2.2 | A-Ci curve measurements
Between 31 August and 14 September 2017 we measured A-Cicurves on sun-exposed leaves of T. rosea saplings over a wide,
but realistic leaf temperature range (i.e., no temperatures lower than
minimum daytime temperatures were included to avoid cold stress
responses in these lowland tropical plants). In the control dome the
measurement leaf temperature ranged from 26.5 to 43.3°C; in the
treatment dome the range was 28.0–44.6°C.
Target
temperatures varied based on the leaf temperatures reached and the
requirements to fill the gaps in the curves. Dome temperatures were
maintained as close as possible to target leaf temperatures to minimize
the temperature differential between the measured leaf and the rest of
the plant. The block temperature of the leaf cuvette was controlled
using the Peltier cooling/heating capacity of the LI-6400XT portable
photosynthesis system (LI-COR Biosciences, Lincoln, Nebraska, USA). Leaf
temperature always exceeded air temperature because of the heat
generated by the light source; hence the lowest measurement temperature
was higher than the lowest ambient daytime air temperature.
Fully-expanded mature sun-exposed
leaves that had developed under the growth conditions in the dome were
selected from the second-or third most recently emerged leaf pair and
each leaf was measured 2–18 times (average 12, median 14). Measurements
were made on leaves between 0.5 and 1.0 m above ground level.
A-Ci curves were measured at a pre-determined light
saturation level of 1500 µmol m–2s–1 that was provided by the red:blue LED light
source of the LI-6400XT. Light-saturated photosynthesis rates were
determined at ≥13 CO2 concentrations between 50 and 1950
ppm, using the built-in CO2 mixer of the LI-6400XT to
control CO2 concentrations of the incoming air, which
was delivered at a flow rate of 500 µmol s–1. Leaf
temperature during measurements was monitored abaxially with a Type E
thermocouple inside the leaf cuvette. After equilibration at ambient
[CO2] in the dome, photosynthesis was recorded at
50, 100, 200, 300, 400, 475, 550, 675, 800, 950, 1200, 1500 and 1950 ppm
CO2 of the incoming air. Measurements were discontinued
whenever a precipitous decline in stomatal conductance was observed and
photosynthesis rates failed to stabilize at a given target
CO2 concentration.
After completing these measurements of plants at their respective growth
conditions, control plants were moved to the treatment dome, and
treatment plants to the control dome. At this point 46 useable
A-Ci curves had been collected for control plants, and
46 curves for treatment plants. After one week at their new condition,
we started re-measuring the same plants (and the same leaves). The
post-transition measurements yielded another 46 A-Cicurves for control plants transferred to treatment conditions and 44
curves for treatment plants transferred to control conditions.
Before calculating VCMax and JMax, we
corrected photosynthesis rates and Ci values for
diffusion errors associated with CO2 concentration
gradients between the leaf cuvette and the atmosphere. To do so, we
measured CO2-response curves in both domes for leaves
that were killed by submersion in boiling water for ~5
minutes. These measurements were taken at 30°C, as previous tests showed
no temperature effect on the diffusion error (Slot & Winter
2017b). Applying the correction changed VCMax and
JMax estimates by 5 and 7% (median), respectively, but
had no effect on the temperature responses of these parameters.