Salvia L. is a species-rich and easily recognizable genus within the subtribe Salviinae (tribe Mentheae, subfamily Nepetoideae, family Lamiaceae), comprising approximately 1,088 species (Kriebel et al. 2019; Drew 2020; POWO 2025). The genus has an almost cosmopolitan distribution (Hu et al. 2018). Salvia has undergone significant species radiations in three global biodiversity hotspots: the Americas (Central to South America, with 510 species, and North America, with 94 species), followed by Asia (270 species in Western Asia and 97 species in East Asia), and Europe, hosting 117 species (Wei et al. 2015; Rose et al. 2021). At the country level, Mexico harbors the highest number of species (322), followed by Russia (109), Turkey (88), the United States (85), and China (82) (Wei et al. 2015; POWO 2025).

This study focuses on the genus Salvia in Japan, which comprises 12 species (Murata and Yamazaki 1993; Inoue 1997; Inoue and Ozawa 1998; Hihara et al. 2001; Takano and Okada 2011; Takano et al. 2014; Hu et al. 2018; Hayakawa et al. 2024). Among the 12 Salvia species, eight are endemic, while the remaining four have broader distributions (Takano et al. 2014; Hu et al. 2018; Hayakawa et al. 2024). These include S. japonica Thunb., also found in Korea, Taiwan, and China; S. nipponica Miq., present in both China and Japan; and S. plebeia R. Br., which is widely distributed across Asia and Australia (Murata and Yamazaki 1993; Hu et al. 2018). Most recently, Hayakawa et al. (2024) reported the first naturalization of Salvia lyrata L. from a roadside on Niijima Island, Izu Islands, Japan. This species, native to the United States (Marín et al. 1996; POWO 2025), is cultivated as a garden herb in Japan and has shown the potential to escape from cultivation and become naturalized in the wild (Hayakawa et al. 2024).

Japanese Salvia has traditionally been classified into three subgenera: subg. Salvia, subg. Sclarea Mill., and subg. Allagospadonopsis Briq. (Wu 1977; Murata and Yamazaki 1993). Most recently, based on molecular and morphological evidence, Hu et al. (2018) reclassified all East Asian Salvia into a newly recognized subgenus, Glutinaria (Raf.) G.X. Hu, C.L. Xiang & B.T. Drew, and further recognized eight sections within this subgenus. All native Japanese Salvia species were placed in subg. Glutinaria and assigned to three sections. Sect. Sobiso (Raf.) G.X. Hu, A. Takano & B.T. Drew comprises Salvia akiensis A. Takano, Sera & N. Kurosaki, S. isensis Nakai ex H. Hara, S. japonica Thunb., S. lutescens (Koidz.) Koidz., S. omerocalyx Hayata, S. pygmaea Matsum., and S. ranzaniana Makino. Sect. Glutinaria (Raf.) G.X. Hu, C.L. Xiang & B.T. Drew includes S. koyamae Makino, S. glabrescens Makino, S. nipponica Miq., and Salvia × sakuensis Naruh. & Hihara, whereas sect. Notiosphace Benth. includes S. plebeia R. Br. Furthermore, the recently introduced S. lyrata was originally placed in sect. Heterosphace Benth. (Benth. & Hook.f., Gen. Pl. 2: 1196, 1876), within subgenus Leonia Benth., a subgeneric name later noted as invalidly published (Marín et al. 1996; Govaerts 2003; Kalnyuk et al. 2025). However, recent molecular phylogenetic studies instead place S. lyrata within an informally named “Heterosphace” clade (Kriebel et al. 2019; Rose et al. 2021), treated by some authors as an unranked subgenus pending formal validation.

