Morphometric Investigation of Mitochondrial Ultrastructure in Etiolated Pea Seedlings

This study focuses on the ultrastructural and functional organization of mitochondria from etiolated 6-day-old pea seedlings (Pisum sativum L., cultivar ‘Aksayskiy Usatyi 55’); a morphometric profile of a typical pea organelle was compiled and compared with available data from other organisms. It was shown that in the absence of respiratory substrates and ADP, 93.6% of isolated mitochondria retained a condensed, functionally competent conformation. The organelles exhibited classical ultrastructure with a homogeneous electron-dense matrix, clearly distinguishable inner and outer mitochondrial membranes, and dilated cristae with predominantly narrow crista junctions. A representative pea mitochondrion measures approximately 0.80 × 0.64 µm with a cross-sectional area of 0.39 µm2 and possesses a well-developed cristae network with inner-to-outer and cristae-to-boundary membrane ratios of 4.44 and 5.18. The surface density of the inner membrane averages 25.91 µm2/µm3. The enlarged cristae with preserved narrow crista junctions indicate a low-active but potentially functional ‘ready for OXPHOS’ state of mitochondria, which was confirmed by their high oxidative phosphorylation activity upon addition of respiratory substrates and ADP. Fragments of dimeric ATP synthase arrays were traced along the cristae edges. The dimensions and arrangement of dimers suggest a small angle between monomers and relatively loose organization in rows, which could promote the formation of flat ridges and widened cristae. Comparative cross-taxon analysis of the morphometric mitochondrial parameters indicates high energy metabolism in pea organelles. Open-access database comprising ultrastructural and functional characteristics of mitochondria from various species and organisms under diverse physiological conditions is considered as a prospective resource for multiple research purposes.