B. Rising CO 2 and Climate Warming
There have been a number of reviews regarding the effects of temperature on leaf photosynthesis
[127–129] and the effects of interactions of rising atmospheric [CO 2 ] and temperature on growth, func-
tion, and development in C 3 plants [4,46,130,131]. Photosynthesis of C 3 plants, in addition to CO 2 , is in-
fluenced by high growth temperature regimes, and Rubisco plays a central role in these responses [130].
Unfortunately, there is little experimental information on possible mechanisms of Rubisco regulation un-
der interacting CO 2 -temperature growth conditions [4]. Temperature and CO 2 have interactive effects be-
cause a rise in temperature reduces the activation state of Rubisco [64,132,133] (also see Table 2) and de-
creases both the specificity for CO 2 and the solubility of CO 2 , relative to O 2 [130,134,135], resulting in
increased photorespiratory CO 2 losses as the temperature rises. Consequently, a doubling of atmospheric
[CO 2 ] and the concomitant inhibition of the Rubisco oxygenase reaction could partially offset the adverse
effects of increased global temperature on C 3 photosynthesis [130]. However, the data in this regard are
equivocal [53], and species-specific differences may be partially accounted for the differing results. In ad-
dition, these photosynthetic gains may or may not be realized in long-term growth and yield because
growth and reproduction reflect the integrated temperature response of metabolism and developmental
processes, not just photosynthesis [46]. In soybean, the enhancement effect on leaf photosynthetic rate
due to doubling the growth [CO 2 ] increased linearly from 32 to 95% with increasing day temperatures
from 28 to 40°C, whereas with rice it was relatively constant at 60% from 32 to 38°C [64]. In addition,
although both elevated [CO 2 ] and temperature reduced Rubisco protein and activity, the reduction by ei-
ther factor was greater for rice than for soybean [64]. Even within the same species, however, plant
biomass and grain yield respond differently to increasing growth temperature. In the case of rice, plants
grown at 34°C accumulated biomass and leaf area faster than plants at 28°C, but grain yield declined by
about 10% for each 1°C rise above 26°C [136–138]. Similar scenarios have been reported for soybean
[139] and wheat [140].
In citrus, the net CER measured at the [CO 2 ] used for growth is substantially enhanced by elevated
[CO 2 ] [141–144]. At elevated growth [CO 2 ], the inhibitory effects of high leaf-to-air vapor pressure dif-
ference and decreased available soil water on citrus CER are lessened, and the CO 2 assimilation rate does
not exhibit the midday depression commonly observed in trees grown under ambient [CO 2 ] [144,145]. In
addition, elevated [CO 2 ] can compensate for the adverse effects of high temperature relative to the net
photosynthetic rate [142,146], as seen in other crops [64]. In sour orange grown in Phoenix, Arizona, the
mean daily leaf CER under summer conditions was about twofold greater for the elevated (700 ppm) CO 2
treatment in comparison with the control at 400 ppm CO 2 [142]. CO 2 enrichment enhanced sour orange
leaf CER by 75% at a leaf temperature of 31°C, 100% at 35°C, and 200% at 42°C [146]. These degrees
of enhancement are in the range of the predictions for an idealized C 3 plant, showing that a rise in tem-
perature from 28 to 40°C increases enhancements in CER from 66 to 190% when atmospheric [CO 2 ] is
raised from 350 to 650 ppm [130]. This is substantially greater than the 32–95% enhancement found with
42 VU ET AL.TABLE 2 Activation of Rubisco Extracted from Leaves of Soybean Plants
Grown at 350 and 700 ppm CO 2 and Under Varying Day/Night
Maximum/Minimum Air Temperature Regimesa
Temperature [CO 2 ] Degree of activation (%)
regime (C) (ppm) Midday Predawn
28/18 350 66.3 4.7 31.4 2.2
700 66.9 4.2 39.0 3.1
32/22 700 67.3 2.8 31.9 1.3
36/26 700 64.7 3.9 21.9 1.3
40/30 350 63.3 4.1 19.3 1.7
700 65.2 1.1 20.1 0.7
44/34 700 52.7 3.4 20.4 1.3
48/38 700 41.5 2.9 17.2 1.2
aUppermost, fully expanded leaves were sampled at predawn and midday, 48 days after
planting. Activation is computed as the ratio of the initial to the corresponding total activity
of midday-sampled leaves. Values are the mean standard error.