Photosynthesis and Respiration

7.2.1 Carbon dioxide

(a) Photosynthesis

Probably more data have been collected worldwide on photosynthesis and respiration of soybean than of any other crop, certainly of any other grain legume. Cure (1985) provided a detailed review of responses of ten important crops worldwide - wheat, maize, rice, barley, soybean, sorghum (Sorghum bicolor [L.] Moench), lucerne (Medicago sativa L.), cotton (Gossypium hirsutum L.), potato (Solanum tuberosum L.) and sweet potato (Ipomoea batatas [L.] Lam.) - to a doubling of carbon dioxide concentration ([CO2]). Many of the data sets had few observations from even fewer experiments. Only those responses calculated from three or more experiments will be considered in this chapter. Based on this criterion for elimination of responses, soybean ranked highest among all crops in percentage response to doubled [CO2] for short-term CO2 exchange rate (CER), (+70 ± 20%), acclimated CER (+42 ± 10%), initial net assimilation rate (NAR), (+35 ± 6%) and long-term NAR (+23 ± 5%). Cotton led in biomass accumulation and yield.

Based on earlier reports (Kramer, 1981), much attention was brought to bear on the acclimation of photosynthesis to elevated [CO2] (i.e. photo-synthetic inhibition, loss of photosynthetic capacity, down-regulation of photosynthesis) in certain plants (Oechel and Strain, 1985; Sasek et al., 1985; Tissue and Oechel, 1987). However, much of the work on soybean photosynthesis suggests that soybean does not lose photosynthetic capacity with long-term exposure to elevated [CO2].

Soybean photosynthetic rates increase with increasing [CO2] at both the leaf (Valle et al., 1985a) and canopy levels (Acock et al., 1985; Jones et al., 1985a). Valle et al. (1985a) exposed soybean leaflets grown at 330 |mmol CO2 mol-1 or 660 |mmol CO2 mol-1 to short-term concentrations ranging from 84 to 890 |mmol CO2 mol-1 under photosynthetic photon flux densities (PPFD) of 1100-1300 |mmol (photon) m-2 s-1. They fitted leaflet photosynthetic responses to [CO2] using the following non-linear rectangular hyperbola:

where CER = photosynthetic CO2 exchange rate (mmol CO2 m-2 s-1; CO2 = carbon dioxide concentration (mmol CO2 mol-1); Kc = apparent MichaelisMenten constant for CO2; Pmaxc = asymptotic maximum photosynthetic rate (mmol CO2 m-2 s-1); and R = the j-intercept parameter (mmol CO2 m-2 s-1). The parameter R can be regarded as the leaf respiration rate in high light at zero [CO2].

The values of the parameters Pmaxc, Kc, Rc, and the [CO2] compensation point, rc, which are derived from the CER response to [CO2] of two leaves grown at 330 and two leaves grown at 660 mmol CO2 mol-1, are given in Table 7.2. Leakage from the leaf chambers was not a factor in these experiments because the leaf CERs were measured with the leaf chambers mounted inside the respective plant growth chambers. Nevertheless, the parameters might have been slightly different if exposures to [CO2] had been extended to concentrations of at least 1200 mmol CO2 mol-1.

There was some evidence that elevated [CO2] suppressed respiration rates (presumable photorespiration). The average rc and Rc were 63.0 and -7.8 mmol CO2 m-2 s-1, respectively, for the leaves exposed to 330 mmol CO2 mol-1, but only 42.4 and -4.6 mmol CO2 m-2 s-1, respectively, for leaves exposed to 660 mmol CO2 mol-1.