Previous studies have demonstrated the taxonomic significance of nutlet morphology in several Salvia taxa (Ryding 2001; Özkan et al. 2009; Ifrim 2012). In addition to its importance for species identification, nutlet morphology has increasingly been recognized as an important source of taxonomic information supporting existing sectional circumscriptions within Salvia (Oran 1997; Kahraman et al. 2009; Kahraman 2011; Polat et al. 2015; El-Rabiai and Elfaidy 2021). Nutlet characters, including size, shape, surface ornamentation, and abscission scar morphology, provide relatively conservative and independent morphological evidence that complements the floral and staminal characters traditionally used for taxonomic circumscription of sections and subgenera of Salvia (Özkan et al. 2009; Kahraman et al. 2010a, 2010b; Büyükkartal et al. 2011; Ifrim 2012; Ananthalakshmi 2020). Although Bentham’s (1876) traditional classification of Old World Salvia was primarily based on staminal and corolla morphology, subsequent studies have demonstrated that nutlet morphological characters, particularly surface ornamentation, correspond closely with sectional boundaries and therefore have considerable systematic importance (Oran 1996; Kahraman 2011; Mousavi et al. 2013; Celep et al. 2014; Alimpić et al. 2021). Among these characters, nutlet surface ornamentation has consistently been identified as one of the most informative diagnostic features for sectional delimitation in Salvia (Oran 1996; Kahraman 2011; Salimpour et al. 2014; Kılıç and Kılıç 2021). The nutlet morphology of Japanese Salvia remains poorly documented despite its systematic importance (Li et al. 2016). To date, only two species, S. lyrata and S. plebeia, have been investigated (Marín et al. 1996; Li et al. 2016), both of which are also examined in the present study. A detailed investigation of nutlet morphology across the Japanese taxa is therefore necessary to assess the systematic significance of these characters.

The present study describes the nutlet morphology of 19 taxa (species and varieties) of Japanese Salvia, representing four sections: Glutinaria, Heterosphace, Notiosphace, and Sobiso. Nutlet micromorphological characters were investigated to evaluate their taxonomic utility and systematic significance for species identification at the sectional level. Specifically, the present study aims to (a) document the variation in nutlet morphology among the examined taxa; (b) assess whether nutlet micromorphological characters provide reliable information for the current sectional circumscription of Japanese Salvia; and (c) classify nutlet surface ornamentation into distinct descriptive categories and evaluate their taxonomic significance for existing sectional classifications.

In the present study, the nutlet morphology of 19 taxa (species and varieties) assigned to the sections Glutinaria, Heterosphace, Notiosphace, and Sobiso was examined. These four groups were easily distinguishable based on micromorphological characteristics, particularly nutlet shape, size, color, abscission scar, and surface ornamentation. The diagnostic characters are summarized in Tables 1, 2 and illustrated in Figs 3, 4, 5, 6, 7, 8, 9.

Even with a limited number of taxa, the present results demonstrate that nutlet morphology can contribute significantly to the existing infrageneric classification of Salvia. This study provides the first comprehensive account of nutlet morphology in Japanese Salvia, representing four sections: Glutinaria, Heterosphace, Notiosphace, and Sobiso. Significant differences were observed across all examined nutlet characters, establishing a robust morphological framework for evaluating both interspecific and intraspecific variation within Japanese Salvia. The findings of the present study and previous literature show that nutlet morphological characters are useful not only for species identification but also for distinguishing sections and subgenera across the genus (Marín et al. 1996; Moon et al. 2009; Özkan et al. 2009; Kahraman et al. 2010a, 2010b; Kahraman 2011; Büyükkartal et al. 2011; Mousavi et al. 2013; Celep et al. 2014; Salimpour et al. 2014; Polat et al. 2015; Li et al. 2016; Alimpić et al. 2021; Kılıç and Kılıç 2021; Celep et al. 2022).

Mature nutlets of Salvia usually vary from light brown-yellow and brown to blackish-brown, often with darker stripes or a mottled appearance across a remarkably broad taxonomic range within the genus, including sections Aethiopis, Angulatae, Calosphace, Drymosphace, Hemisphace, Holochilus, Horminum, Hymenosphace, Plethiosphace, and Salvia, as well as subgenera Salvia, Sclarea, and Heterosphace (Özkan et al. 2009; Moon et al. 2009; Kahraman et al. 2010a; Kahraman 2011; Ifrim 2012; Salimpour et al. 2014; Ananthalakshmi 2020; Celep et al. 2022). In addition, specifically pale brown nutlets have been reported in sect. Plethiosphace and sect. Salvia (Büyükkartal et al. 2011; Alimpić et al. 2021), and blackish to brown-yellowish nutlets have been reported within sect. Salvia (Kılıç and Kılıç 2021). The present observations align with this consensus, indicating that brown coloration with darker markings is a broadly conserved feature across Old World Salvia rather than a section-specific diagnostic character, and its taxonomic importance mainly relies on fine-scale variation (marking patterns) rather than nutlet color patterns.

