Ovule and Female Gametophyte Development in Fertile and Sterile Safflower Plants (Carthamus tinctorius L.)

Development of safflower ovules and female gametophytes was compared in fertile and genetically sterile F2 and backcross segregants from the cross between 'US-10' and '57-147' genotypes. Fertile plants formed normal anatropous ovules with eight-nucleate embryo sacs, typical of the angiosperms. One week before anthesis, the eight-nucleate embryo sac is well developed and undergoes rapid elongation and expansion during the 24 h prior to anthesis, accompanied by a doubling in length of the florets. Sterile plants also formed normal ovules, but apparently with a delayed initiation of meiosis which was subsequently arrested at Metaphase I. Embryo sacs did not form in sterile florets except for rare observations of uninucleate embryo sacs which began to degenerate before anthesis. The integumentary tapetum which normally developed upon completion of meiosis in fertile plants, was well developed during Prophase I of megasporogenesis in sterile plants. This observation suggests that cell differentiation and development of this nutritive jacket is basically controlled by the age of the ovules rather than initiated by appearance of the functional megaspore. Failure of both female and male gametogenesis seems to result from interaction of three independently segregating genes.