O 2 , becoming similar to that of C3 plants [59]. Therefore, the higher O 2 requirement for optimal C4 pho-
tosynthesis must be specifically associated with the C4 function. With the Rubisco-limited Flaveria
bidentis(an antisense transformation of the small subunit of Rubisco as a C3 cycle–limited transformant),
there was less inhibition of photosynthesis by supraoptimal levels of O 2 than in the wild type. The opti-
mum O 2 partial pressure for C4 photosynthesis at 30°C, atmospheric CO 2 levels, and half-full sunlight
(1000mol quanta m^2 sec^1 ) was about 5–10 kPa [68]. Photosystem II activity, measured as chloro-
phyllafluorescence, however, was not inhibited by O 2 levels above the optimum for CO 2 assimilation
but was inhibited by suboptimal ones [68].
Photorespiration, by definition, is a light-dependent evolution of CO 2 and thus it can be traced by a
CO 2 evolution signal instantly following a light phase. This signal usually lasts for up to 1–2 min. The
rate and time of photorespiratory CO 2 postirradiation burst in wheat leaves are suppressed by the PSII in-
hibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) [7]. This postirradiation burst is dependent on
the electron transport system and on the PSII activity. However, the relationship between PSII electron
transport and CO 2 assimilation remained similar throughout state transitions in maize leaves [69]. In
wheat leaves, the carbon needed for long-term CO 2 evolution in the CO 2 -free air might be derived not
only directly from the pool of intermediates in the Calvin cycle but also indirectly from a remotely fixed
reserve of photosynthates in the leaf via a photorespiratory carbon oxidation–mediated mobilization pro-
cess [7]. Such a mobilization process of photosynthates probably played an important role in the coordi-
nation of photochemical reactions and carbon assimilation during photosynthesis in C3 plants under pho-
toinhibitory conditions. In addition, photorespiratory losses of CO 2 in transgenic tobacco plants or subunit
1 of cotton seed (Gossypium hirsutum) were significantly reduced with increasing catalase activities at
38°C, indicating that the stoichiometry of photorespiratory CO 2 formation per glycolate oxidized nor-
mally increases at higher temperatures because of enhanced peroxidation [70]. The Calvin cycle metabo-
lites, and especially those requiring ATP and/or NADPH for their metabolism such as 3-PGA or triose-
P, would control the photosynthetic electron transport capacity when photorespiration is blocked. Under
low-temperature conditions (18°C), there was nearly complete loss of O 2 sensitivity of photosynthesis at
normal ambient levels of CO 2 in the flag leaf of rice (Oryza sativa), in contrast to the large enhancement
of photosynthesis by supra-atmospheric levels of CO 2 and subatmospheric levels of O 2. These conditions
induce a suppression of photorespiration; i.e., there is no limitation in utilizing the initial product of CO 2
assimilation (triose-P) as predicted from the kinetic properties of Rubisco [71].
C. Maintenance Respiration
In general, respiration can be defined as a mechanism to gain energy equivalents from the oxidation of an
appropriate substrate. The energy is then used for various physiological demands with respect to the over-
all energy budget of a growing cell. The utilization of assimilates for the synthesis and maintenance of
plant materials can be described by two respiratory components: growth respiration and maintenance res-
piration [72]. A third component of respiration can be related to energy costs for ion uptake against a con-
centration gradient, and this is termed ion respiration. Growth respiration represents the cost of convert-
ing assimilates into new structural plant constituents [73,74], while the maintenance coefficient represents
the energy required to maintain biomass. It is likely that maintenance respiration is dependent upon the
tissue composition, the growth environment, and the temperature in particular [72]. The most important
processes utilizing energy of maintenance respiration may be protein turnover, compartmentation, and se-
cretion and repair of membranes.
Three methods for determining the maintenance respiration coefficient are described in the literature,
each of which is based on a different rationale. These methods are the dark decay method [72], the dy-
namic method [72], and the zero-growth-rate method [75]. In the dark decay method, the plants are kept
in the dark and respiration rates are followed. During the dark period, respiration rates decrease with time
until a minimal steady state is attained. Such decline in respiration is ascribed to the fact that under dark
conditions the available substrate pools (sugars, organic acids, fatty acids, etc.) are gradually consumed.
Following the quantitative depletion of these pools, respiratory rates become minimal just to keep the
cells alive. In other words, no respiratory energy is diverted for growth or yield. Under specific conditions
and in comparison with earlier work in the literature [e.g., 72], 60 hr of darkness was sufficient for the ex-
perimental plants to reach such a minimal rate of respiration. For guidance, these values were about 27,
26, and 18 (mg CO 2 (g dry wt)^1 d^1 ) for sunflower, maize, and broad bean plants, respectively [76]. The
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