On this individual leaf basis, there is no indication of a down-regulation of photosynthesis in soybean. In fact, Allen et al. (1990b) reported that soybean individual-leaf CER vs. [CO2] was linear over the range of 300-800 mmol CO2 mol-1 when plotted at PAR (photosynthetically active radiation) of 300, 600, 900 or 1500 mmol (photon) m-2 s-1. When plotted on an intercellular [CO2] basis, Campbell et al. (1988) reported that soybean leaflet CER grown at 660 mmol CO2 mol-1 was about twofold that of CER of a leaflet grown at 330 mmol CO2 mol-1. Also, ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein of soybean was a rather constant fraction of leaf soluble protein (55.2 ± 1.3%) from leaves grown at [CO2] ranging from 160 to 990 mmol mol-1. Furthermore, Rubisco activity was nearly constant at 1.0 mmol CO2 min-1 mg-1 protein across this range of [CO2], and leaf-soluble protein (g m-2) was almost constant (Campbell et al., 1988). However, since specific leaf weight (SLW; leaf laminae weight/leaf area) increased from 20.3 to 30.5 g m-2 with [CO2] increasing from 160 to 990 mmol mol-1, the leaf soluble

Table 7.2. Mean asymptotic maximum photosynthetic rate (Pmaxc) with respect to y-intercept parameter (Rc), apparent Michaelis-Menten constant for CO2 (Kc), and CO2 compensation point (rc) for soybean leaves grown at two CO2 concentrations and subjected to different short-term CO2 exposures across the range of 84-890 mmol mol-1. (Adapted from Valle et al., 1985a.)

Table 7.2. Mean asymptotic maximum photosynthetic rate (Pmaxc) with respect to y-intercept parameter (Rc), apparent Michaelis-Menten constant for CO2 (Kc), and CO2 compensation point (rc) for soybean leaves grown at two CO2 concentrations and subjected to different short-term CO2 exposures across the range of 84-890 mmol mol-1. (Adapted from Valle et al., 1985a.)

CO2 concentration

maxc

Rc

Kc

Gc

(mmol mol-1)

(mmol m-2 s-1)

(mmol m-2 s-1)

(mmol mol-1)

(mmol mol-1)

330a

52

-7.8

359

63

660

127

-4.6

1133

42

aMean values within each CO2 level of Pmaxc/ Kc and Tc were significantly different at P = 0.05 as tested by a f-test.

aMean values within each CO2 level of Pmaxc/ Kc and Tc were significantly different at P = 0.05 as tested by a f-test.

protein (expressed as mg g-1, dry weight) decreased, and Rubisco activity (expressed as ||mol CO2 min-1 g-1, dry weight), decreased concomitantly. Soybean leaf photosynthetic rates at elevated [CO2] were probably maintained higher because of an increase of one layer of palisade cells in the leaves (Thomas and Harvey, 1983; Vu et al., 1989). This modification of leaf structure provided more surface area of cells within the leaf with a similar amount of Rubisco per unit leaf area, as well as possibly providing better light distribution within the leaf.

An extensive set of data of soybean leaf photosynthesis versus [CO2] was obtained by Harley et al. (1985), and included responses to temperature and PPFD. Leaf photosynthetic rates increased almost linearly with increasing intercellular [CO2] up to 600-700 |mmolmol-1, and then increased slowly toward an asymptotic level above that concentration. Allen et al. (1990b) reported that soybean leaf photosynthetic rates increased linearly with external leaf [CO2] to 800 |mmol mol-1, which is consistent with the data of Harley et al. (1985).

Increased photosynthetic rates per unit leaf area were also reflected in increased photosynthetic rates per unit ground area of soybean plant canopies (Jones et al., 1984, 1985a,b,c). Jones et al. (1984) found that midday canopy CER was about 60 |mmol m-2 s-1 for soybean grown at 330 |mmol CO2 mol-1, but was about 50% higher (90 |mol m-2 s-1) at 800 |mol CO2 mol-1. Similar results were found in other experiments. Likewise, the daytime total CO2 exchange averaged 0.92 and 1.72 mol CO2 m-2 for the 330 and 800 |mol CO2 mol-1 treatments, respectively, for data collected during 8 clear days. Cumulative daytime CER of soybean for the season was 53.0 and 84.7 mol m-2 for canopies grown at 330 and 660 |mol CO2 mol-1, respectively (Jones et al., 1985c).

