Oct 25, 1977 - chlorophyU content per unit area, and photosynthetic unit density per unit area. ... We also studied the effects of irradiance on certain growth.
Plant Physiol. (1978) 61, 402-405
Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton1 Received for publication September 22, 1977 and in revised form October 25, 1977
DAVID T. PATTERSON, STEPHEN 0. DUKE, AND ROBERT E. HOAGLAND Southern Weed Science Laboratory, Agricultural Research Service, United States Department of Agriculture, Stoneville, Mississippi 38776 were grown in a
ABSTRACT We grew velvetleaf (Abutilon theophrasd Medic.) and cotton (Gossypium hirsutm L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 peinsteins m-2sec-'. In air containing 21% 02, velvetleaf and cotton grown at 750 peinsteins m-2sec-I had maximm photosynthetic rates of 18.4 and 21.9 mg of CO2 dm-2hr-', respectively. Maximum rates for leaves grown at 320 and 90 peinsteins m-2sec' were 15.3 and 10.3 mg of C02 dm-2hr-' in velvetleaf and 12 and 6.7 mg of CO2 dm-2hr-' in cotton, respetively. In 1 O2, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air conting 21% °2, and plants grown at medium and high irradiance dW not differ in rate. In both species, stomatal conductance was not signflcantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with diferences in mesophyl conductance. Mesophyll conductance increased with growth irradiance and correlated positively with mesophyll thickness or volume per unit leaf area, chlorophyU content per unit area, and photosynthetic unit density per unit area. Thus, quantitative changes in the photosynthetic apparatus help account for photosynthetic adaptation to irradiance in both species. Net assimilation rates calculated for whole plants by mathematical growth analysis were closely correlated with single-leaf photosynthetic rates.
Velvetleaf is a malvaceous annual weed of increasing importance to cotton growers (6). Chandler (6) reported cotton yield reductions of over 90%o with full season competition from dense infestations of velvetleaf. Both Chandler (6) and Bjorkman (4) have shown the need for comparative physiological studies to help explain competitive and environmental responses of closely related plants. Comparisons of such closely related weed and crop plants as velvetleaf and cotton should lead to a better understanding of weed-crop interactions. As part of a series of investigations of the comparative ecophysiology of weeds and crop plants, we studied the effects of irradiance during growth on the photosynthetic characteristics of velvetleaf and cotton. We determined the degree of adaptability of the photosynthetic apparatus to irradiance and the changes in some leaf characteristics which help to account for the adaptability. We also studied the effects of irradiance on certain growth parameters determined from mathematical growth analysis and related the differences in the growth parameters to differences in
photosynthetic characteristics. MATERIALS AND METHODS Plant Material. Seedlings of velvetleaf (Abutilon theophrasti Medic.) and cotton (Gossypium hirsutum L. var. Stoneville 213)
' Mississippi Agricultural and Forestry Experiment Station cooperating. 402
1:1 (v/v) mixture of Bosket2 sandy loam and sand in a controlled environment chamber at a 31/25 C day/night temperature, 80%o relative humidity, and a 15-hr photoperiod. Light was supplied by a mixture of fluorescent and incandescent lamps. The temperature regime was chosen according to the method of Went (20) to represent average July day/night temperatures for Stoneville, Mississippi. The plants were grown in 15-cm pots and watered daily to field capacity with Hoagland solution. Screening in the chamber provided three irradiance levels during growth: 90, 320, and 750 ,tE m-2sec-1 PAR,3 400 to 700 nm. Measurement of CO2 Exchange. The rates of CO2 uptake of single leaves of cotton and velvetleaf were determined at ambient C02 and 02 concentrations with an IR gas analyzer and pincer cuvette described previously (16). Photosynthetic rates were determined in air containing 21% 02 at 30 C at six irradiances ranging from 90 to 2,000 ,uE m-2sec' PAR, provided by a 300-w incandescent lamp with a water filter. Rates of CO2 uptake for the same