Styrian oil pumpkin pollen germinability at higher irradiation doses: optimization of the in vitro germination protocol and irradiation procedure

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.