Allen et al. (1987) used a non-linear model similar to equation 7.1 to express whole-canopy relative photosynthetic rates as a function of [CO2]. This same formulation was also used to express relative biomass yield and relative seed yield. Since all photosynthetic rate data were expressed relative to ambient rates of soybean growing at 330 or 340 | mol CO2 mol-1, this equation was reformatted as:

where R = the relative photosynthetic response (or relative biomass or seed yield response, as discussed in the later section on crop yield) to CO2; CO2 = carbon dioxide concentration; Rmax = the asymptotic relative response limit of (R - Rint) at high CO2 concentration; Rint = the intercept on the y-axis; and Km = the value of CO2 concentration at which (R - Rint) = 0.5 Rmax. The model parameters for midday relative photosynthetic rate data shown in Allen et al. (1987) are Rmax = 3.08, Km = 279 |mol CO2 mol-1, and Rint = -0.68. From equation 7.1a, the value computed for the CO2 compensation point, r, was 79 |mol CO2 mol-1.

(b) Respiration

It is well known that elevated [CO2] decreases photorespiration or the photo-oxidative pathway of C3 plants by increasing the reactivity of Rubisco for CO2. In fact, this is the underlying mechanism for the response of C3 plants, such as soybean, to increasing concentrations of CO2, and the concepts of this mechanism have now been extended to modelling crop responses to elevated CO2 (Boote et al., 1997). Also, many studies have been conducted on the effects of elevated CO2 on dark respiration of plants (e.g. Bunce and Ziska, 1996; Gonzales-Meler et al., 1996; Amthor, 1997). The magnitude of the short-term (direct) effect of elevated CO2 on dark respiration is still being researched. Respiration rate per unit mass of plants appears to decrease with elevated CO2, probably because of the greater relative accumulation of both structural and non-structural carbohydrates and the lesser relative accumulation of proteins. Protein content appears to be the controlling factor for the direct effect of elevated CO2 on dark respiration. Baker et al. (1992) found that dark respiration of rice per unit land area was linearly related to the amount of plant N per unit land area.

7.2.2 Solar radiation

Valle et al. (1985a) reported leaf CER vs. PPFD for eight soybean leaves grown in 330 and 800 |mmol CO2 mol-1. A Michaelis-Menten type of rectangular hyperbola was fitted to the CER data as follows:

CER = [CPmaxL X PPFD)/(PPFD + K)] + RSPl, (Eqn 7.2)

where PmaxL is the asymptotic maximum of CER with respect to the j-intercept parameter, RSPL; and KL is the apparent Michaelis-Menten constant. The parameter RSPL can be viewed as the extrapolated respiration rate at zero light. These parameters are shown in Table 7.3. An apparent quantum efficiency, Q, was computed from the slope of the CER vs. PPFD curves. Values of Q evaluated at PPFD equal to zero are also shown in Table 7.3.

In summarizing another large data set, Allen et al. (1990b) found that soybean leaf CER vs. PPFD followed a similar trend to that reported by Valle et al. (1985a). However, Allen et al. (1990b) found that the leaf response to [CO2] was linear over the range of 330- 800 |mmol mol-1.

Table 7.3. Photosynthetic asymptotic ceiling (PmaxL) with respect to /-intercept parameter (RSPL), apparent Michaelis-Menten constant (KL), light compensation point (rL) and apparent quantum yield (Q) for soybean leaves (cv. Bragg) grown and measured at two CO2 concentrations. The parameters estimated by an overall analysis using data for four leaves at the same CO2 are presented. Average temperature and standard deviations for leaves grown at 330 and 660 |mmol mol-1 were 28.4°C ± 2.8 and 29.6°C ± 2.6, respectively. (Adapted from Valle et al., 1985a.)

Table 7.3. Photosynthetic asymptotic ceiling (PmaxL) with respect to /-intercept parameter (RSPL), apparent Michaelis-Menten constant (KL), light compensation point (rL) and apparent quantum yield (Q) for soybean leaves (cv. Bragg) grown and measured at two CO2 concentrations. The parameters estimated by an overall analysis using data for four leaves at the same CO2 are presented. Average temperature and standard deviations for leaves grown at 330 and 660 |mmol mol-1 were 28.4°C ± 2.8 and 29.6°C ± 2.6, respectively. (Adapted from Valle et al., 1985a.)

