Photosynthesis

Cotton canopies grown continuously in elevated [CO2] responded to increasing solar radiation with increasing rates of photosynthesis (Fig. 8.1).

Table 8.1. Treatment structures for experiments conducted on cotton in naturally sunlit environment chambers (SPAR units) for the last 10 years.

Table 8.1. Treatment structures for experiments conducted on cotton in naturally sunlit environment chambers (SPAR units) for the last 10 years.

Year

Cultivar

Temperatures (°C) day/night

(|mmol mol 1)

Comments and references

Expt 1-1989

DPL 50

15/7, 20/10, 25/15, 30/20, 35/25

350, 700

70 days from emergence, well-watered and fertilized (V.R.

Reddy etal., 1994a,b, 1995a,b)

Expt 2-1989

DPL 50

30/22

Several [CO2]s

Short-term, few weeks during flowering, well-watered and

fertilized (K.R. Reddy etal., 1995b)

Expt 3-1989

DPL-50

Several high temperatures

600

4 weeks during the fruiting period, flower abscission study

(V.R. Reddy et al., 1994a,b, 1995a)

Expt 4-1990

Pima-S-6

20/12, 25/17, 30/22, 35/27, 40/32

350, 700

64 days from emergence, well-watered and fertilized (K.R.

Reddy etal., 1995a,b)

Expt 5-1990

Pima-S-6

25/17, 30/22, 35/27

350, 700

Flowering to maturity, well-watered and fertilized (K.R.

Reddy etal., 1995a,b,c)

Expt 6-1990

Pima-S-6

Four high temperatures

700

Flowering to end of season, fruit retention study (A.R. Reddy

etal., 1997; K.R. Reddy etal., 1993)

Expt 7-1991

Pima-S-6

30/22

350, 450,700

95 days from emergence, three drought stress levels (K.R.

Reddy and Hodges, 1998)

Expt 8-1991

DES 119

30/22

350

Four nitrogen levels (K.R. Reddy etal., 1997b; V.R. Reddy

etal., 1997)

Expt 9-1992

DPL 5415

26/18, 31/23, 36/18

350, 450, 700

60 days from emergence, well-watered and fertilized (A.R.

Reddy etal., 1998)

Reddy etal., 1998)

Expt 10-1992 DPL 5415 26/18,31/23,36/18 350,450,700

Expt 1 1-1993 DES 1 19 30/22 350,700

Expt 12-1993 DES 1 19 30/22 350,450,700

Expt 13-1994 AcalaMaxxa 20/1 2, 25/1 7, 30/22, 35/27, 40/32 360,720 HS-26, DPL 51

Expt 14-1994 DPL 51 Temperatures: long-term MS July 360, 700

Expt 15-1995 DPL 51 Temperatures: 1 995 ambient, 1 995 360, 720

ambient-2, and 1995 ambient plus 2, 5 and 7

Expt 16-1996 NuCot33 30/22 360,720

Expt 17-1996 NuCott33 26/26 360

Expt 18-1997 Nucot33 30/22 360,720

Expt 19-1997 NuCot33 Several short-term temperature Several [C02]s treatments

Flowering to maturity, well-watered and fertilized.

49 days from emergence, five N levels (K.R. Reddy ef a/., 1997b)

80 days from emergence, three drought stress levels (K.R. Reddy ef a/., 1997c)

46 days from emergence, well-watered and fertilized (K.R. Reddy ef a/., 1997d)

4 weeks, flowering period, well-watered and fertilized (K.R. Reddy ef a/.,1997a)

Full-season, well-watered and fertilized (K.R. Reddy ef a/., 1997a; 1998).

84 days, five K levels

Manual de-leafing and de-fruiting study, well-watered and fertilized

Several water deficient studies

Short-term, few days to weeks, well-watered and fertilized a-* a-*

Table 8.2. Treatment structures for experiments conducted on cotton in open-top chambers (OTCs) and free-air CO2 enrichment (FACE) studies.

