Monoterpene emissions in relation to foliar photosynthetic and structural variables in Mediterranean evergreen Quercus species

The relationships between foliar photosynthetic variables and monoterpene emission rates (E) are reported here for two sclerophylls, Quercus coccifera L. and Q. ilex L., which lack specialized foliar monoterpene-storage structures. Photosynthesis rates and E were measured within a forest canopy, and in leaves of different exposure in a garrigue ecosystem. Leaf dry mass per unit area (LMA) was used as a proxy of long-term leaf light availability. Maximum photosynthetic electron transport rate (J(max)), Rubisco carboxylase activity (V-cmax) and light-saturated E (E-max) per unit area increased strongly with increasing LMA. The positive effect of LMA on area-based E-max resulted from an accumulation of enzymes responsible for monoterpenoid synthesis per unit area. Positive relationships were observed between both area- and mass-based E-max and J(max), but not with photosynthesis rates, suggesting a possible control of terpene synthesis by foliar electron transport rates. Results suggested that monoterpene volatility limitations at low temperatures might also constrain E. Nonspecific storage may explain bursts of monoterpene emission on warm days following cold days, and monoterpene emission in low quantities during night. Although the correlations between E-max and J(max) do not necessarily give mechanistic insight into the physiology of monoterpene emission, they provide a basis for scaling up leaf emissions to a canopy level. However, both the leaf-to-leaf differences in monoterpene synthase activities, and monoterpene storage may perplex the relations between the electron transport rates and E.