Despite previous reports of no apparent photorespiration in C4 plants
based on measurements of gas exchange under 2 versus 21% 02 at varying
[CO2], photosynthesis in maize (Zea mays) shows a dual response to va
rying [O2]. The maximum rate of photosynthesis in maize is dependent o
n 02 (approximately 10%). This 02 dependence is not related to stomata
l conductance, because measurements were made at constant intercellula
r CO2 concentration (C(i)); it may be linked to respiration or pseudoc
yclic electron flow. At a given C(i), increasing [02] above 10% inhibi
ts both the rate of photosynthesis, measured under high light, and the
maximum quantum yield, measured under limiting light (PHI(CO2)). The
dual effect of 02 is masked if measurements are made under only 2 vers
us 21% 02. The inhibition of both photosynthesis and PHI(CO2), by 02 (
measured above 10% O2) with decreasing C(i) increases in a very simila
r manner, characteristically Of 02 inhibition due to photorespiration.
There is a sharp increase in 02 inhibition when the C(i) decreases be
low 50 mubar of CO2. Also, increasing temperature, which favors photor
espiration, causes a decrease in PHI(CO2) under limiting CO2 and 40% O
2. By comparing the degree of inhibition of photosynthesis in maize wi
th that in the C3 species wheat (Triticum aestivum) at varying C(i), t
he effectiveness of C4 photosynthesis in concentrating CO2 in the leaf
was evaluated. Under high light, 30-degrees-C, and atmospheric levels
of CO2 (340 mubar), where there is little inhibition of photosynthesi
s in maize by 02, the estimated level of CO2 around ribulose-l,5-bisph
osphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartme
nt was 900 mubar, which is about 3 times higher than the value around
Rubisco in mesophyll cells of wheat. A high [CO2] is Maintained in the
bundle sheath compartment in maize until C(i) decreases below approxi
mately 100 mubar. The results from these gas exchange measurements ind
icate that photorespiration occurs in maize but that the rate is low u
nless the intercellular [CO2] is severely limited by stress.