leaves were also measured in a gas mixture containing 310 Au/1 CO2 and 1.5% 02 in N2 at 2,000 ,uE m-2sec-' and 30 C. Dark respiration rates were determined for the same leaves at 25 C. All measurements were made on the most recently fully expanded leaf on each of three 30-day-old plants of each species from each of three irradiance treatments. Leaf Resistances. Stomatal resistances of the upper and lower surfaces of each leaf to H20 flux were determined at 2,000 ttE m 2sec'I with a Lambda autoporometer by the method of Kanemasu et al. (10). The sum of the resistances of both leaf surfaces was multiplied by 1.605 to convert to resistance to CO2 flux (9). Total resistance was calculated for CO2 exchange in both normal and low 02 air, treating the light-saturated photosynthetic rate as the flux and the external CO2 concentration as the CG2 concentration gradient (22). We used wet filter paper as a leaf model to determine boundary layer resistance for the pincer cuvette. Stomatal and boundary layer resistances were subtracted from the total resistance to yield residual or mesophyll resistance. This calculation of mesophyll resistance includes differences in carboxylase activity as well as all other nonboundary layer and nonstomatal effects (16). Leaf Anatomy, Specific Weight, and Chi Content. Samples of leaf tissue were collected from each leaf used in the CO2 exchange measurement, fixed in formalin-aceto-alcohol, mounted in the medium described by Wills and Briscoe (21) and sectioned in a cryostat. The 40-,um sections of velvetleaf leaves and 20-,um sections of cotton leaves were examined at 400x and the total thickness of the mesophyll layer was determined with an ocular 2 Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable. 3 Abbreviations: PAR: photosynthetically active radiation; PSU: photosynthetic units.
Plant Physiol. Vol. 61, 1978
403
PHOTOSYNTHESIS IN VELVETLEAF AND COTTON
micrometer. We multiplied the mesophyll thickness in
mm
by 10
volume/unit area in the high irradiance plants resulted in greater specific leaf weights and Chl contents/unit leaf area in both
to calculate the mesophyll volume in cubic cm' per dmi leaf. This calculation simply converts thickness in mm to volume in cm3 per
species.
dm2. Specific leaf weights were calculated from leaf discs collected from the same leaves and dried at 65 C. Leaf Chl contents and a to b ratios were determined in 80% acetone extracts by the method of Arnon (3). Characterization of Chloroplast Lamella. Chloroplast lamellae were prepared from the same leaves according to Alberte et al. (2) and Shiozawa et al. (19). Light-induced oxidation of P700 was measured in triton extracts (19) and the size of the PSU was calculated from the ratio of total Chl to P700 in the lamellae preparations. The PSU densities per unit leaf area or mesophyll volume were calculated from PSU sizes and Chl contents per unit area or volume. Growth Analysis. Net assimilation rates, relative growth rates, and leaf area ratios were calculated for both species by the interval technique described by Kvet et al. (11). Samples of five plants were harvested at the beginning and end of an 8-day interval terminating at the time of the gas exchange measurements, and the growth parameters were calculated for paired replicates. Leaf areas were determined with a Lambda leaf area meter (model LI3000); leaf and total plant dry weights were determined after drying at 65 C. Statistical Analyses. Analyses of variance for each species were computed for the physiological responses, leaf characteristics, and growth parameters; irradiance during growth was considered the treatment effect. In accordance with the recommendations of Carmer and Swanson (5) the LSD test was used to determine significant differences among the different treatments whenever F values were significant. Within each irradiance treatment, differences between the two species were determined using the t test. Correlation analyses were used to determine the relationships among the physiological responses, leaf characteristics and growth parameters. Coefficients of determination were calculated as indicators of the amount of variation in the dependent variables accounted for by the independent variables (8).
