`Gulfstream' and `Charentais' muskmelons (Cucumis melo. L.) plants were regenerated by in vitro culture to increase their genetic variability for resistance to root-knot nematodes (Meloidogyne spp.). While no genetic variability for root knot resistance was found, regenerated plants exhibited other traits that varied from the donor cotyledons. Chromosome counts confirmed that >75% of the somaclonal variants were tetraploid (2n = 24; 4n = 48). Tetraploids consistently exhibited micro- and macroscopic morphological changes that enabled distinction between tetraploids and diploids without chromosome counts; tetraploids contained enlarged stomates with more chloroplasts in the guard cells and pollen with a high percentage of square-appearing shapes. Tetraploids exhibited distinctive macroscopic morphological changes, including differences in leaf structure, fruit shape, blossom-end scar, number of vein tracts, and seed size.

Interspecific crosses between Cucumis metuliferus Naud . and C. anguria L . were obtained through embryo culture . Embryos in the rabbit-ear to advanced fluke-shaped stages were rescued 34-99 days after pollination . Plants were obtained through direct embryo culture, and through somatic embryogenesis from immature embryos . For direct embryo culture, fluke-shaped embryos were stored in sterile water in darkness for three days at 25C prior to transfer on Murashige and Skoog (MS) culture medium plus 1 .0µM 6-benzylamino-purine . Multiple plants were obtained from single embryos through somatic embryogenesis of rabbit-ear stages on MS plus 10 µM indole-3-acetic acid and 5 µM 6-benzylamino-purine . Evidence of hybridization included leaf shape intermediate between the two parents, penduncle shape prior to fertilization which resembled the male parent, low pollen viability and isoelectric focussing of protein bands for acid phosphatase of leaf extracts .

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Bush type snap beans (Phaseolus vulgaris L.) with resistance to the root-knot nematode, Meloidogyne incognita (cotton strain) are being developed by using PI-165426 as the resistant parent. PI-165426 (resistant), ‘Black Valentine’ (susceptible) and F5 breeding line B3864 (resistant) were inoculated with second-stage larvae. There were no significant differences in larval penetration of roots. Root tips showed slight swellings at infection loci of resistant and susceptible plants. Necrosis was evident in the resistant lines 4 days after inoculation. Histological studies of early infections showed that resistance was due to absence of adequate giant cell development and to hypersensitive reaction within the infected portion of the root. When soil temperature was changed from 16 to 28°C, galling, female development, and egg mass production in the resistant plants were increased.

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