Submitted to: HortScience
Publication Type: Abstract Only
Publication Acceptance Date: 4/27/2016
Publication Date: N/A
Interpretive Summary: Pollen grains of various species of watermelon (Citrullus) were examined using an electron microscope and a light microscope. All of the known watermelon species related to watermelon were included in the study, and also one sample of a plant related to watermelon – but believed to belong to a different genus (Acanthosicyos). Pollen grains of all the plant materials were quite similar in terns of their appearance (size, shape, and general physical characteristics). Two unusual types of pollen with a surface that was distinctly different from all other samples, were detected. Both of these were citron type watermelons. Pollinations were made to intercross (hybridize) the watermelon-related species Citrullus colocynthis with Acanthosicyos naudinianus. The hybridizations were successful and two fruit were produced. The seed from threse hybridizations germinated and plants were grown in the greenhouse. The hybrid plants had several characteristics in common with each of the parents. Tis indicates that Citrullus colocynthis may be useful in efforts to transfer characteristics associated with Acanthosicyos naudinianus to the cultivated waterrmelon.
Technical Abstract: 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. 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 (3) 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 C. colocynthis and A. naudinianus 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.