Photosynthesis and Respiration

Maize and sorghum differ from the other arable crops reviewed in this book because they use the C4 photosynthetic pathway. This pathway confers potentially more efficient use of CO2, solar radiation, water and N in photosynthesis relative to C3 crops. The possession of C4 photosynthesis explains why these crops differ substantially from many C3 crops in their photosynthetic

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Fig. 6.2. Total production of (a) maize and (b) sorghum by global region in 1997. (FAO, 1998.)

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Fig. 6.2. Total production of (a) maize and (b) sorghum by global region in 1997. (FAO, 1998.)

and production responses to CO2, solar radiation, water and N deficits (Brown, 1999; Long, 1999).

6.2.1 Carbon dioxide

The primary physiological effect of the unique combination of photosynthetic metabolism and leaf anatomy that characterizes C4 plants (including maize and sorghum) is elevation of the CO2 concentration at the site of Rubisco in the bundle sheath (Hatch, 1992). The C4 dicarboxylate cycle in the these plants serves to concentrate CO2 from an atmospheric partial pressure of CO2 (pCO2) of about 35 Pa at sea level to an equivalent pCO2 of about 700 Pa (c. 20 x) at the site of RubP carboxylase-oxygenase (Rubisco) in the bundle sheath. This elevated concentration has two effects. Firstly, competitive inhibition of the oxygenase reaction of Rubisco eliminates most of the C2-photosynthetic oxidative or photorespiratory pathway (PCO) activity and its expenditure of energy associated with photorespiration. Secondly, it allows Rubisco to approach its maximum rate of catalysis despite its low affinity for CO2. Phosphoenolpyruvate carboxylase (PEPc) from C4 photosynthetic tissues has a high affinity for CO2, such that photosynthesis is saturated at a low pCO2 of typically 15-20 Pa (Long, 1999).

The pCO2 of the atmosphere is rising by 0.4-0.5% per year, and should approximately double by the year 2100 (Houghton et al, 1996). While this increase is expected to increase leaf photosynthesis of C3 species by c. 58% in the absence of limitation on rooting volume, no direct effect of this increase in pCO2 is expected in C4 species (Drake et al., 1997). Nitrogen deficiency and drought stress can increase leaking of CO2 from the bundle sheath cells of C4 leaves, thus possibly allowing a direct response of photosynthesis to elevated pCO2 (Long, 1999). Some long-term field studies have measured increased photosynthesis of C4 grasses, including sorghum and maize, under elevated pCO2 (Samarakoon and Gifford, 1996). However, a complication is that elevated pCO2 lowers stomatal conductance (gs) in C4 as in C3 species. Thus, C4 plants grown at elevated pCO2 show improved water status and this in turn will allow increased rates of CO2 assimilation whenever there is any shortage of water. Particularly significant is the observation that leaf and canopy photosynthesis have not been affected by doubled pCO2 over 10 years in the C4 grass Spartina patens growing on a marsh in Maryland, USA. The absence of any significant effect on photosynthesis in this environment is consistent with the hypothesis that the apparent response of photosynthesis of C4 species to elevated pCO2 in other environments is an indirect response to decreased transpiration (Long, 1999).

There are several reports that mitochondrial respiration is partially and significantly inhibited, on average by c. 20%, with a doubling of pCO2 above the current ambient level (Drake et al., 1997). The basis of this decrease is uncertain, with several potential sites of CO2 effect suggested. Both C3 and C4 species show this response (Drake et al., 1999).

6.2.2 Solar radiation

One of the first physiological features noted of C4 plants, following the discovery of their unique biochemistry in 1965/66, was their high rate of photosynthesis at full sunlight under tropical conditions (Hatch, 1992). By avoiding photorespiration, C4 species have potentially higher rates of net photosynthesis in full sunlight. Although additional energy is required to assimilate CO2 via the C4 pathway, this becomes irrelevant at light-saturation since light will by definition be in excess of requirements. In dim light when photosynthesis is linearly dependent on the photon flux, the rate of CO2 assimilation depends entirely on the energy requirements of carbon assimilation (Long et al., 1993). The additional two molecules of ATP required for assimilation of one molecule of CO2 in photosynthesis in sorghum and maize, compared with C3 plants, increases their photon requirement (Hatch, 1992). However, in C3 species at 30°C, the amount of light energy diverted into photorespiration in photosynthesis will considerably exceed the additional energy required for CO2 assimilation in C4 photosynthesis.

