QUANTIFYING IMMEDIATE C EXPORT FROM LEAVES 413
Figure 2 Counting efficiency of the GM detectors in experiments with Panicumspecies. Radioactivity was
measured nondestructively by monitoring^14 C accumulation in the leaf with a GM detector and by scintillation
counting after destructive sampling of the leaf. Data shown are those for two C 3 species,P. laxum(A) and P.
bisulcatum(B); a C 3 -C 4 intermediate species, P. milioides(C); an NAD-ME C 4 species,P. capillare(D); an
NADP-ME C 4 species,P. bulbosum(E); and a PEP-CK C 4 species,P. maximum(F). Each point is the mea-
surement of one leaf. The data are from measurements made at different CO 2 levels (35 and 90 Pa) and at the
end of different experimental periods (30, 60, 90, 120 min and 17 hr). Feeding periods were usually 120 min,
but in pulse-chase experiments we frequently extended the period of noninvasive monitoring of^14 C retention
to 17 hr. Respiration data in these pulse-chase experiments were used to correct for total export (see Refs. 63,
79, and 80). Counting efficiency (CE) is the value of radioactivity obtained by the GM detector divided by the
radioactivity determined after destructive analysis of the leaf tissue times 100. Values for CE are means SE.
The variation explained by a linear model fitted to the data is indicated by the coefficient of determination (r^2 ).
IV. CASE STUDIES
A. Photosynthesis and Export Under Stress
As already pointed out, the primary functions of a “source” leaf are to fix light energy and provide that
energy in the form of photoassimilates for plant growth. The leaf is not a homogeneous structure, and in