model 991, Dev-All, EG&G, Waltham, MA). The flow rate in each chamber was maintained constant
between 0.25 and 0.70 L min^1 by flow meters (Cole-Parmer Instrument Comp., Niles, IL) and pre-
cisely measured by an electronic flowmeter (Top-Trak, Sierra Instruments, Inc., Monterey, CA). A per-
sonal computer was used to control, monitor all devices, and log data.
B.^14 CO 2 Feeding and Monitoring of^14 C Retention
An acclimation period of 15–30 min was usually required before a steady rate of photosynthesis was mon-
itored after inserting the leaf in the chamber. Only after this period was the^14 CO 2 supplied. During
steady-state^14 CO 2 labeling,^14 CO 2 generated from NaH^14 CO 3 was injected into the gas stream by using
a precision syringe pump (model 341A, Sage Instruments, Cambridge, MA). The specific activity of
(^14) CO 2 in the gas stream was measured at regular intervals (20 min) by trapping 3 mL of the inlet gas in
2.5 mL of ethanolamine/ethylene glycol monomethyl ether (1:2 v:v). This specific activity varied among
experiments from 0.3 to 3.0 kBq mol^1 C depending on the leaf photosynthetic rate or the CO 2 con-
centration. However, during each feeding the specific activity of the^14 CO 2 was constant.
The GM detector positioned under the leaf surface was used to monitor the radioactivity accumulated
in the source leaf during the feeding period. The GM output through a rate meter (model 8731-32, Nu-
clear Chicago Co., Des Plaines, IL) was recorded and the counts were corrected for the total radioactiv-
ity recovered in the leaf at the end of the experiment. The total radioactivity recovered in the leaf extract
QUANTIFYING IMMEDIATE C EXPORT FROM LEAVES 411
Figure 1 A simplified schematic of the open-flow gas analysis and^14 CO 2 labeling system used to measure
photosynthesis and C export rates in source leaves. Gas, water, and electrical lines are represented by solid,
dashed-dotted, and dotted lines, respectively.