Physiological and ultrastructural responses during drying of recalcitrant seeds of Araucaria angustifolia

The intense exploration of forests for wood production in Brazil over the past decades has led to the reduction of the population of native trees with economic importance and many efforts have been carried out to conserve these species. The purpose of this study was to characterise the germination process and evaluate the effect of different drying rates on physiological and ultrastructural changes of Araucaria angustifolia seeds from two provenances. The imbibition pattern was linear during germination and when seeds were cut, water uptake increased, indicating that the seed coat inhibits water absorption although final germination percentage was not affected. Seeds subjected to fast drying with silica gel or slow drying in a cold room did not show interaction between water content (WC) and drying rate, and total viability was lost when seeds were dried to 20% WC. Under slight drying at a fast rate, seeds survived dehydration to 34% WC (85% germination). The ultrastructure of mature embryos indicated active metabolism and starch appears to be the main storage reserve. Drying to lower WC promoted abnormal appearance of cell walls, deformation of organelles and subcellular deterioration which was followed by an increase in electrical conductivity.