Year

Cultivar

Water supply

Nutrient supply

[CO2], (mmol mol-1)

Comments and references

Open-top chamber experiments

1983

DPL 70

Well-watered

Well-fertilized

Ambient, 500, 650

Ambient-no-chamber treatment also included (Kimball etal., 1992b; Kimball and Mauney, 1993)

1984

DPL 61

Well-watered Drought-stressed

Well-fertilized

Ambient, 500, 650

Ambient-no-chamber treatment also included (Kimball etal., 1992b; Kimball and Mauney, 1993)

1985

DPL 61

Well-watered Drought-stressed

Well-fertilized

Ambient, 500, 650

Ambient-no-chamber treatment also included (Kimball etal., 1992b; Kimball and Mauney, 1993)

1986

DPL 61

Well-watered Drought-stressed

High-N Low-N

Ambient, 650

Kimball etal. (1992b); Kimball and Mauney (1993)

1987

DPL 61

Well-watered Drought-stressed

High-N Low-N

Ambient, 650

Barley N-removal crop grown during winter of 1986-1987 (Kimball etal., 1992b; Kimball and Mauney, 1993)

Free-air CO2 enrichment (FACE) experiments

1989 DPL 77 Well-watered Well-fertilized Ambient, 550

1990 DPL 77 Well-watered Well-fertilized Ambient, 550

Drought-stressed

1991 DPL 77 Well-watered Well-fertilized Ambient, 550

Drought-stressed

Hendrey (1993); Dugas and Pinter (1994) Hendrey (1993); Dugas and Pinter (1994)

Hendrey (1993); Dugas and Pinter (1994)

0 500 1000 1500 2000

Photosynthetic photon flux density (^mol rrr2 s~1)

Fig. 8.1. Influence of carbon dioxide and solar radiation (photosynthetic photon flux density, PPFD) on cotton canopy gross photosynthesis (Pg) at 60 DAE on SPAR experiments. Pg = net photosynthesis plus respiration. (K.R. Reddy et al., 1995c.)

0 500 1000 1500 2000

Photosynthetic photon flux density (^mol rrr2 s~1)

Fig. 8.1. Influence of carbon dioxide and solar radiation (photosynthetic photon flux density, PPFD) on cotton canopy gross photosynthesis (Pg) at 60 DAE on SPAR experiments. Pg = net photosynthesis plus respiration. (K.R. Reddy et al., 1995c.)

Both the initial slopes of the light response curves and the asymptotes were greater in elevated [CO2]. This indicates that in nearly optimum growth conditions, additional [CO2] resulted in higher rates of photosynthesis due to the efficiency of the carboxylase fixation rates (higher rates where light is limiting) and diffusion of the gas to the fixation sites (higher rates where CO2 is the primary limiting factor). This point was further illustrated by the photo-synthetic (Pnet) responses of cotton canopies to CO2 and radiation at different air temperatures (Fig. 8.2). This experiment was conducted by growing plants in ambient and twice-ambient [CO2] at five temperatures. The 1995 temperature in Mississippi, USA, was used as a reference, with the other temperatures being 1995 minus 2°C, and 1995 plus 2, 5 and 7°C. Daily and seasonal variation and amplitudes were maintained. Net photosynthesis was less at both higher and lower temperatures than at optimum. The trends of the responses to air temperature and [CO2] were similar to those reported by K.R. Reddy et al. (1995c). At the three lower temperatures and ambient CO2, Pnet response to increasing radiation decreased as temperature increased. This apparently reflected the higher respiration caused by higher temperatures of the plant canopies. The opposite effect was found with high CO2, but this was w

1995 ambient minus 2°C

• >

• 720 pmol C02 mol'1 o 360 pmol C02 mol"1

1995 ambient temperature

1995 ambient plus 5°C

, «

1995 ambient plus 2°C

»v • • V» 9

1995 ambient plus 7°C

0 500 1000 1500 2000 0 500

1000

1500 2000

0 500 1000 1500 2000 0 500

1000

1500 2000

Fig. 8.2. Effect of solar radiation (PPFD), [CO2] and temperature on cotton canopy net photosynthesis (Pn) in 1995 SPAR experiments. Plants were grown from emergence in their respective [CO2] and temperature environments. Data represent 80 days after emergence when the canopies were intercepting about 95-98% of the incoming solar radiation.

the result of greater growth in the elevated [CO2]-grown plants and more young leaves that contributed to higher photosynthetic rates.

