Photosynthetic capacity and terpene production in populations of Lippia graveolens (Mexican oregano) growing wild and in a common garden in the Yucatán peninsula

•Photosynthetic capacity and essential oil yield of Lippia graveolens was assessed.•Physiological traits of two chemotypes (T and S) were measured in the field.•Photosynthetic capacity (PN, WUE and CE) was correlated with terpene content.•Terpene content among populations was unrelated to either locations or rainfall.•In the field, the T populations produced more terpene than the S populations.

Plants of Lippia graveolens (Mexican oregano) growing in the wild are harvested for their essential oils, which are appreciated in the pharmaceutical industry. In order to establish a relationship between photosynthetic capacity and terpene yield and the possible effects of differences in rainfall on these variables, plants of three chemotypes, accumulating mainly sesquiterpenoids (S), carvacrol (C) or thymol (T), were studied during the rainy season in the field as well as a common garden in the Yucatán peninsula. Physiological variables were measured in the field in two low-rainfall (S and T chemotypes) and two high-rainfall (S and T chemotypes) populations. Significant variations were found in water potential (Ψw) among the four populations studied, with no evident relationship to rainfall regime; terpene content bore no relationship to Ψw. The highest terpene content was found in plants from the high-rainfall T population with the highest photosynthetic rate (PN), water-use efficiency (WUE) and carboxylation efficiency (CE), and carbon isotopic composition (δ13C) among the highest. Differences in terpene production among populations of L. graveolens were generally unrelated to either site of origin, rainfall or chemotype, although among the four populations selected for physiological measurements in the field, the T populations produced more terpene than the S populations. Terpene content, which was higher in plants sampled in the field than in the common garden, was determined by PN and WUE in the field, and only by PN in the common garden. The underlying reasons for the difference between field and common garden results, being apparently not microclimatic, need to be established.