Natural Etiolation in Spearmint (Mentha spicata L. var. crispa) Shoots Emerging From Rhizomes-Plastids, Glandular Hairs, and Phytochemical Profile.

Spearmint (Mentha spicata L. var. crispa) is a widely cultivated aromatic herb valued both as a culinary ingredient and as a source of bioactive compounds, such as essential oils and phenolics, which contribute to its traditional uses and potential medicinal applications. In this work, we provided the first evidence of natural etiolation with etioplast and prolamellar body formation in leaves and stems of spearmint shoots emerging in the soil from underground rhizomes. Soil-grown and hydroponically dark-forced shoots exhibited comparable etiolation symptoms, plastid ultrastructure, and chlorophyll biosynthesis as revealed by 77 K fluorescence emission spectroscopy, establishing laboratory etiolated plants as reliable models for natural skotomorphogenetic processes. Comparison of 14-day-old light-grown and dark-forced shoots developing from the rhizomes of hydroponically cultured adult plants revealed that the slightly smaller etiolated leaves had significantly smaller peltate glandular hairs than light-grown ones (diameter: 54-56 μm vs. 59 μm, respectively), but their number and density were not influenced by light and they were always present in much higher numbers on the abaxial side than on the adaxial side of the leaf surfaces. Principal component analysis of the volatile organic compound and phenolic profiles distinctly separated light-grown and dark-forced shoots, with fewer compounds being present and in lower relative abundance in etiolated tissues, and more pronounced differences observed in the phenolic composition. These data underscore the value of etiolation and greening research, provide new models for studying development and environmental interactions under naturally light-deprived conditions, and offer novel insights into primary and secondary metabolism in hidden spearmint organs.