The genus Baccharis L. belongs to the family Asteraceae, subfamily Asteroideae and tribe Astereae. It has more than 400 species distributed in almost the entire American continent. This paper is about Baccharis ‌notosergila ‌Griseb. which is distributed in Argentina, Brasil, Paraguay and Uruguay. It is a perennial woody shrub with high adaptive capacity to different environmental conditions. In Argentina, it is a problematic weed that produces strong competition for water, nutrients and light with the species with forage value in areas with natural grassland, generating a process of degradation and reducing the livestock productivity (Sione et al. 2006). Many species of Baccharis have also been used in traditional medicine for treatment of diverse pathologies, such us gastrointestinal ailments, ulcers, fever, rheumatism, and infectious diseases. Alvarenga et al. (2018) evaluated the immunomodulatory activity of methanolic extracts from Baccharis ‌notosergila, B. ‌punctulata ‌DC., and B.trimera ‌(Less.) DC. The three species are a potential source of chemical compounds which had immunomodulatory activity on human mononuclear cells. In a comparative study of therapeutic activity of Baccharis genus Petroche Torres et al. (2022) expressed that some flavonoids would have anti-inflammatory and anti-oxidant activities. Baccharis exhibits an interesting secretory tissue. Today many papers referred specifically to secretory structures in Asteraceae, notably in the genus Baccharis (e.g., Tetley 1925; Ragonese 1988; Lersten & Curtis 1986, 1987, 1988, 1989; Werker et al. 1994; Poli et al. 1995; Castro et al. 1997; Simon et al. 2002; Melo-de-Pina & Menezes 2003; Łotocka & Geszprych 2004; Cury & Appezzato-da-Glória 2009). However, great variability exists among the species of Asteraceae coincidently as reported by Metcalfe & Chalk (1950). The most frequently internal secretory structures mentioned in Asteraceae are ducts and cavities (Castro et al. 1997; Budel et al. 2012, 2015, 2018; Bobek et al. 2015), whose location is different. Milan et al. (2006) cited secretory ducts on the xylem side in Mikania ‌glomerata ‌Spreng. (Eupatorieae). Plos et al. (2011) mentioned the presence of secretory cavities and ducts in the genus Ophryosporus ‌Meyen (Eupatorieae) on the xylem and phloem sides. Delbón et al. (2012) in Flourensia ‌Cambess. (Heliantheae) found ducts on the abaxial and adaxial sides of the vascular bundles, depending of the species. Similar encounter was reported by Hernández & Arambarri (2019) in leaves of Pascalia ‌glauca ‌Ortega (Heliantheae). In the leaf of Baccharisobovata ‌Hook. et Arn., Molares et al. (2009) indicated presence of cavities on abaxial side, whereas Ornellas et al. (2019) in leaves of B.stylosa ‌Gardner reported ducts on the abaxial and adaxial sides and mesophyll. There are still many doubts in the anatomical interpretation of the secretory structures that exert an important ecological role in plants (Sharifi-Rad et al. 2017). Further studies are needed to extend the knowledge of specialized secretory tissue as well as the chemical nature of secreted compounds which are useful for plant and humanity. Results would be useful in taxonomy, for medicine, the mechanism of defense against biotic and abiotic factors that each species possesses, and in management and conservation of species. The aims of this research were: 1) to deepen the knowledge of internal secretory structure types in leaves of Baccharis ‌notosergila and 2) to check the main chemical substances secreted and/or accumulated in the secretory structures.

Plants of B. ‌notosergila having fully developed leaves were collected during the spring of 2018 and 2020 in Vieytes, Magdalena party (Pdo.), Buenos Aires province (Prov.), Argentina. The botanical material was identified and the vouchers were deposited in the Facultad de Agronomía, Universidad Nacional de La Plata (LPAG) herbarium.

