[Polyploidy in melon (Cucumis melo L.)]

Tetraploids from five melon commercial varieties were obtained by means of seed immersion in a colchicine solution, at a rate of 8 per cent of the total treated material (32 per cent of the material that could be grown in the field). All varieties showed very low fertility. Triploids were obtained by crossing tetraploid with diploid plants, 2X x 4X crosses gave no offspring and just a few weak plants were obtained from 4X x 2X crosses. Factors involved in the sterility of these polyploids were studied. Delay and decrease in germinative capacity of the polyploid pollen grains, double germinations, bifurcations and hyperdeveloping of the tubes into the ovules were observed, which may suggest a loss in the functionality of such tubes. The fertility of this pollen has been proved, so that loss of functionality cannot explain by themselves the lack of success in the 2X x 4X crosses. A delay of the pollinic tubes has been observed in the latest stages of growing inside the ovary, a fact that may decrease the probability of fertilization. Also, the internal conditions of the endosperm, altered because of the different parental contributions, may influence the lack of viability of the zygote. A comparative study between tetraploids and their corresponding diploids was made. The most useful characteristics for the recognition of tetraploids are its broader floral parts, like petals and ovary, bigger pollen grains and the presence of four germinal pores in most of them, instead of the usual three. Tetraploid fruits showed increased soluble solids content, flesh consistence and skin thickness. However, some problems, particularly those related to high sterility and low yield, must be solved before they can be commercially used. Possible use of triploids will require previous obtention of fertile vigorous parentals, and this seems to need a more intense previous work of selection.