In the 1995 plus 5 and plus 7°C temperature treatments, the photo-synthetic rates of plants in the 760 mmol CO2 mol-1 environments were also high. These plants abscised their bolls soon after anthesis. Cotton is a perennial plant that is managed for commercial cotton production as an annual. Cotton plants typically set their first fruit on a fruiting branch near the bottom of the plant (nodes 5-8) (K.R. Reddy et al., 1997b). In optimum conditions, a new fruiting site is produced at the next higher mainstem node 3 days later and at the next node of the same fruiting branch 6 days later. Cotton plants progressively add bolls until the available photosynthetic supply will no longer support additional fruit. As the nutrient requirements for supporting a high population of bolls on the plant increases, the nutrients for vegetative growth become less available. This causes slowing and eventual cessation of stem and leaf growth. Reduced stem growth is reflected in both slower expansion of internodes and fewer nodes produced. As fewer nodes are produced, fewer fruiting branches and therefore fewer sites for additional fruit are available. Even under good conditions, nutrient requirements decline as bolls mature and vegetative growth resumes. In high-temperature environments (above 32°C), fruits were abscising 3-5 days after anthesis, thus voiding typical nutrient sinks and therefore promoting luxuriant vegetative growth. The high photosynthetic rates in these high-temperature environments (1995 plus 5 or plus 7°C) resulted from the relatively young population of leaves continuously being added to the top of the canopy.

The impact of [CO2] on photosynthetic efficiency is shown in Fig. 8.3. By interpolating from that figure, one can determine that cotton plants were fixing CO2 at 4.3 g CO2 m-2 MJ-1 in ambient (360 |mmol mol-1) and 6.3 g CO2 m-2 MJ-1 in twice-ambient [CO2]. This represents a 25% increase in photosynthetic efficiency caused by doubling the [CO2]. Both of these values represent highly efficient fixation rates in non-stressed environments. The photosynthetic efficiency was limited at the low [CO2] by the available CO2. The efficiency of CO2 fixation increased as [CO2] increased, but at a diminishing rate. At twice today's ambient [CO2], essentially all the feasible gain in photosynthesis was accomplished.

The information provided in Fig. 8.4 shows changes in canopy photosynthesis throughout the season with normalized radiation of 1200 |mmol m-2 s-1, and naturally varying temperature. These data illustrate photosynthetic rates over time with solar radiation normalized so that seasonal variation in radiation was not a confounding factor. Photosynthesis increased until about 80 days after emergence, and then decreased throughout the rest of the growing season. Crops growing in ambient [CO2] decreased their photosynthetic rates more than crops growing in twice-ambient [CO2]. This was caused by the increasing age of the light-intercepting canopies. In the high CO2 environment, photosynthesis was still proceeding at a very healthy

0 200 400 600 800 1000

Carbon dioxide concentration (irmol C02 mol-1)

Fig. 8.3. Effect of atmospheric [CO2] on radiation use efficiency of cotton canopies. (K.R. Reddy et al, 1997c.)

0 200 400 600 800 1000

Carbon dioxide concentration (irmol C02 mol-1)

Fig. 8.3. Effect of atmospheric [CO2] on radiation use efficiency of cotton canopies. (K.R. Reddy et al, 1997c.)

Fig. 8.4. Temporal trends in net photosynthesis (Pn) of cotton canopies grown in 1995 ambient temperatures in 360 and 720 ||mol CO2 mol-1 at Mississippi State, Mississippi. Photosynthesis was measured continuously, summarized at intervals of 900 s, and normalized at 1200 |mmol m-2 s-1 each day throughout the season. Time of first flower and first mature boll is indicated. (K.R. Reddy et al., 1998b.)

Fig. 8.4. Temporal trends in net photosynthesis (Pn) of cotton canopies grown in 1995 ambient temperatures in 360 and 720 ||mol CO2 mol-1 at Mississippi State, Mississippi. Photosynthesis was measured continuously, summarized at intervals of 900 s, and normalized at 1200 |mmol m-2 s-1 each day throughout the season. Time of first flower and first mature boll is indicated. (K.R. Reddy et al., 1998b.)

rate even at the end of the season and provided enough reduced carbon to support some new leaf growth during the fruiting period.