CO2

1 max

RSPl

Kl

Tl

Q

concentration

(mmol CO2

(mmol CO2

(mmol photon

(mmol photon

(mol CO2

(mmol mol-1)

m-2 s-1)

m-2 s-1)

m-2 s-1)

m-2 s-1)

mol-1 photon)

330

37

-1.7

713

35

0.052

660

76

-2.4

844

27

0.091

Soybean leaf Rubisco activity (bicarbonate-magnesium activated) of 21- and 39-day-old plants decreased in proportion to elevated [CO2] (450 and 800 mmol mol-1) compared with plants grown at 330 |J.mol mol-1 (Vu et al., 1983). However, leaves from 59-day-old plants showed no differences (Table 7.4). Light was necessary for full activation of Rubisco. When soybean plants were kept in darkness from pre-dawn to noon, both the non-activated and the bicarbonate-magnesium-activated Rubisco activities remained at the low pre-dawn values. As soon as the plants were uncovered, the Rubisco activity increased to about the same value as that of plants not kept in darkness. Bicarbonate-magnesium-activated Rubisco increased with PPFD up to 400-500 mmol (photon) m-2 s-1. Campbell et al. (1988) found that leaf-soluble protein was relatively stable in soybean leaves, and that Rubisco protein was a constant 55% of leaf-soluble protein. These two reports showed that Rubisco activity per unit protein was not down-regulated in activity or amount in soybean grown in elevated CO2. In studies of rice plants (Rowland-Bamford et al., 1991), both Rubisco protein concentration and enzyme activity were down-regulated in leaves of plants grown in elevated CO2.

7.2.3 Temperature

More studies have been conducted on the effects of low rather than of high temperature on photosynthesis of soybean. Harley et al. (1985) compiled extensive data on soybean leaf photosynthesis versus temperature (and CO2 and PPFD). Their light-saturated and CO2-saturated photosynthetic rates increased with temperature to about 40°C. From these data, Harley and Tenhunen (1991) developed equations for modelling leaf photosynthetic responses to both temperature and CO2 concentration. Probably the most definitive studies on temperature effects on soybean canopy photosynthesis throughout the whole season were reported by Pan (1996). In general, whole-canopy photosynthesis rates (and vegetative growth) were maintained without failure up through at least 44/34°C sinusoidal maximum/minimum (day/night) temperatures.

Table 7.4. HCO3-/Mg2+-activated Rubisco values from leaves of soybean grown under three CO2 concentrations. Enzyme activities were determined on leaves sampled about 0830 EST at 21 and 39 days after planting (DAP). At 21 DAP, leaves were expanding; at 39 DAP, leaves were fully expanded. Data at 59 DAP were collected at four times during the day. (Adapted from Vu et al.,1983.)

Rubisco activity (|mmol CO2 mg-1 Chl h-1)

CO2 concentration -

330 450 800

Jones et al. (1985b) conducted experiments on photosynthetic rates of soybean canopies with short-term (1 day) changes in temperature. They found no differences in canopy CER from 28 to 35°C. Plots of CER vs. PPFD at different temperatures were indistinguishable within CO2 levels of 330 or 800 |mmol mol-1.

7.2.4 Water deficits

Jones et al. (1985c) conducted a drought study on soybean and found that both an early and a late stress diminished the seed and biomass yield of beans grown at both 330 and 660 |mmol CO2 mol-1. Plants grown under doubled [CO2] and subjected to short but severe drought cycles yielded about 20% more than the non-stressed plants grown under ambient [CO2]. Based on the same experiment, Allen et al. (1994) showed that leaf temperatures were about 2°C warmer when exposed to 660 compared with 330 |mmol CO2 mol-1. As a 13-day drying cycle progressed, leaf temperatures of drought-stressed plants increased by about 7°C compared with non-stressed plants. The residual internal leaf conductance (so-called mesophyll conductance) for CO2 of drought-stressed plants decreased throughout the drying cycle. However, the intercellular CO2 concentration (Ci) remained nearly constant throughout the drought period. Furthermore, Ci/Ca ratios remained steady throughout the drought, where Ca is the ambient [CO2].

