Growth and defense enzyme responses of grafted cucurbit crops to varying initial populations of Meloidogyne incognita

Root-knot nematode (Meloidogyne incognita) is among the most serious constraints to cucurbit crop production worldwide. This study evaluated the effects of different initial population densities (0, 1,000, 2,000, 4,000, and 8,000 second-stage juveniles [J ₂]/plant) on nematode development, reproduction, plant growth, and defense-related enzyme activities in watermelon (Citrullus lanatus, cv. Aswan F1) and sweet melon (Cucumis melo, cv. Faransawy) hybrids, their grafted combinations, and the resistant rootstocks 6001 F1 and BS F1. Plants were grown in sterilized sandy-loam soil under greenhouse conditions, and nematode parameters, including gall index, females per root system, egg masses per root, final population (Pf), reproduction factor, penetration rate, and reproduction rate were assessed 60 days after inoculation. Both host genotype and inoculum density significantly (P < 0.001) affected nematode reproduction and plant growth traits. Resistant rootstocks and their grafted scions consistently exhibited lower Pf/Pi ratios, fewer galls, reduced shoot weight losses, and significantly higher peroxidase and polyphenol oxidase activities compared to susceptible cultivars. These findings demonstrate that combining resistant rootstocks with grafting can effectively suppress M. incognita multiplication and associated growth losses, while enzyme activity profiling provides a useful biochemical indicator of partial resistance in cucurbit breeding and management programs.