stress [3] , highlighting finger millet's strength in this trait. Rsoot hairs are metabolically inexpensive and effective at short-distance P capture, making them efficient adaptations for low-input systems [4] . Acid phosphatase activity provided biochemical confirmation the 83.6% rise in GPU-28 suggests active mining of organic P pools, which can represent 30-65% of total soil P in Ultisols [6] .

HR-374 lacked both morphological plasticity and enzymatic compensation, indicating system-wide P inefficiency [7] . However, this research used greenhouse rhizotrons, and root behavior in undisturbed field soil may differ due to microbial interactions and spatial P heterogeneity [12] .

Conclusion

GPU-28 emerged as the most P-efficient finger millet genotype, combining large increases in root length, root hair density, and acid phosphatase activity under deficiency. HR-374 showed minimal plasticity across all traits. Tolerant genotypes used coordinated morphological and biochemical responses for P acquisition. Root hair density increase and acid phosphatase induction together explained the majority of genotypic variation in P uptake, offering practical selection criteria for breeding finger millet adapted to acidic, P-poor soils. Field validation across multiple soil types and seasons would strengthen these findings.

References

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