Responses of tomato plants bearing Mi-1 exposed to high soil temperatures for varied time periods to Meloidogyne incognita

Tomato is one of the most important vegetables grown in the world. Root-knot nematodes are considered as economic pests of tomato. The use of tomato varieties resistant to root-knot nematodes is the most effective management method. In tomato, resistance to root-knot nematodes is controlled by the Mi-1 gene which confers resistance to Meloidogyne incognita, M. javanica and M. arenaria. But Mi-1 gene is not effective against virulent root-knot nematode populations. Moreover, resistance provided by this gene is broken down at high soil temperature. There is contradictory information about the breaking duration of resistance and the performance of Mi-1 gene at high soil temperature. In the present study, responses to M. incognita of susceptible (mimi), heterozygote resistant (Mimi) and homozygote resistant (MiMi) tomato varieties subjected to four different soil temperatures (25°C, 28°C, 30°C and 32°C) for time periods (6, 12, 24, 48, 120 and 168 h) were separately investigated. In all experiments, tomato plants with four true leaves stages were inoculated with 1000 second-stage juveniles of M. incognita. Plants were inoculated with M. incognita in two different experimental designs. First, plants were separately exposed to different soil temperatures for time periods mentioned above. Then, the plants were transferred to another growth chamber. When soil temperature was at 25°C, the plants were inoculated with M. incognita. Resistance of tomato varieties was not broken down in the tested all soil temperatures. Second, the plants were separately exposed to different soil temperatures mentioned above for time periods (6, 12, 24, 48, 120 and 168 h). When soil temperature reached separately at 25°C, 28°C, 30°C and 32°C, plants were simultaneously inoculated with M. incognita. Inoculated plants were held in soil temperatures mentioned for time periods and then, transferred to another growth chamber at 25°C soil temperature. The number of juveniles in soil, and galls and egg masses on root of resistant plants held at 30°C soil temperature for time periods 120 and 168 h increased a few values. But Pf/Pi ratio was < 1. The number of egg masses on root of resistant plants held at 32°C high soil temperature for time periods 6 and 12 hours did not change. But, the number of galls and egg masses on root of resistant plants held at 32°C high soil temperature for 24 hours were statistically significant. When same plants were held at 32°C soil temperature for 48 hours, the number of juveniles in soil, gall and egg masses on root of them increased more than for 24 hours. Pf/Pi ratio of heterozygote resistant plants subjected to at 32°C soil temperature for 48 hours and more than 48 hours was >1. However, Pf/Pi ratio of homozygote resistant plants subjected at 32°C soil temperature for 120 and 168 hours was >1. This study showed that resistance was broken in plants held at 32°C soil temperature for 48 hour and more. The findings could help the efficient use of tomatoes bearing Mi-1 gene grown in fields which are at high soil temperatures (≥32°C).