In both species, PSU size tended to decrease with increased irradiance during growth, though the differences were not statistically significant. As PSU size decreased, the Chl a to b ratio increased. This increase in a to b ratio is associated with changes in the light-harvesting Chl a to b protein as reported by Alberte and Thornber (1). The combination of smaller PSU values and greater Chl contents/unit leaf area in the plants grown at higher irradiance resulted in greater PSU densities/unit leaf area. There was little difference between cotton and velvetleaf in PSU density/unit area, but in all irradiances velvetleaf had a greater PSU density/unit of mesophyll volume. CO2 Exchange Rates. Increased irradiance during growth resulted in higher photosynthetic rates/unit area in both species at all measurement irradiances greater than 320 ,uE m-2sec-' (Table II). The stimulating effect of high irradiance during growth on photosynthetic rate was greater in cotton than in velvetleaf. Only when measured at 90 ,uE m-2sec-' did the cotton plants grown in low irradiance have higher photosynthetic rates/unit area. When the photosynthetic rates were expressed on the basis of mesophyll volume, a completely different pattem emerged (Table II). The plants of both species grown in low irradiance generally had higher rates/unit of mesophyll volume at measurement irradiances of 1,000 ,uE m 2sec-' or less, when compared with plants grown at high irradiance. At 1,500 and 2,000 t,E m-2sec' there were no differences in photosynthetic rate/unit volume within each species. At all measurement irradiances above 90 ,uE m velvetleaf had significantly higher photosynthetic sec rates/unit mesophyll volume than cotton. Dark respiration rates/unit area measured at 25 C (Table II) were significantly higher in the plants grown at 750 ,IE m 2secthan in those grown at 320 or 90,uE m-2sec'. When grown at the low and medium irradiances, cotton had significantly higher dark respiration rates than velvetleaf. The rates of CO2 uptake measured in low 02 at 2,000 ,iE m-2sec-' were about 1.5 to 2.3 times the rates in normal air. The high and medium irradiance plants had similar rates/unit area, and these rates were significantly greater than the rates of the low irradiance plants. In low 02, the photosynthetic rates expressed/unit of mesophyll volume did not differ significantly among plants of the same species grown at the three irradiances (Table II). However, for all three irradiances velvetleaf had higher photosynthetic rates/unit of mesophyll volume than cotton in low ,
RESULTS Leaf Characteristics. In both species, higher irradiances during growth resulted in thicker leaves with greater mesophyll volume/unit area (Table I). The relative increases in thickness were greater in cotton: the high irradiance plants had, respectively, 1.5 and 2.5 times the mesophyll volume/unit area of the medium and low irradiance plants. In velvetleaf the corresponding ratios in mesophyll volume were 1.3 for high to medium irradiance and 1.8 for high to low irradiance. The greater mesophyll thickness or
02.
Leaf Conductances. Stomatal conductance did not differ significantly among the three growth irradiances (Table II). Therefore,
Characteristics of leaves of velvetleaf and cotton grown at three irradiances: 750 (H), 320 (M), and 90 (L) wEm72sec-1. For each species, values within each row sharing the same letter are not significantly different at P - 0.05. A significant species effect within an irradiance treatment is indicated by underscoring of the larger value.
Table I.
Cotton Growth irradiance M
Velvetleaf Leaf clIaracteristic
Mesophyll thickness (mm)
Mesophyll volume per unit leaf
Growth irradiance M
L
H
L
0.122
a
0.093 b
0.069
c
0.280
a
0.186 b
0.112 c
1.22
a
0.93 b
0.69
c
2.80
a
1.86
b
1.12
c
5.43 4.45
a a
4.95 ab 5.33 b
3.96 5.71
b b
4.96 1.82
a a
4.52 2.43
ab b
4.15 3.73
b
3.28
a
3.00 b
2.87
b
3.40
a
3.19
ab
3.03
b
(cm3/dm2)
area
Chlorophyll content mg/dm2 leaf mg/cm3 mesophyll
Chlorophyll
a/b
H
ratio
PSU size
(wmole Chl/P700)
335
a
366
a
PSU density
nmole nmole
P700/dm2 P700/cm3
16.8 14.7
a a
13.9 16.2
ab a
379 12.3 16.2
a
b a
341
17.9 6.1
a
a a
370
15.0 7.5
a
a
b
403
11.2 11.1
c
a
b b
Plant Physiol. Vol. 61. 1978
PATTERSON, DUKE, AND HOAGLAND
404 Table II.
Gas exchange parameters of leaves of velvetleaf and cotton grown at three irradiances: 750 (H), 320 (M), and 90 (L) oEm-2sec1l. pa = photosynthetic rate per unit leaf area; P = photosynthetic rate per unit mesophyll volume; Cs = stomatal conductance; Cm = mesophvll conductance; the same letter Rn = dark respiration at 25 C. For each species, values within each row sharingwithin an are not significantly different at P = 0.05. A significant species difference irradiance treatment is indicated bv underscoring of the larger value.
