Proteins and triacylglycerols (TAGs) are the major reserves of loblolly pine (Pinus taeda L.) seeds. They are located within protein and lipid bodies found within storage parenchyma of the megagametophyte and embryo. At maturity, more than 70% of these reserves are located within the megagametophyte, a non-embryonic tissue. Consequently, the developing seedling is dependent upon the megagametophyte for much of its nutrition until photoautrophic independence is attained. Rapid breakdown of megagametophyte reserves occurs following germination; depletion of materials within protein bodies coincides with the biochemical breakdown of buffer-insoluble storage proteins. Amino acids and carbohydrates, generated from reserve breakdown, are immediately exported from the megagametophyte to the developing seedling. This indicates that the developing seedling is a very efficient sink; the megagametophyte is closely appressed to the seedling, seemingly to facilitate metabolite transport. The seedling is required to maintain rapid reserve breakdown in the megagametophyte. In relation to this, the seedling influences the level of activity of several key enzymes of megagametophyte TAG breakdown. The reverse is also true. The megagametophyte not only provides nutrition for the seedling during its early development, it also exerts control over the utilization of megagametophyte-derived nutrients by the seedling; it does this through the induction and maintenance of seedling enzymes involved in nutrient assimilation. Clearly, in loblolly pine, germination and early seedling growth involves a complex set of interactions between the megagametophyte and seedling that are bi-directional.

Key words: germination, loblolly pine, megagametophyte, Pinus taeda, seed, seedling growth