Nutlet shape has long been regarded as a taxonomically informative character in Salvia (Oran 1996; Kahraman et al. 2010a). The diversity reported in earlier work includes elliptic, widely elliptic, ovate, lance-ovate, oblong, obovate, ovoid, suborbicular, and trigonous forms across Salvia (Harley et al. 2004; Moon et al. 2009; Kahraman et al. 2010b; Büyükkartal et al. 2011; Mousavi et al. 2013; Celep et al. 2014; Polat et al. 2015; Li et al. 2016; Alimpić et al. 2021). These observations corroborate the present study. However, the present findings diverge from the widely reported pattern of assessing circular (spheroidal) and prolate shapes primarily via the L/W ratio (Özkan et al. 2009; Kahraman et al. 2010a; Kılıç and Kılıç 2021), suggesting that nutlet shape descriptions in Japanese Salvia contradict this trend. In the studied taxa, the most common nutlet shape is elliptic, which has also been reported in some members of sections Aethiopis, Holochilus, Hemisphace, Horminum, Hymenosphace, Plethiosphace, and Salvia (Salimpour et al. 2014; Ananthalakshmi 2020). According to Celep et al. (2022), the basic nutlet shapes reported across subgenera Salvia, Sclarea, and Heterosphace include elliptic, widely elliptic, circular, ovate, and obovate forms. Notably, the long-ovoid nutlet shape observed in S. plebeia corresponds precisely with descriptions of this species by Sales et al. (2010) and Li et al. (2016), providing independent cross-study confirmation.

Nutlet size has repeatedly proven to be a diagnostic character in Salvia (Moon et al. 2009; Kahraman et al. 2010b). Nutlet length and width ranges vary among the infrageneric groups of Salvia: 1.6–3.5 mm in length and 1.0–2.9 mm in width across taxa of sections Plethiosphace, Salvia, Hemisphace, and Hymenosphace (Özkan et al. 2009); 1.53–2.64 mm in length and 1.00–2.39 mm in width across taxa of sections Aethiopis, Holochilus, Hemisphace, Hymenosphace, and Plethiosphace (Salimpour et al. 2014); 2–3.5 mm in length and 1.5–2.5 mm in width across sections Salvia, Plethiosphace, Aethiopis, and Horminum (Ananthalakshmi 2020); 3.02–6.47 mm in length and 1.78–5.10 mm in width in taxa of sect. Salvia (Kılıç and Kılıç 2021); and 1.61–8.44 mm in length and 1.02–4.20 mm in width, with L/W ratios of 1.02–3.06, across subgenera Heterosphace, Salvia, and Sclarea (Celep et al. 2022). These measurements are comparable to the present findings. Nutlet size ranges of 1.5–2.5 × 1.0–2.0 mm in sect. Plethiosphace and 2.2–2.5 × 1.3–1.5 mm in sect. Hemisphace (Büyükkartal et al. 2011) are compatible with those observed in the present study. The smallest nutlets recorded in S. plebeia (0.45–0.60 × 0.38–0.40 mm) correspond with the measurements reported by Sales et al. (2010) and Li et al. (2016) for the same species, suggesting that nutlet size may represent a relatively consistent character at the species level.

Abscission scar morphology has been considered systematically useful in Salvia (Harley et al. 2004; Moon et al. 2009; Özkan et al. 2009). Several studies have reported abscission scar position, shape, and diameter as descriptive characters across different sections of the genus: triangular scars in sections Aethiopis and Plethiosphace; orbicular scars in sections Aethiopis, Hemisphace, Holochilus, Hymenosphace, and Plethiosphace; round scars in sections Salvia and Hemisphace; triangular scars in sections Plethiosphace and Eusphace; and irregular scars in sections Holwaya and Drymosphace (Kahraman 2011; Büyükkartal et al. 2011; Ifrim 2012; Salimpour et al. 2014; Alimpić et al. 2021). Furthermore, an areolate to round abscission scar has been reported in subg. Salvia and a triangular scar in subg. Sclarea (Celep et al. 2022). Additionally, Li et al. (2016) described a convex-to-round abscission scar in S. plebeia, whereas a triangular scar was observed in the same species in the present study. Abscission scar diameter has been reported to range from 0.50–1.28 mm, with L/W ratios of 1.01–1.88 among taxa of sect. Hymenosphace (Kahraman et al. 2010b; Kahraman 2011), and scar diameters of 0.41–0.83 mm have been reported among species of sect. Salvia (Büyükkartal et al. 2011). These findings lend additional support to the present results. However, these studies suggest that the abscission scar is a consistent, apomorphic trait that carries taxonomic information at the species level, extending to the sectional and even subgeneric levels within Salvia.