Parame
r
Velvetleaf Growth irradiance ;
Measurement
*
a
D--ter o
c-
a a a a a a
0.8 7.1 14.0 14.7 15.3 15.3
a a a
1.0 6.1
b b b
9.9 10. 3 10.3
90 320 750 1000 1500 2000
0.5 5.9 12.5 13.3 14.9 15.1
a a a a a a
0.9 7.6 15.0 15.8 16.5 16.5
b ab b b a
1.4 8.9 13.5 14.3 14.8 14. 8
dmihr-l)
2000
31.3
a
26.6
ab
16.9
(low 0 ) (mg cm- hr l)
2000
25.7
a
28.5
a
24.4
(mg C02
Pv
cm
3hr-1)
a
Cotton Growth irradiance M.1
H
L
0.7 7 .2 15.2 16.2 18.1 18.4
Pv (normal 02)
a
H
90 320 750 1000 1l00 2000
Pa (normal ° ) (mg C02 dmn2hr-1)
Pa
irradiance \ (oEm-2sec-)
a
-1. 3
a
a
6. 2 1 3. 7
a
9.3
cb
15.7
20.0 21.9
cc
b
0.6 6.8 8.0
8.2
aaa
9.7
a
12.(0
-0. 5 2. 3
a a
5.0
a
0. 3 4. 0 4. 3
ab a
b b b b
L
b
1. 4 ; .4 6 .7
a
b
6.7 6.7 6. 7
c
b b
1.2
b b b
4.8
7. 3 8.0
aa
5. 2 6 . '-
a a a a
33.0
a
28.0
a
13.0
h
12.0
a
15.0
a
11.7
a
ab
ab
abs a a
6. 1 6. 1
a a
6. 1 6.1
(low 0)
C, (cm
sec
1)
(cm sec-1)
Cm (low 02) (cm
sec-1)
Rn (mg C02 dmn2hr-1)
0.45
a
0.45
0.098 b
0.057
c
0.169 a
0.072 b
a
0.263
a
0.115 b
0.441
a
0.296 a
a
0.2
b
0.1
1.3
a
0.6
0.60
a
0.49
2000
0.121
a
2000
0.318 1.2
Cm (normal 02)
a
0.57
2000
a
b
mesophyll conductance, calculated from CO2 uptake rates in normal air at 2,000 ,uE m-2sec', increased with growth irradiance in both species, but the relative increases were greater for cotton than for velvetleaf. Mesophyll conductance was higher in cotton than in velvetleaf for the high irradiance treatment, but higher in velvetleaf than in cotton for the medium and low irradiance treatments. When mesophyll conductances were calculated from CO2 uptake rates in low 02, the values were similar in the high and medium irradiance plants and were lowest in the plants grown at the lowest irradiance. A significant difference between species was apparent only in the plants grown in low irradiance where velvetleaf had the higher values. Growth Analysis. Average net assimilation rates calculated from harvest data (Table III) were highly correlated with average singleleaf photosynthetic rates measured at irradiances corresponding to the growth irradiances (Table IV). Variation in single leaf photosynthetic rates accounted for about 99% of the variation in net assimilation rates for both species. Relative growth rates were 50 to 130%7o higher in velvetleaf than in cotton, at all three irradiances, primarily because of the 50 to 70%o higher leaf area ratios in velvetleaf (Table III). Because of its greater relative growth rates velvetleaf's total dry weight exceeded that of cotton by 25 days after emergence in the high irradiance treatment, despite cotton's 10-fold greater seed weight at planting. Correlation Analyses. For all irradiance treatments the photosynthetic rates of both cotton and velvetleaf in normal air at 2,000 ,uE m-2sec-' were best correlated with the calculated mesophyll conductance (Table IV). About 99% of the variation in maximum photosynthetic rate could be accounted for by the variation in mesophyll conductance. Mesophyll conductance in turn was highly correlated with mesophyll volume/unit area, specific leaf weight, Chl content/unit area, and PSU density/unit area. Neither photosynthetic rate nor mesophyll conductance was significantly correlated with stomatal conductance.
