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Chemosystematic significance of flavonoids isolated from Diplotaxis acris (Brassicaceae) and related taxa

Abstract The chemosystematic relationship of four Diplotaxis species; Diplotaxis acris, Diplotaxis erucoides, Diplotaxis harra and Diplotaxis muralis were surveyed from the flavonoids point of view. These species were found to produce 33 flavonoids (7 flavones and 26 flavonols), including 11 compounds were isolated in the present study from D. acris. Among them, seven flavonoids were identified for the first time; luteolin (4), kaempferol (8), kaempferol 3-O-β-glucopyranoside-7-O-α-rhamnopyranoside (13), quercetin 3-O-β-glucopyranoside (16), quercetin 7-O-β-glucopyranoside (20), isorhamnetin (22) and isorhamnetin 3-O-β-glucopyranoside-7-O-α-rhamnopyranoside (32). Their structures were recognized on the basis of chemical and spectroscopic techniques (1D & 2D NMR, UV, EI & ESI/MS). The isolated flavonoids may provide useful taxonomic characters at the infraspecific levels of classification where the flavonoid profile of D. acris and D. harra is similar and different from the other species.


Introduction
The genus Diplotaxis DC. belongs to the most economically important tribe of family Brassicaceae; Brassiceae (Warwick et al. 2009), the number of species vary from 27 to 36 and they are distributed from Europe, Mediterranean region to north-west of India (Boulos 1999;The Plant List 2010;Grillo et al. 2012;Oueslati et al. 2015). Four taxa occurred in Egypt; Diplotaxis acris (Forssk.) Boiss. Diplotaxis erucoides (L.) DC. Diplotaxis harra (Forssk.) Boiss. and Diplotaxis muralis (L.) DC. (Boulos 1999). D. acris is a winter annual herb, known as salad rocket; it tastes like Eruca sativa Mill. and grazed by animals, it grows in desert slopes, stony and sandy desert valleys (Boulos 1999;Warwick et al. 2009). In addition to its economic importance, D. acris has antioxidant, hepatoprotective and antinociceptive activities (Atta & Abo 2004;Atta et al. 2006). Little publications dealt with isolation and characterisation of flavonoids from the studied taxa, flavonol and flavone compounds and their glycosides were identified (Hussiney et al. 1998;El-Sayed et al. 1999).
To the best of our knowledge, the chemosystematic value of D. acris was not discussed before, moreover as the global interest in finding a new valuable medicinal and food sources was increased, the present study aims to reinvestigate the phytochemical constituents of D. acris and to study the infraspecific relationship with three related species; D. erucoides, D. harra and D. muralis from the flavonoids point of view.

Chemosytematic significance
Taxonomically, the genus Diplotaxis belongs to family Brassicaceae (3709 species, 338 genera and 13-19 tribes); tribe Brassiceae (242 species & 48 genera) and considered as one of the core genera of subtribe Brassicinae (Warwick et al. 2009). On the basis of the molecular characters, the species of Diplotaxis are divided into three subgenera; Hesperidium  (Hussiney et al. 1998;Mohammed et al. 2013) while six compounds were found in D. acris and characterized by three core flavone nucleus; apigenin, luteolin and diosmetin as well as their glycosylation at position 7 (Hussiney et al. 1998;El-Sayed et al. 1999). All the studied taxa have flavonol compounds; D. harra is the most rich species in flavonol content (19 compounds; Table 1) (Sánchez-Yélamo 1994;Hussiney et al. 1998;Atta et al. 2011;Kassem et al. 2013;Mohammed et al. 2013). The flavonol profile of D. erucoides and D. muralis is different. The former is characterised by the presence of -OCH 3 group at position 7; rhamnetin 3, 3′-di-O-glucoside (Salah et al. 2015) and the gylcosylation take place at position 3 or 7. In D. muralis, the -OCH 3 group is found at position 3′ (isorhamnetin compounds) and the gylcosylation at position 3 is predominant (Sánchez-Yélamo & Martinez-Laborde 1991). The present study agrees with Gómez-Campo (1999) in the separation of the studied taxa into three  subgenera, where the flavonoid profile of D. acris and D. harra is similar and the flavonoid pattern of D. erucoides is different from D. muralis, moreover, more investigation is needed to include the rest species.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the National Research Centre, Egypt [grant number 10010002].