DEVELOPMENT OF ASPARAGUS MICROSPORES IN-VIVO AND IN-VITRO IS INFLUENCED BY GAMETOGENIC STAGE AND COLD TREATMENT

Development of asparagus microspores in cold-treated buds of varying s izes and shed microspores from these buds in in vitro culture were obs erved cytologically for the G459 genotype. Before cold pretreatment, m ore than 75% of the microspores in flower buds of the 1.4-1.6, 1.7-1.9 , 2.0-2.2, 2.3-2.5, and 2.6-2.8 mm size classes were at the early-, mi d-, late-uninucleate, early-, and late-binucleate stages, respectively . After 7 d in cold treatment, percentages of microspores at different stages changed in all flower buds. Most notable was the appearance of binucleate microspores resulting from symmetric rather than asymmetri c division. For flower buds of 1.7-1.9, 2.0-2.2, and 2.3-2.5 mm size c lasses, 4.9%, 2.2%, and 11.4% of the microspores had divided symmetric ally, respectively. When microspores from buds of each size category w ere cultured in androgenesis induction medium, only microspores comple ting symmetric pollen mitosis I during cold treatment were observed to divide further, and calluses were only obtained from microspores of f lower bud size classes where symmetric divisions were observed after s everal days of cold treatment. Significant correlations existed among microspore callus yield, the percentage of late-uninucleate microspore s in vivo before cold treatment, and the frequency of symmetric pollen mitosis I after 7 d of cold treatment. Consequently, asparagus micros pore androgenesis may occur through one developmental pathway, where a symmetric first mitotic division is a prerequisite for continued deve lopment.