For anatomy study, fresh mature leaves from basal, middle and apical stem regions were fixed in FAA (a solution of formaldehyde, glacial acetic acid, and 70% ethyl alcohol, Johansen 1940), then stored in 70% ethanol. To analyze the structures, freehand cross sections (CS) of the petiole and leaf-blade middle part were cut; the selected sections were bleached in (50%) sodium hypochlorite (NaClO), washed thrice with distilled water (DW), then were stained with an alcoholic solution (80%) safranin or (0.05%) Brilliant cresyl blue (Pérez & Tomasi 2002). For the paradermal view (PW), thirty leaves were made transparent using the method of Franklin (1945) modified, for that, they were boiled in ethyl alcohol 96º for 20 min, allowed to cool then washed and bleached in 50% (NaClO) for 2 h, then they were washed twice and submerged in a solution of (10 vol.) hydrogen peroxide + (5%) glacial acetic acid (1: 1 v/v) for 48 h, after that washed twice in DW and bleached again in (50%) NaClO. At the completion of the bleaching process, five washes were carried out to remove the NaClO, and samples were transferred into a solution of (5%) chloral hydrate for a minimum of 48 h. To complete the process, leaves were stained using different dyes and reagents, Safranin, Toluidine blue “O”, and Oil red “O”. The sections and diaphanous leaves were mounted in gelatin-glycerin on glass slides and sealed with nail polish. Secretory spaces dimensions in CS and PW of 20 samples were taken using ImageJ program (González 2018), the values were expressed in micrometer (µm).

The histochemical analysis was performed on leaves CS and PW. To analyze lipid compounds an alcoholic solution of Oil red “O” (ORO test) was used (Gurr 1971), the orange to red-color indicates positive test and essential oils were seen as droplets. Detection of tannins was performed using Ferric chloride (FeCl3) test (Zarlavsky 2014), a green-blue color is a positive test. Resins were detected using a saturated solution of copper sulphate and heated gently (Cosa et al. 2014) a color emerald green indicates positive test. Toluidine blue “O” (TBO test) was used to contrast phenolic compounds a green turquoise color indicates positive test, and lilac-colored carbohydrates (Tapia-Torres et al. 2014).

Anatomical structures were examined and microphotographs were taken by using a stereoscopic microscope Bausch & Lomb, stereo zoom 5, and a digital camera, resolution 12MP; light microscopes Nikon E200 LED equipped with Micrometrics SE Premium software and a Lancet XSP-136D optical microscope equipped with digital camera solution disk.

For analyzing the secretory spaces we consider cavity and duct (= canal) according Metcalfe & Chalk (1989) who wrote: “Secretory cavities are intercellular spaces of various sizes and shapes in which chemical substances secreted from the surrounding cells are deposited. Secretory canals differ from cavities in being more elongated, and both are sometimes lined by epithelial cells from which secretion takes place. The space of ducts and cavities are commonly formed by the separation of cells that were previously in contact with each other and when formed in this way they are described as schizogenous”. In our understanding the ducts from different plants and families are large intercellular spaces and have continuity with their main function to conduct substances from one place to another, whereas in cavities the intercellular space is shorter than duct and their main function is to store chemical compounds. These were named reservoirs by Lersten & Curtis (1989).

In the leaf of Baccharis ‌notosergila false ducts formed by cavities and cavities themselves were found. False duct is a long and extended space lined by very long epithelial cells, developed from a row of elongated cavities during the leaf growth. They are located on the abaxial vascular bundle side from 1st to 3rd order veins. The cavity itself is a short intercellular space lined by short epithelial cells. Cavities are isolated or in small clusters. They are located on lateral-abaxial side and in the angles of 1st to 5th order veins, at the end of veinlets, and in the mesophyll. False ducts and cavities contain a complex mixture of lipids, resins, phenolic compounds, carbohydrates and other substances. Extracts of these substances may be a new source of medicinal compounds or useful in organic control of pathogens or as herbicide to improve sustainable agriculture systems.

We are grateful for the comments and useful suggestions of Drs Susana Freire (Universidad Nacional de La Plata) and Thierry Deroin (MNHN) that allowed us to improve the manuscript.