Treatment-dependent responses of Opuntia ficus-indica to electron-beam-irradiated superabsorbent polymer, zeolite, and biostimulants at a reduced soil-water target

Abstract

A reduced soil-water target may constrain cactus-pear establishment, but the responses to combinations of water-retentive amendments and biostimulants remain uncertain. In a 90-day greenhouse experiment, five physicochemical regimes (none, electron-beam-irradiated superabsorbent polymer at 1–2 g kg⁻¹ soil [EISAP₁ or EISAP₂], and zeolite at 1–2 g kg⁻¹ [Z₁ or Z₂]) were crossed with all eight presence/absence combinations of arbuscular-mycorrhizal inoculum (My), seaweed extract (SW), and humic acid (HA). Three independent pots were used per combination. Pots were maintained gravimetrically at a target derived from 70% of the field capacity of the unamended soil. Continuous responses were analyzed with a 5 × 8 fixed-effects model, followed by Tukey-adjusted comparisons when appropriate; cladode number was analyzed with a Poisson model. Physicochemical regime, biostimulant regime, and their interaction affected most continuous traits (interaction P < 0.05 for 15 of 17 responses); the interaction was not detected for chlorophyll b or total phenolics. Relative to the untreated control, the highest observed shoot fresh mass occurred under My + SW+HA+EISAP₂ (99.50 vs. 45.95 g; Tukey-adjusted P < 0.001), and the highest relative water content occurred under My + SW+EISAP₂ (98.66% vs. 83.06%; P < 0.001). The highest observed total chlorophyll occurred under HA + SW+EISAP₂ (1.835 vs. 0.770 mg g⁻¹ fresh weight; P < 0.001), whereas the highest DPPH value occurred under My + SW+HA + Z₂ (76.89% vs. 65.78%; P < 0.001). PC1 and PC2 described 48.34% of treatment-mean variation; the first four components described 75.02%. Responses were combination- and trait-specific. EISAP₂- and Z₂-containing combinations are candidates for further replicated greenhouse and field evaluation, but the experiment did not establish drought mitigation, irrigation savings, mycorrhizal colonization, or superiority to non-irradiated SAP.