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Production of hull-less seeds of styrian oil pumpkin (Cucurbita pepo ssp. pepo var. styriaca) is increasing as a result of demand for aromatic seed oil and for other uses. Hybrid cultivars have recently been released but a method for haploid induction has not been established. We focused on the development of a parthenogenetic haploid induction protocol based on pollination with pollen irradiated with X-ray radiation at 0, 50, 100, 150, 200, 300, and 350 Gy. Fruit set largely decreased at 200 Gy, whereas a decline in embryo formation was observed at 100 Gy. Various accessions were tested as the female parent or as the pollen donor and large differences were found. The best parthenogenetic response was found in ‘Turkey #2’ (10.0%), ‘Gleisdorfer Ölkürbis’ (4.4%), and ‘Naked Seed’ (3.9%), whereas ‘GL Opal’ and ‘White Acorn’ were efficient as pollen donors. The ploidy level of 3830 putative parthenogenetic embryos was determined using flow cytometry. Four ploidy levels (n, 2n, 3n, and 4n) were found with the majority being diploid. Interestingly, a significant proportion was determined to be tetraploid and this was clearly correlated with increased radiation delivered to pollen grains. Using selected simple sequence repeat markers on diploid embryos, no spontaneous chromosome doubling could be confirmed. In this study, haploid induction in styrian oil pumpkin was elaborated for the first time. We also showed that X-ray pollen irradiation provides an alternative to gamma radiation treatment, yielding a sufficient percentage of haploid plantlets.

Protocols for haploid induction in cucurbits are based on pollination with irradiated pollen, but the induction frequency is low and the majority of obtained embryos are zygotic. The longevity of Cucurbita pepo L. pollen is short even under natural conditions; following irradiation, germinability is decreased even further. This study was initiated to develop an optimal in vitro germination medium for styrian oil pumpkin (Cucurbita pepo L. subsp. pepo var. styriaca Greb.) pollen, which would enable accurate germination testing. Different pH values and the addition of sucrose, mannitol, and polyethylene glycol to the Brewbaker and Kwack germination medium were tested. The optimum medium condition was pH 9 and 12.5% (w/v) sucrose, while other tested components were not efficient. Using the optimized medium, X-ray-irradiated (100-700 Gy) pollen germinability was assessed under 2 air humidity conditions. Germinability of pollen irradiated at room humidity (RH) was generally lower than that of pollen irradiated at high humidity (HH). A major variability in pollen size (diameters ranged from 79.2 to 196.5 µm) and 2 subgroups were found in the pollen population. Following irradiation, HH conditions allowed germination of larger pollen grains than those of the nonirradiated control and RH.

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