Advances in Genetic Study of Cultivar Resistance of Psidium spp. Meloidogyne spp. in a tropical dry forest

In order to study the genetic resistance of Psidium spp. cultivars to nematodes of the genus Meloidogyne, the chromosomal, cytological and morphological constitution of guava trees in a tropical dry forest located in the area of the Centro Fruticola Zulia State-CORPOZULIA (10 degrees 49'47 '', 31914 LN, 71 degrees 46'28 '', 44742 LO), Venezuela was determined. Four tolerant cultivars of Psidium guajava were selected ("AGROLUZ-14", "AGROLUZ-42", "AGROLUZ-45"), one susceptible ("Criolla Roja") and one of Psidium friedrichsthalianum ("Cas") resistant to Meloidogyne spp. (M. mayaguensis and M. incognita). The technique of squashing the root tips was used for the cytogenetic study, in which 1050 cells/tree were analyzed determining: mitotic hour, mitotic index (MI), index of phases and caryological constitution. The mitotic hour was observed in roots collected between 6 and 9 a. m., where they registered the greater values of MI. Mitotic index of tolerant selections were more similar to those of P. friedrichsthalianum, than those of the susceptible selection. All P. guajava cultivars presented 2n = 22 chromosomes with a variable length between 2 and 0.6 mu, while P. friedrichsthalianum presented 2n = 44 with a length of 2.5 to 0.6 mu. The caryological formulae found were: 7m+2sm+2ac for the tolerant selections of P. guajava, 8m+1sm+2ac for the selection "Criolla Roja" and 14m+4sm+4ac for the P. friedrichsthalianum karyotype. In the morphological study several aspects of the leaves, flowers, fruits and seeds were considered. The tetraploidy found in P. friedrichsthalianum agrees with the foliar and fruit characteristics of these plants. The obtained data allows developing programs of genetic improvement with a special view to obtain pest-resistant cultivars that produce fruits of excellent quality to supply the national and world marketplace.