2.1 Plant material and growth conditions
Tabebuia rosea is a common tree species throughout its native range in central America and northern South America where it occurs in dry, moist, and wet forests, in plantations, gardens and farms, and as a common ornamental roadside tree (Condit, Pérez & Daguerre 2010). It is a light-demanding species that forms sparse canopies with minimal self-shading. Seeds of T. rosea were collected near Panama City, Republic of Panama, and germinated in trays with MiracleGrow® potting soil. After germination the seedlings were transferred to individual 61 cm tall, 30.3-liter tree pots (TP1124R, Stuewe & Sons, Tangent, OR) filled with 80:20 ratio of local clay-textured top soil and river sand. After 1 month in these pots, all plants were supplemented with 15:9:12 Osmocote Plus very slow-release (8−9 months) N:P:K fertilizer (Scotts-Sierra, Maryville, OH, USA). Five plants were placed in a temperature- and CO2 controlled 6 m-diameter geodesic dome (Solardomes Industries Limited, Nursling, UK) maintained at current local ambient temperature and ~420 ppm CO2; five other plants were placed in a second dome maintained at ambient temperature +4°C and ~800 ppm CO2. By the time measurements were started, 8 weeks after the pots were moved into the domes, the plants were about 8 months old, 120 cm tall and contained at least five opposite pairs of compound leaves—the first leaves after germination to emerge are simple. Dome temperatures were controlled by split AC units and passive warming (control dome) and heaters (HER 75B 3101, 21.3 Amp, Modine Manufacturing Company, Racine, WI, USA) (treatment dome). The domes operated as semi-closed systems, with enough ventilation to maintain the ambient dome at ~420 ppm. CO2 from a cylinder was injected at a small, constant flow rate to maintain the treatment dome at ~800 ppm during daytime hours. For the duration of the experiment air temperature was on average 4.0°C higher in the treatment dome than in the control dome (3°C at night—between 6 p.m. and 6 a.m.—and 5°C during the day) and mean daily relative humidity (± SD) was 74 ± 7% in the treatment dome and 85 ± 4% in the control dome. Daily mean (± SD) CO2 concentration in the treatment dome was 809 ± 71 ppm during the month of photosynthesis measurements.
After gas exchange measurements were made on the plants grown under these contrasting conditions and samples were collected to assess leaf morphological and chemical traits (see below), the plants were switched between the domes, and after one week the same leaves were measured again. Re-measuring the plants took 9 and 7 days for the plants moved to control and to treatment conditions, respectively, resulting in an average short-term acclimation duration of 11 days. On each day measurements were taken over a range of temperatures, thereby minimizing the risk of introducing bias in the temperature responses based on the number of days since conditions were switched. The experiment was conducted at the Santa Cruz Experimental Field Facility of the Smithsonian Tropical Research Institute in Gamboa, Republic of Panama, where the average 24-hour mean temperature and the average maximum temperature equal 26.9°C and 32.6°C, respectively.