Piriformospora indica improves water stress tolerance in watermelon (Citrullus lanatus (Thunb.) Matsum & Nakai)

An investigation was conducted into how the beneficial fungal root endophyte Piriformosopra indica impacts the growth and physio-biochemical attributes of two watermelon varieties, Sugarbaby and Thar Manak under varying water conditions: well-watered (75% field capacity) and water-stressed (35% field capacity). Piriformosopra indica has stimulated vegetative growth in both varieties of watermelon irrespective of the water treatments. A significantly higher root length of 31.33 cm was observed in P. indica colonized Thar Manak at 35% field capacity. Piriformosopra indica colonization helped in mitigating the increase in leaf temperature and decline in leaf gas exchange parameters, leaf pigment concentration, and relative water content induced by water stress. During water stress, the highest proline content was observed in colonized seedlings of Sugarbaby (119.46 μg g⁻¹ fresh weight) and lowest in non-colonized Sugarbaby seedlings (60.44 μg g⁻¹ fresh weight). Diphenyl picrylhydrazyl free radical scavenging activity and catalase activity were enhanced due to pre-treatment with P. indica. The highest free radical scavenging activity of 60.30% was observed in P. indica colonized Thar Manak at 35% field capacity and the lowest in non-colonized Thar Manak at 75% field capacity (21.79%). Accumulation of harmful reactive oxygen species like H₂O₂ was lower in P. indica colonized seedlings when compared with the non-colonized ones during water stress. Although both varieties benefited due to colonization, the influence of P. indica was more pronounced in the susceptible variety, Sugarbaby.