Substantial resource reallocation during germination of Araucaria bidwillii (bunya pine), an Australian rainforest conifer with large seeds and cryptogeal germination

Key message: Araucaria bidwilliihas cryptogeal germination, with seedlings not starting photosynthesis for several months after imbibition. Our results indicate that this complex process is metabolically relatively efficient. Abstract: Araucaria bidwillii (bunya pine, Araucariaceae), an Australian rainforest conifer, produces heavy (15–20 g fresh mass) recalcitrant seeds that feature an unusual cryptogeal germination sequence. Germination features a substantial reallocation of starch from the massive megagametophyte (12–14 g fresh mass) to the hypocotyl which progressively develops into a tuberous, belowground storage organ. While germination (radicle emergence) is very rapid after imbibition, seedlings do not commence photosynthesis for several months. We were interested in the metabolic efficiency of this lengthy and complex developmental process that depends solely on stored resources. On average, the seeds had a dry mass of 10.1 g (seed coat 29 %, megagametophyte 68 %, embryo 3 %). Over 47 days the megagametophyte lost 91 % of its mass (mainly through the almost complete export of its abundant starch reserves), while the embryo/seedling increased in mass by 19 times and the hypocotyl almost 200 times. In total, a 13–14 % loss of dry mass occurred in converting a large seed at soil level into an established seedling with substantial protected belowground reserves. If only considering the living components of the seed/seedling (i.e. excluding the substantial and woody seed coat) then an 18 % loss occurred. Comparison with its closest cryptocotylar relatives from South America (A. araucana and A. angustifolia) and a range of hypocotylar rainforest angiosperms indicates that the complicated germination sequence of A. bidwillii is relatively efficient. While A. bidwillii now has a highly disjunct and contracted distribution this would appear not to be directly related to the metabolic efficiency of its unusual germination sequence.