Combined Protein Hydrolysate and Seaweed Extract Biostimulants Enhance Nodulation Dynamics and Nodule Development of Green Bean (Phaseolus vulgaris L.) under Semi-Arid Field Conditions

Biological nitrogen fixation is a key process for sustainable legume production, particularly under semi-arid environments where nitrogen availability and water resources frequently limit crop productivity. Although non-microbial biostimulants are increasingly recognized for improving plant growth and stress tolerance, their influence on the temporal dynamics of nodulation under field conditions remains insufficiently documented. The present study evaluated the effects of a combined protein hydrolysate- and seaweed extract-based biostimulant program on the development of nodules in green bean (Phaseolus vulgaris L.) cultivated under semi-arid conditions in central Morocco. Field experiments were conducted using untreated and biostimulants-treated plants, and nodulation was monitored throughout seven phenological stages (T1–T7). At each stage, ten plants per treatment were sampled to determine nodule number and nodule dry biomass. The protein hydrolysate was applied by fertigation (5 L ha⁻¹ at T1, T3 and T4), whereas the seaweed extract was applied as a foliar spray (500 mL hL⁻¹ at T3, T4 and T6). Data were analyzed using Student's t-test after verification of variance homogeneity. No nodules were observed during the first three developmental stages in either treatment. From the pre-flowering stage (T4), biostimulants-treated plants exhibited a marked increase in nodulation compared with untreated plants. Nodule number increased from 2.0 to 25.35 nodules plant⁻¹ at T4, 4.0 to 42.33 at T5, 11.0 to 71.25 at T6 and 13.0 to 119.33 nodules plant⁻¹ at T7, corresponding to relative gains ranging from 548% to 1167%. Nodule dry biomass also increased significantly throughout the reproductive period, indicating enhanced structural development of the symbiotic organs. The stimulation of nodulation is interpreted as an indirect consequence of improved plant physiological performance resulting from the combined action of protein hydrolysates and seaweed extracts on root development, nutrient acquisition and photosynthetic activity. Although biological nitrogen fixation was not directly quantified, the simultaneous increase in nodule number and biomass demonstrates that the formulated biostimulants substantially promoted the establishment and development of the legume–rhizobium symbiosis under semi-arid field conditions. These findings highlight the potential of combining agro-food-derived protein hydrolysates and seaweed extracts as sustainable biostimulants to improve nodulation in common bean while supporting environmentally friendly crop management strategies in water-limited environments.