Intraspecific divergence in phytochemical characteristics and drought tolerance of two carvacrol-rich Origanum vulgare subspecies: subsp. hirtum and subsp. gracile

Oregano, Origanum vulgare L., is a well-reputed medicinal herb used globally in cosmetics, food and pharmaceutical industries. Essential oil (EO) composition and productivity of oregano subspecies are highly affected by water deficit stress. Here, a two-year (2018 and 2019) greenhouse study was conducted to comparatively investigate the impact of water deficit stress on biochemical traits and EO profiles of two carvacrol-rich subspecies of O. vulgare (subsp. gracile and subsp. hirtum). The pot-grown plants were subjected to four water stress treatments based on field capacity (FC) method, i.e., control condition (100 % FC), mild stress (75 % FC), moderate stress (50 % FC) and severe stress (35 % FC). Overall, the EO content of subsp. hirtum (up to 3.25 % (v/w)) was higher than that of subsp. gracile (up to 1.92 % (v/w)). Compared with control, the EO contents increased by over 75 % under moderate water stress. Carvacrol was present in the EO profiles of both subspecies as the major component. Its highest content in the subsp. hirtum was achieved under mild water stress (i.e., 60 % (in 2018) and 68 % (in 2019)), whereas in the subsp. gracile, it was obtained under severe (55 % (in 2018)) and moderate (51 % (2019)) stresses. Overall, the subsp. hirtum showed higher dry weight, EO and carvacrol content than subsp. gracile. Furthermore, a significant positive correlation was observed among dry weight and EO content (r = 0.74, p < 0.01). On the other hand, there was a negative correlation between carvacrol and p-cymene content (r = −0.60, p < 0.01). Water stress treatments inevitably reduced fresh weight, dry weight, relative water content and total chlorophyll content in both subspecies. However, the maximum quantum yield of PSII (Fv/Fm), superoxide dismutase and ascorbate peroxidase activity, and also proline content was changed in a subspecies-specific manner. The results of the present study enabled selection of the subsp. hirtum as a desirable prototype for relevant cultivation and prospective breeding programs.