Notes on Citrullus spp. and Acanthosicyos naudinianus

Scanning electron and light microscopy were utilized to examine pollen of the currently recognized species (and forms) within the genus Citrullus (Cucurbitaceae). Materials examined included: C. lanatus (Thunb.) Matsum. & Nakai including the citron (C. amarus Schrad.) and egusi (C. lanatus subsp. mucosospermus Fursa) forms, C. colocynthis (L.) Schrad., C. rehmii de Winter, C. ecirrhosus (Cogn.) and a member of a closely related genus - Acanthosicyos naudinianus (Sond.) C. Jeffrey (Citrullus naudinianus (Sond.) Hook. F., Cucumis naudinianus Sond.). Pollen of all species and forms were similar in shape differing slightly in their width (W), length (L) and L/W ratio. In general, all were characterized as prolate and tricolpate with a small polar area and reticulate ornamentation. A mutant effecting pollen ornamentation was identified in PI 482261, a citron type from Zimbabwe. Pollen ornamentation on PI 482261 was rugulate. An examination of 15 additional accessions from neighboring areas in Zimbabwe revealed a second mutant of this type in PI 482312. Mutations of this sort could be used to facilitate studies of pollen transmission in this genus. Artificial hybridizations (2) between C. colocynthis (PI 482261) x A. naudinianus (GRIF 14032) produced 84 seed from two fruits. The seed were subsequently germinated in vitro and the seedlings (21) acclimated to greenhouse conditions. The F₁ hybrids exhibited the zig-zag growth pattern of the stem typical of the male parent and other characteristics of the male parent such as the presence of solitary tendrils and storage root formation. These preliminary data indicate a relatively close relationship between A. naudinianus and C. colocynthis and suggest that introgression of traits such as the drought tolerance, frost tolerance, storage root formation and other characteristics associated with A. naudininanus (Gemsbok cucumber) to C. lanatus may be possible using C. colocynthis as a bridge species.