The energy expended in photorespiration as a proportion of photosynthesis rises with temperature. At 25°C and below, the energy required for the net assimilation of one CO2 molecule is higher in C4 than in C3 photosynthesis, but above 25°C the situation is reversed (Ehleringer and Monson, 1993). Thus, under warm conditions, the efficiency of photosynthetic CO2 uptake should always be higher in maize and sorghum compared with a similar C3 crop canopy. This difference is apparent in the efficiency of light use at the level of production (Monteith, 1978).

6.2.3 Water deficits

Because C4 photosynthesis is saturated with CO2 at partial pressures well below the current ambient level, some stomatal closure can occur without any effect on assimilation (A). In maize, following witholding of water, there was a progressive decrease in gs, evaporation (E) and leaf intercellular pCO2 (p) with little decrease in A until pi was approximately half that of the controls. Further analysis suggested that decreased A resulted from reduced physical conductance of CO2 during drought stress (Lal and Edwards, 1996).

In theory, because C4 plants lack photorespiration, they could be more prone to photoinhibition of photosynthesis when drought-induced closure of the stomata prevents CO2 uptake and the absence of photorespiration prevents internal cycling of CO2. In practice, significant internal cycling of CO2 in droughted leaves of maize is implied in the study by Lal and Edwards (1996). Even though sorghum subjected to drought did show increased photoinhibition of photosynthesis in sunlight compared with controls, there was no significant contribution of this photoinhibition to dry matter production (Ludlow and Powles, 1988).

6.2.4 Nutrient deficits

As a direct result of CO2 concentration at the site of Rubisco in C4 species, their theoretical requirement for N in photosynthesis is lower than that in C3 species. Furbank and Hatch (1987) calculated a CO2 concentration at the site of Rubisco in C4 species of 10-100 times that found in C3 species. At these CO2 concentrations a C4 leaf at 30°C would require 13.4-19.8% of the amount of Rubisco in a C3 leaf to achieve the same ^sat (Long, 1999). The decreased requirement for N in Rubisco will be partially offset by the requirement of N for the enzymes of the photosynthetic C4 dicarboxylate cycle, in particular PEPc. However, because of its tenfold higher maximum catalytic rates (kcat) of carboxylation, much smaller concentrations of PEPc are required, relative to Rubisco. Together, Rubisco and PEPc in C4 species constitute less than half the amount of N invested in Rubisco in C3 species (Sage et al, 1987).

Lower leaf N and higher leaf photosynthetic rates of C4 species result in a photosynthetic N use efficiency (PNUE) which is about twice that of C3 species (Long, 1999). Most comparisons of NUE have been undertaken in well-fertilized conditions. On N-deficient soils, NUE of C4 crops can be twice that of C3 (Brown, 1999). By decreasing the requirement for Rubisco in C3 species, rising atmospheric pCO2 may erode the PNUE advantage of maize and sorghum (Hocking and Meyer, 1991).

6.2.5 Temperature

In much of the developing world sorghum has been replaced by maize, except in areas that are too hot, dry or infertile for present maize cultivars. Consequently both crops are grown at temperatures closer to their upper limit. Progressive temperature increases associated with climatic change increase the risk of temperature stress for these crops. C4 photosynthesis is more tolerant of high temperature than is C3, due to the absence of photorespiration, which increases rapidly with temperature. However, C4 photosynthetic efficiency declines with temperature above c. 35° C, with some inactivation at 40oC and above (Maiti, 1996). In much of the developed world, growth is at higher latitudes and these risks are much reduced. At the highest latitudes, elevated temperature may increase photosynthetic efficiency during cooler periods, extending the range and economic viability of these crops in temperate climates (Wittwer, 1995).

6.2.6 Ozone and UV-B radiation

Ozone and other atmospheric pollutants gain access to and damage the photosynthetic apparatus following their entry into the intercellular air space via the stomata. Because C4 species have inherently lower stomatal conductances than equivalent C3 species, they will show a lower rate of pollutant uptake. This explains why they are generally among the more pollutant-tolerant plants, such that the effect of the same level of ozone on C4 maize is about half the effect on C3 wheat (Rudorff et al1996).

While C4 crops may be more resistant to increases in tropospheric ozone, a decline in the stratospheric ozone layer and the concomitant rise in surface UV-B will affect their photosynthesis. Sorghum grown in the field under supplemental levels of UV-B, simulating a 20% reduction in the stratospheric ozone column, showed significant decreases in photosynthetic CO2 uptake that were apparently linked to increased stomatal resistance. Enhanced UV-B also caused reductions in chlorophyll and carotenoid pigments and caused increases in UV-B absorbing pigments and peroxidase activity after 60 days of exposure (Ambasht and Agrawal, 1998). These changes suggest that damage was accompanied by partial acclimation. Similar reductions in photosynthetic CO2 uptake in maize have been observed with similar UV-B doses (Mark and Tevini, 1997).

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