Anatomical changes in corn (Zea mays L.) roots caused by Meloidogyne javanica.

The use of corn as a rotation crop in integrated programs for controlling p lant- parasitic nematodes has been widely recommended. In Brazil, where mos t of the commercial varieties and hybrids have been rated as resistant to r oot-knot nematodes for many years, its inclusion in cropping sequence strat egies for the control of Meloidogyne incognita and mainly M. javanica becam e almost obligatory. At present, however, it is known that many genotypes a re actually tolerant hosts for M. javanica, allowing the nematode to develo p and reproduce in the roots, without causing root galls or plant growth re duction. A greenhouse/laboratory study was carried out to increase our know ledge of the anatomical changes induced by M. javanica in corn roots. Corn hybrid BR 106 (a suitable host) seedlings were individually inoculated with 10 000 second-stage juveniles and maintained in a greenhouse. After 60 day s, slightly swollen and galled root pieces (1-3 cm long) were processed for light microscopy histological studies. Typical multinucleated giant cells limited to the vascular tissue were observed in association with M. javanic a in all infected roots. Even with several mature females and egg masses em bedded in their tissues, most root galls formed were rather inconspicuous, and frequently occurred close to the root apex. In some cases cortical pare nchyma cells were hypertrophied. The presence of giant cells and hypertroph ied parenchyma cells in the stele region obliterated part of the xylem vess els causing disruption and disorganization of the vascular cylinder. Staini ng techniques allowed the detection of a high protein content in the giant cells as well as a deposition of pectic substances in their thickened walls .