Hypocotyl of seedlings of the large-seeded species Araucaria angustifolia: an important underground sink of the seed reserves

Abstract

Araucaria angustifolia exhibits cryptogeal germination, where the root-hypocotyl axis emerges first and penetrates into the soil. In Araucaria bidwillii, the whole process of transferring reserves from the seed to the seedling takes place before shoot emergence, and there is a major storage of these reserves in the underground hypocotyl, which assumes a tuberous form. In A. angustifolia, the shoot emerges before seed reserves are depleted. Though it does not grow like a tuber, the hypocotyl of A. angustifolia grows thicker than the adjacent taproot during initial growth, and we hypothesize that it may act as a major sink for seed reserves during this stage. The study tests this hypothesis by evaluating changes in the mass of different plant parts during initial growth. Four harvests were conducted during a *6-month period to compare the dry mass of different fractions (attached seed, seedling, its shoot and root and the hypocotyl) of seedlings growing under darkness and high light. While seed reserves were still being depleted, the hypocotyl mass showed an initial increase and then a reduction. This was more abrupt when light was available. After seed mass had stabilized, the mass of the hypocotyl continued to decrease in the darkgrown seedlings, but showed a second increase in the lightgrown ones. Results confirm the hypothesis that the hypocotyl represents a major sink for the seed reserves of A. angustifolia, acting as an underground storage structure for the growing seedling. Its reserves seem to be important for sustaining initial shoot growth and might also act as a storage sink for photosynthates.

The development of seedlings from fragments of monoembryonic seeds as an important survival strategy for Eugenia (Myrtaceae) tree species

“…For some species, the reserves are stored belowground, where they are probably safer from herbivores. However, even in these conditions, as long as the cotyledons do not persist below the ground, the great amount of reserves can be very important in sustaining initial shoot growth, for example in Hymenaea courbaril (Santos and Buckeridge 2004) and Araucaria angustifolia (Dillenburg et al 2010). In fact, most of the Eugenia species show few seeds per fruit, but these seeds are often large and heavy.…”

Growth inhibition of an Araucaria angustifolia (Coniferopsida) fungal seed pathogen, Neofusicoccum parvum, by soil streptomycetes

“…The plant growth is strongly dependent on carbohydrate import from seed until 70 – 80 days after germination [17] and the seed reserves are apparently exhausted approx. 100 days after planting [40]. …”

“…The pathogenicity test was performed by using healthy seeds excised from mature cones collected in 2011. Seeds were disinfected as previously described and scarified by removing the integuments from the seed tip [40], exposing the megagametophyte. Scarified seeds were incubated at 25°C in darkness with the fungus.…”

The periodic wetting of leaves enhances water relations and growth of the long‐lived conifer Araucaria angustifolia

“…Plants were grown on laboratory benches placed by a glass window and were well watered during the first 6 months after sowing. This period ensured complete exhaustion of the seed reserves (Dillenburg et al. 2010).…”