Reproductive biology of the "Brazilian pine" (Araucaria angustifolia-Araucariaceae): the pollen tube growth and the seed cone development.

KEY MESSAGE: In Araucaria angustifolia, the seed scale is part of the ovule, the female gametophyte presents a monosporic origin and arises from a coenocytic tetrad, and the pollen tube presents a single axis. The seed cone of conifers has many informative features, and its ontogenetic data may help interpret relationships among function, development patterns, and homology among seed plants. We reported the seed cone development, from pollination to pre-fertilization, including seed scale, ovule ontogeny, and pollen tube growth in Araucaria angustifolia. The study was performed using light microscopy, scanning electron microscopy, and X-ray microcomputed tomography (µCT). During the pollination period, the ovule arises right after the seed scale has emerged. From that event to the pre-fertilization period takes about 14 months. Megasporogenesis occurs three weeks after ovule formation, producing a coenocytic tetrad. At the same time as the female gametophyte's first nuclear division begins, the pollen tube grows through the seed scale adaxial face. Until maturity, the megagametophyte goes through the free nuclei stage, cellularization stage, and cellular growth stage. Along its development, many pollen tubes develop in the nucellar tissue extending straight toward the female gametophyte. Our observations show that the seed scale came out of the same primordia of the ovule, agreeing with past studies that this structure is part of the ovule itself. The formation of a female gametophyte with a monosporic origin that arises from a coenocytic tetrad was described for the first time in conifers, and the three-dimensional reconstruction of the ovule revealed the presence of pollen tubes with only one axis and no branches, highlighting a new pattern of pollen tube growth in Araucariaceae.

Ultrastructural analysis of Rhynchospora ovules: The first record of Cyperaceae megagametophyte on transmission electron microscope.

Microsporogenesis and microgametogenesis are unusual in sedges (Cyperaceae), the third largest monocotyledonous family, as three microspores are aborted in favor of a single functional microspore. However, studies using light microscopy show that megasporogenesis and megagametogenesis occur normally. Nevertheless, the lack of ultrastructural details limits our knowledge of female gametophyte development in this family. Given the importance of morphological studies of reproductive structures, ovules and megagametophytes of Rhynchospora pubera were analyzed under transmission electron microscopy for the first time. Overall, ovules presented features similar to those described for the family, but ultrastructural details revealed an absence of a clear boundary between the egg cell and the central cell cytoplasm. Most interestingly, antipodal and nucellar cells showed several signs of vacuolar cell death, which suggest that programmed autolysis in sporogenous and gametophytic tissue is common in gametophyte development in the Cyperaceae. This may be related to the reproductive success of this family.

Heat transfer in the tank-inflorescence of Nidularium innocentii (Bromeliaceae): Experimental and finite element analysis based on X-ray microtomography.

In Bromeliaceae, various traits have evolved for the uptake and storage of water; however, their roles in bromeliad inflorescences remain unresolved. This study investigates the role of water in the flowers and inflorescences of Nidularium innocentii, and describes water as a protection mechanism. Individuals were divided into groups with and without water provision in inflorescences. Both groups were maintained with water in soil and leaves under the same environmental conditions. During anthesis, individuals were collected, and inflorescences were measured. Another specimen was prepared and scanned using X-ray microtomography (µCT), generating a high-resolution 3D model that was converted into a discretized geometry. Heat transfer finite element analysis (FEA) of the µCT-based geometry was then performed to simulate external temperature dissipation with the presence and absence of water in 3D. Flower size in the control group was significantly larger, and many injuries were observed in the drought group. FEA data indicated that the water environment led to lower temperature variation when compared to the air environment by significantly alleviating thermal amplitude. Water acted as a temperature stabilizer for the inflorescence, while its absence initiated physiological stress responses.