Average photosynthetic rates of the canopies at both ambient and twice-ambient [CO2] were summarized as rates at 720/360 ||mol CO2 mol-1 and plotted against average seasonal temperatures (Fig. 8.5). The seasonal average photosynthetic rate at 720 |mmol CO2 mol-1 was about 140% of that at 360 |mmol mol-1 at 20°C and 32°C, but the response increased to more than 180% at nearly optimum temperatures (26-28°C). Figure 8.6 provides the relative photosynthetic response for cotton canopies grown at elevated [CO2] compared with crops grown at ambient [CO2], but in a range of temperatures and water-deficient conditions. The data in this figure represent photosynthetic rates of both Pima and Upland cotton canopies, and although there was considerable variability among individual data points, the regression line indicated that photosynthesis at 720 |mmol CO2 mol-1 was 156% of that at 360 |mmol CO2 mol-1. The photosynthetic response to twice-ambient [CO2] appeared to be linear even at high rates.

These results from controlled-environment chambers are generally consistent with observations of the effects of elevated [CO2] on photosynthesis of field-grown plants. Radin et al. (1987) reported that CO2 enrichment to 650 |mol CO2 mol-1 in the 1985 open-top chamber experiment (Table 8.2) increased leaf photosynthesis of well-watered cotton more than 70%. Hileman et al. (1994) found that both leaf and canopy photosynthetic rates in well-watered conditions were increased by about 27% when [CO2] was

DC

Temperature (°C)

Fig. 8.5. Effect of temperature on the relative response of cotton canopy photosynthesis to doubling ambient [CO2]. Temperature and photosynthetic rates were averaged over the season from the time 95% of the radiation was intercepted by the canopy until first open boll.

0 20 40 60 80 100 120 140 160 180 Net photosynthesis at ambient [C02] (g C02 rrr2 day-1)

Fig. 8.6. Relation between daily total net canopy photosynthesis of cotton plants grown at ambient (360 |mmol mol-1) and elevated [CO2]. The data are from five temperature treatments of Upland cotton and three water-deficit treatments of Pima cotton collected over several days during fruiting.

enriched to 550 |mmol mol-1 in the 1989 FACE experiment (Table 8.2), and canopy rates were increased by about 31% in 1990.

Cotton plants grown in water-deficient conditions in controlled-environment chambers decreased their photosynthetic rates as midday leaf water decreased regardless of the [CO2] (Table 8.3; K.R. Reddy et al., 1997c). Plants grown in 720 |mmol CO2 mol-1 fixed an average of 48% more CO2 than

Table 8.3. Parameters for equations regressing several developmental events/rates and growth of cotton grown in elevated (700 or 720 |mmol CO2 mol-1) atmospheric CO2 (y) as a function of plants grown in ambient (350 or 360 |mmol CO2 mol-1) CO2 (x) (y = b0 + b1x- b2x2). The data were obtained from plants grown at a range of temperatures and nutrient and water deficient conditions.

Table 8.3. Parameters for equations regressing several developmental events/rates and growth of cotton grown in elevated (700 or 720 |mmol CO2 mol-1) atmospheric CO2 (y) as a function of plants grown in ambient (350 or 360 |mmol CO2 mol-1) CO2 (x) (y = b0 + b1x- b2x2). The data were obtained from plants grown at a range of temperatures and nutrient and water deficient conditions.

