Currently, there is increasing demand for brown kelp, especially Macrocystis spp., to support the development of the abalone industry and for use as raw material for the production of alginate. Thus, there is a need to supplement the naturally available kelp biomass with the development of mariculture techniques. In this study, we investigated whether it was possible to cultivate Macrocystis pyrifera sporophytes in the laboratory using gametophyte clones. We propagated gametophytes by periodic fragmentation and then mixed aseptically an equal number of male and female gametophytes to induce fertilization. Approximately 60 days after fertilization, numerous embryonic sporophytes emerged on the seed fibre. By using chlorophyll fluorescence technology, we investigated variation in the photosystem II (PSII) characteristics of M. pyrifera during development from the gametophyte to juvenile sporophyte stage. During this growth period, both the optimal (Fv/Fm) and effective (YII) PSII quantum yields increased significantly from the haploid gametophyte to diploid sporophyte stages. In addition, by comparing female gametophytes with male gametophytes, we found that female gametophytes were not only larger and more pigmented than the males, but also recorded higher values of Fv/Fm and YII. Our results supplemented information on the life cycle and PSII photosynthetic performance during the early development and confirmed the possibility of artificial seeding technology, both of which may contribute to mariculture of M. pyrifera in China in the near future.