Structural and biochemical changes in loblolly pine (Pinus taeda L.) seeds during germination and early seedling growth.: II.: Storage triacylglycerols and carbohydrates

Triacylglycerols (TAGs) comprised 59% of the total storage reserve in mature loblolly pine (Pinus taeda L.) seeds; 80% of these TAGs were stored in the megagametophyte. The TAG breakdown in the seedling was initiated before radicle emergence (during germination), while in the megagametophyte breakdown occurred after radicle emergence (during early seedling growth). In both seed tissues, the majority of TAG breakdown took place during early seedling growth. Within the seedling, the most rapid rate of TAG breakdown occurred in the radicle and hypocotyl. Unlike TAGs, there was very little carbohydrate stored in loblolly pine seeds at maturity. Levels of 80% ethanol-soluble carbohydrate in the megagametophyte and seedling decreased during germination and then increased during early seedling growth. This increase coincided with the period of rapid TAG depletion. Accompanying the increase in 80% ethanol-soluble carbohydrate level was a corresponding increase in 80% ethanol-insoluble carbohydrates, such as starch, in the seedling during early seedling growth. Accumulation of starch occurred in both the cotyledons and hypocotyl. Starch accumulation in the megapametophyte was more transient, occurring around germination. The megagametophyte was important for;he growth and nutrition of the seedling. In the presence of the megagametophyte, the seedling accumulated sucrose during early seedling growth. However, in the absence of the megagametophyte, loblolly pine seedlings failed to accumulate carbohydrates to any great extent.