Distribution of exchangeable Ca²⁺ during the process of Larix decidua Mill. pollination and germination

The involvement of Ca²⁺ ions in angiosperms sexual processes is well established, while in gymnosperms, such knowledge remains limited and is still a topic of discussion. In this study, we focused on Larix decidua, using Alizarin-red S staining and the pyroantimonate method to examine the tissue and subcellular distribution of free and loosely bound Ca²⁺ ions at different stages of the male gametophyte's development and its interaction with the ovule. Our findings show that in larch, both the germination of pollen grains and the growth of pollen tubes occur in an environment rich in Ca²⁺. These ions play a crucial role in the adhesion of the pollen grain to the stigmatic tip and its subsequent movement to the micropylar canal. There is a significant presence of free and loosely bound Ca²⁺ ions in both the fluid of the micropylar canal and the extracellular matrix of the nucellus. As the pollen tube extends through the nucellus, we observed a notable accumulation of Ca²⁺ ions just above the entry to the mature archegonium, a region likely crucial for the male gametophyte's directional growth. Meanwhile, the localized presence of free and loosely bound Ca²⁺ ions within the egg cell cytoplasm may inhibit the pollen tubes growth and rupture, playing an important role in fertilization.