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.