Abstract
The host status of 17 medicinal plant species was assessed to the root-knot nematode, RKN (Meloidogyne incognita) susceptibility. Most tested plant species were incompatible with nematode infection according to the nematode reproductivity and ranked as follows, periwinkle, geranium, peppermint, and sage were immune (1), common mint, horsemint, marjoram, rosemary, and goldenrod were highly resistant (HR), coriander, dill, and thyme were ranked as resistant (R), caraway, parsley, and chamomile were moderately susceptible, sweet basil was susceptible and calendula was the highly susceptible host. The RKN was able to infect and reproduce efficiently on sweet basil, causing reductions in essential oil (E.O). In a four-month greenhouse experiment, the impact of four inoculum levels (3000, 6000, 9000, and 12000 J2/plant) on sweet basil, E.O quantity, and chemical structure were measured. Significant reductions were achieved in galls, eggs numbers as the inoculum level increased. E.O quantity reductions in basil leaves ranged from 31-53% compared to those obtained from healthy plants. GC-MS analysis of the E.O revealed different quantitative and qualitative alterations in basil E.O chemical profile due to different levels of nematode infestation, resulting in some constituents' ratios which went up or down and some others disappeared or even newly introduced. Generally, these changes did not follow a regular pattern. However, there was a proportional negative relationship between the major compound (linalool) with inoculum level and a converse situation that occurred in the case of eucalyptol. As E.O constituents are produced through the secondary metabolic pathway, it can be suggested that these alterations may be due to the reallocation of the secondary metabolic pathway products in basil as defensive molecules against nematode attack instead of producing E.O.