Analysis of volatile terpenes was performed as described by Schilmiller et al. (2009) , with minor modifications. Four-week-old plants were used to obtain trichome exudates from either whole leaves (leaf dip method) or isolated type VI trichomes. For the former method, single leaflets were incubated at room temperature in 1 ml of methyl tertiary-butyl ether (MTBE) containing 10 ng μl−1 of tetradecane internal standard. Following a 5 min incubation period with gentle shaking, the leaf was removed and its dry weight was determined. The resulting MTBE solution (2 μl) was used directly for capillary gas chromatography–mass spectrometry (GC-MS) analysis as described below. For direct analysis of type VI glands, a stretched Pasteur pipette was used to collect type VI glandular heads from the adaxial leaf surface. Collected glands, which readily stick to the glass surface, were dissolved in 100 μl of MTBE containing 10 ng μl−1 tetradecane as an internal standard. A small portion (2 μl) of this extract was analysed by GC-MS on a DB-5 fused-silica column (10 m length, 0.1 mm i.d., 0.34 μm thick stationary phase; Agilent, Santa Clara, CA, USA). The GC program used an injector temperature of 280 °C. The initial column temperature was held at 40 °C for 1 min and then ramped at 40 °C min−1 to 90 °C, 15 °C min−1 to 110 °C, 25 °C min−1 to 250 °C, and finally at 40 °C min−1 to 320 °C, which was maintained for 2 min. The helium carrier gas flow was set to 0.4 ml min−1. All compounds were analysed with an Agilent 6890N GC system interfaced to a 5975B quadrupole mass spectrometer (Santa Clara, CA, USA) operated using 70 eV electron ionization and mixed selected ion monitoring (m/z 85 and 93) per scan (m/z 33–350) mode. The terpene content in leaf dip samples was normalized to the dried weight of the tissue used for each extraction. The terpene content in type VI gland exudates was normalized to a specific number of isolated glands. Under the GC conditions employed, β-phellandrene co-eluted with minor amounts of limonene (data not shown). 2-Carene and α-humulene were used as standards to determine response factors for monoterpenes and sesquiterpenes, respectively.