During the seed-fill (R5-R6 stages of development; Fehr and Caviness, 1977) period, Allen et al. (1998) showed that soybean grown under elevated [CO2] and exposed to a drying cycle maintained a higher leaf turgor pressure by maintaining a higher (less negative) water potential rather than by maintaining a lower osmotic potential. The small change in osmotic potential during seed-fill indicates that osmotic adjustment was suppressed, probably because leaf resources were being remobilized and transported to the seed.

Huber et al. (1984) reported that drought stress decreased CER more in soybean plants that were not enriched with CO2 than in those that were enriched. As CER declined with increasing drought, the internal [CO2] concentration remained relatively constant despite decreased stomatal conductance. This behaviour was similar to the results reported by Allen et al. (1994). Thus, internal factors, rather than stomatal closure itself, caused the decreases in CER under progressive drought stress.

7.2.5 Nutrient deficits

Nutrient deficiency for agricultural crops under potential climatic change conditions is probably of no consequence where fertilizers are heavily used. With regard to photosynthesis, leaf N concentration has long been known to govern leaf photosynthetic rates, presumably through its strong relationship to

Rubisco. In a 1983 experiment, soybean grown in pots without adequate N doubled leaf photosynthetic rates in 3 days after application of ammonium nitrate (R.R. Valle and L.H. Allen, Jr, unpublished).

Leaf total N and Rubisco per unit leaf area decreased under elevated [CO2] in rice (Rowland-Bamford et al., 1991) but not in soybean (Campbell et al., 1988). Elevated [CO2] seemed to have little effect on the C/N ratios of tissues at final harvest. DeWitt et al. (1983) measured N2 fixation of whole soybean canopies throughout the growing season using the acetylene reduction method. The soybean canopies were grown in soil-plant-air research (SPAR) chambers at [CO2] of 330, 450 and 800 ||mol mol-1. They found that long-term CERs, long-term acetylene reduction, root nodule numbers on the glass face of the rooting volume, nodule dry weights at final harvest and whole-plant N contents (g m-2) at final harvest were proportional to CO2 concentration. For these reasons and other experimental data, Allen et al. (1988) concluded that legume crops such as soybean would always supply a sufficient and balanced amount of N through symbiotic N2 fixation to meet the requirements of vegetative growth and seed yield.

7.2.6 Pollutants such as ozone

Ozone is generally considered to be the primary gaseous pollutant of agricultural crops and forests (Heck et al., 1982; Heagle et al., 1983). In many regions of the USA, ozone concentrations are about double what they would be without anthropogenic influences (Heck et al., 1984). In open-top chambers, Heagle et al. (1998a) found that increasing O3 concentrations from 0.020 to 0.080 |ml l-1 (12 h exposure day-1) caused increased soybean foliar injury, decreased leaf chlorophyll and starch content, increased chlorophyll a/b ratio, and decreased SLW. In general, all these impacts on leaves became more severe with time from 43 to 113 days after planting (DAP).

Booker et al. (1997) found that the photosynthetic rates of soybean leaves near the mainstem terminal were the same (about 28 |mmol CO2 m-2 s-1) at 54-62 DAP, when exposed to either the low ozone of charcoal-filtered air (0.020 |ll O31-1) or air enriched to 1.5 times daytime ambient ozone (0.070 |l O3 l-1) in exposures of 12 h day-1 at ambient [CO2] (364 |mmol mol-1). However, the photosynthetic rates in ambient [CO2] were suppressed by 15% at 75-83 DAP and by 45% at 97-106 DAP by the 1.5 daytime ambient ozone level. Under 726 | mol CO2 mol-1 treatments the adverse impacts of high ozone were much less on leaf photosynthesis, as was also shown by Reid and Fiscus (1998). Also, Rubisco activity decreased with level of ozone, especially with increasing leaf age (Reid et al., 1998). Clearly, ozone has an increasingly suppressive effect on photosynthesis as the leaf ages.

Elevated [CO2] generally decreased the impact of ozone on all leaf responses (Booker et al., 1997; Heagle et al., 1998a; Reid et al., 1998). Allen (1990) speculated that partial stomatal closure or additional photoassimilate under elevated [CO2] might decrease ozone stress on plants.

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