Regression parameters

Parameter

bo

b1

b'2

r2

Developmental events

Emergence to square (days)

-0.595

1.016

-

0.99

Square to flower (days)

-1.184

0.944

-

0.92

Flower to open boll (days)

-1.615

0.996

-

0.99

Mainstem nodes (no. plant-1)

0.313

0.998

-

0.98

Fruiting sites (no. plant-1)

-0.999

1.384

-

0.95

Roots (no. m-2 day-1)

-2.460

1.160

-

0.94

Stomatal density (no. mm-2)

-9.365

0.954

-

0.99

Growth and other processes Plant height (cm plant-1) Branch length (m plant-1) Leaf area (m2 plant-1) Total plant weight (g plant-1) Stem weight (g plant-1) Leaf weight (g plant-1) Root weight (g plant-1) Fruit weight (g plant-1) Net photosynthesis

2.419 -103.88 -0.022 2.352 -0.0427 1.936 0.156 0.216 -107.129

1.026 1.168 1.248 1.324 1.434 1.240 1.298 1.256 9.012

-0.167

0.93 0.96 0.98 0.88 0.97 0.97 0.89 0.98 0.94

(g m-2 day-1) Transpiration

2.7799

0.923

-

0.83

(kg H2O m-2 ground area day-1) Transpiration

36.741

0.510

-

0.99

1 eaf N (%)

-0.46

0.954

-

0.95

plants grown in 360 |mmol mol-1 in several water-deficient conditions. The midday leaf water potential varied from approximately -1.2 MPa to -3.0 MPa. There was no significant interaction between the photosynthetic response of plants grown in elevated [CO2] and water-deficient conditions. Plants grown in both ambient and twice-ambient [CO2] decreased photosynthesis by about 43% as their midday leaf water potential decreased from -1.2 MPa to -3.0 MPa. Field results have been similar. Radin et al. (1987) measured a 52% increase in net leaf photosynthesis when [CO2] was at 650 | mol CO2 mol-1 under drought-stressed conditions in the 1995 open-top chamber experiment (Table 8.2). Hileman et al. (1994) reported a 21% increase in canopy photosynthesis with enrichment to 550 |mmol CO2 mol-1 under drought stress in the 1990 FACE experiment.

Cotton plant responses to N deficiency in controlled-environment chambers were somewhat similar to plant responses to water deficits (Table

8.3; K.R. Reddy et al., 1997b,c). Photosynthesis decreased as the leaf nitrogen decreased. Plants grown in twice-ambient [CO2] decreased their photosynthesis by about the same amount in N-deficient conditions as plants grown in ambient [CO2]. Plants grown with 2% leaf N in both 360 and 720 |mmol CO2 mol-1 had 38% lower photosynthetic rates than plants with 4% leaf N, and plants grown at twice-ambient [CO2] only fixed 41% more CO2 than plants grown in ambient [CO2]. Canopy-level photosynthesis was greater in plants grown in higher N with both ambient and elevated [CO2]. Photosynthesis declined rapidly during the fruiting period when canopies became N deficient, but if N was sufficient (leaf N about 3%), the canopy photosynthetic rates maintained about 50% higher rates in twice-ambient [CO2] than in ambient [CO2] (K.R. Reddy and H.F. Hodges, unpublished data). Cotton canopies maintained 90% maximum photosynthetic rates with lower leaf N in a high-[CO2] environment than in ambient [CO2]. Similar gains in efficiency of N utilization have been found in rice (P.J. Conroy, NSW, Australia, personal communication, 1999). These results suggest that higher yields may be obtainable in high-[CO2] environments with increasing N fertilization. Field-grown cotton exposed to 650 |mmol CO2 mol-1 in OTCs increased net leaf photosynthetic rates by 45-70%, depending on the year, and there were no significant effects of N stress on the response (Table 8.2; Kimball et al., 1986, 1987).

Potassium deficiency caused 9.1% and 6.8% declines in photosynthetic rates as leaf K decreased from 4% to 2% in ambient and twice-ambient [CO2] environments, respectively (K.R. Reddy et al., 1997c). However, photosynthesis decreased more rapidly as leaf K decreased from 2% to 1%. Photosynthesis was 10.6% and 15.7% lower in the 1% leaf K plants compared with the 2% leaf K plants in ambient and twice-ambient [CO2], respectively. Potassium nutrition and [CO2] did not interact in their effect on photosynthesis when the leaf K concentration was above 2%, but photosynthesis declined to nearly zero in plants grown with the extremely low K. Thus, it appears that whether stresses are caused by water deficits or N or K deficiencies, there is little or no interaction with the effect of [CO2] on photosynthesis. Barrett and Gifford (1995) found similar results with P nutrition and [CO2] enrichment.

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