Jm. Mjwara et Cej. Botha, THE INTERACTIVE EFFECTS OF LIGHT, TEMPERATURE AND CO2 O2 RATIOS ON PHOTOSYNTHESIS OF COIX-LACHRYMA-JOBI L/, South African journal of botany, 59(4), 1993, pp. 377-384
A portable infra-red gas analyser was used to investigate the interact
ive effects of light, temperature, and CO2/O2 ratios under controlled
environmental conditions in an attempt to model gas exchange character
istics of Coix lachryma-jobi L. Plotting light response curves as a fu
nction of temperature (20, 25, 30 and 35-degrees-C) revealed no sign o
f light saturation even at a photosynthetic photon flux density (PPFD)
close to 2000 mumol m-2 s-1. High net assimilation rates (A) of appro
ximately 24 mumol CO2 m-2s-1 were realized at 30 - 35-degrees-C. Assim
ilation (A) versus internal CO2 partial pressure (C(i)) curves showed
a steep rise in A with increase in C(i) but saturated at approximately
150 mul l-1 and the effect was similar in either the absence or prese
nce of O2, under all temperature regimes. C. lachryma-jobi exhibited l
ow CO2 compensation points (GAMMA) between 0 and 10 mul l-1 at either
0 or 21% O2. The slopes of double reciprocal plots of 1/A versus 1/C(
i), were nearly identical and crossed the y-intercept at almost identi
cal points under all O2 concentrations. This data indicated that there
was no apparent O2 inhibition and that the apparent inhibitor constan
t (K(i)) for O2 at the site of carboxylation did not change with an in
crease in [O2] from 0 to 21%. These observations were further confirme
d by results obtained from the analysis of apparent carboxylation effi
ciency where no inhibition of A with increase of [O2] occurred. These
characteristics are thus consistent with typical features of C4 photos
ynthesis.