Nutlet surface sculpturing has been utilized as a key diagnostic character for species identification at the sectional level within Salvia (Kahraman et al. 2010a; Kahraman 2011; Kılıç and Kılıç 2021). The present results showed some inconsistencies with the findings of previous studies (Moon et al. 2009; Özkan et al. 2009; Kahraman et al. 2010b; Mousavi et al. 2013; Salimpour et al. 2014). In contrast, Kılıç and Kılıç (2021) reported a colliculate pattern in sect. Salvia, which is consistent with the present findings. Similarly, Celep et al. (2022) described subgenus-specific ornamentation patterns: colliculate-verrucate with small knobs in subg. Heterosphace, colliculate with rounded knobs in subg. Sclarea, and a more complex colliculate-reticulate or verrucate pattern with pentagonal-hexangular, sinuate-walled cells in subg. Salvia, which broadly correspond with the ornamentation patterns observed in the present study. At the species level, the rough, non-reticulate or uneven nutlet surface of S. plebeia agrees with Li et al. (2016). The tuberculate ornamentation with small knob-like projections observed in S. lyrata is consistent with the original description by Marín et al. (1996). These comparisons demonstrate that nutlet ornamentation is a relatively stable character with considerable taxonomic importance for species identification and infrageneric classification in Salvia. The present results extend this pattern to the previously uncharacterized Japanese lineage.

The multivariate analysis of nutlet micromorphological traits provides additional information on patterns of variation among the investigated Japanese Salvia taxa. Because the PCA was based on taxon means, these patterns should be regarded as comparative evidence among taxa rather than as direct evidence of within-taxon morphological variation or taxonomic distinctness. PC1 was primarily associated with size-related traits, with nutlet and abscission scar dimensions contributing strongly to the separation of taxon means, making overall nutlet/scar size the single largest source of morphological differentiation. This size axis separates large-nutlet taxa (Salvia koyamae, S. glabrescens var. glabrescens, and S. nipponica) from small-nutlet taxa (Salvia akiensis, S. isensis, S. pygmaea var. pygmaea, S. pygmaea var. simplicior, S. plebeia, and S. lutescens varieties). PC2 was mainly associated with proportional characters, particularly the abscission scar and nutlet L/W ratios, and captured shape independently of size. It distinguished S. glabrescens var. glabrescens, S. glabrescens var. repens, and S. japonica by their unusually elongate scars, while S. pygmaea var. pygmaea, despite differing greatly in size, was characterized by narrowly elongate nutlets with proportionally squarish-rounded scars. This shows that size and shape vary independently among the studied taxa, and taxa can resemble each other on one axis while differing on the other. The close grouping of the S. lutescens varieties with S. isensis and S. pygmaea var. simplicior along both axes supports treating them as a morphologically cohesive complex, whereas the strong isolation of S. koyamae on both axes reinforces its status as a well-differentiated taxon.

The present study demonstrates that nutlet morphology, particularly surface ornamentation, exhibits a high degree of infrageneric stability among Japanese Salvia. Four distinct types of surface ornamentation were recognized, corresponding closely with the four investigated sections: reticulate in sect. Sobiso, colliculate in sect. Glutinaria, verrucate in sect. Notiosphace, and tuberculate in sect. Heterosphace. The consistency of these ornamentation patterns, together with other nutlet morphological characters, provides additional support for the existing sectional circumscription of Japanese Salvia and is congruent with the recently proposed infrageneric classification (Hu et al. 2018). In contrast, some nutlet characters, such as nutlet shape, size, and abscission scar morphology, were more informative for species identification than for supporting the current sectional classification of Japanese Salvia.