Table III.
DISCUSSION Capacity for Adaptation to Irradiance in Cotton and Velvetleaf. Both species exhibit the positive photosynthetic adaptation to high irradiance characteristic of sun plants (18). Increasing the growth irradiance from 90 to 320 to 750 ME m 2sec' resulted in significant increases in the maximum photosynthetic rates/unit leaf area.
a
0.38
a
a
0.037 c
b
0.088
b
0.2
b
Growth parameters for velvetleaf and cotton at three irradiances: 750 (H), 320 (M), and 90 (L) oEm-2sec-1. For each species values within each row sharing the same letter are not significantlv different at P = 0.05. A significant species effect within an irradiance treatment is indicated bv underscoring of the larger value. Velvetleaf Growth irradiance M
H
Parameter
Relative growth rate (g g-ldav 1)
Cot ton
Growth irradiance L
H
L
1
0.098
0.225
a
0.172 b
0.147 c
0. 098 a
0.103 a
0.139
a
0.071 b
0.034 c
0.107
a
0.065
1.582 a
2.396 b
4.401 c
0.940
a
1.596 b
2.956
c
1.19
c
3.43
b
a
Net assimilation
dme2dayvl)
rate (g
Leaf
(dm2
g-l)
(g)
Dry weight at
Leaf at
3333
ratio
area
dav 31
19.47
a
4.32
b
0.51
c
8.00
a
4. 32
(dm2) day 31
19.47
a
10.58
b
2.18
c
7.80
a
6.84
area
Table IV.
a
Correlation analysis: r2 x 100 = coefficient of determination expressed as percent; Pa Max = 3hotosynthetic rate per unit leaf area (mg C02 dm- hr-') measured at 2000 oEm-2sec-l; Cm = mesophyll conductance (cm sec-1); Cs = stomatal conductance (cm sec-1); NAR net assimilation rate (g dm-2day-1); RGR = relative growth rate (g g1ldav-1); SLW = specific leaf weight. Model: Y = mX + b.
Velvetleaf X
Y
r2
x
100
Cotton r2
X
100
99.
99.0
89.8
88. 3
98 1
80.8
84. 1
90. 5
84.6
88. 5
0. 3
2 3. 5
Cm
mesophyll volume (cm3/dm2)
87. 3
87. 1
Cm
SLW (g/dm2)
89. 7
96. 5
Cm
Chl content
C
PSU density 2 (nmoles P700/dm2)
85.9
89 .4
NAR
Pa at growth irradiance
99.2
100.0
cm
Pa max
Pa
max
volume mesophyll (cm3/dm2)
Pa
max
SLW (g/dm2)
P
max
Chl content
P
max
PSU density (nmoles P700/dm3
Pa
max
(mg/dm2)
C.
(cm/sec)
(mg/dm2)
86.6
Plant Physiol. Vol. 61, 1978
PHOTOSYNTHESIS IN VELVETLEAF AND COTTON
There was little evidence of positive photosynthetic adaptation to growth at low irradiance in either species. However, growth in low as against high irradiance did result in about a 3-fold increase in leaf area ratio in both species. Relative growth rate is the product of leaf area ratio and net assimilation rate (11). The increase in leaf area ratio with decreasing growth irradiance is evidence of positive adaptation to low irradiance at the wholeplant level. Mechanisms of Adaptation. As shown by the correlation analyses, the higher maximum photosynthetic rates/unit area of the plants of both species grown at high irradiances were associated primarily with increases in mesophyll conductance. The increases in mesophyll conductances were in turn shown to be related to increases in the amount of photosynthetic tissue/unit of leaf area. These increases in the amount of photosynthetic tissue included increases in mesophyll thickness (or volume/unit area), Chl content/unit area, PSU density/unit area, and specific leaf weight. Other workers have also shown that quantitative changes in the photosynthetic apparatus are important in accounting for photosynthetic adaptability to irradiance (7, 13, 14, 16, 17). The differences in mesophyll thickness or volume/unit leaf area also help to explain the differences in light saturation commonly observed between plants grown in low and high irradiance. Since the thicker leaves of the high irradiance plants contain more Chl and more chloroplasts/unit area, light saturation of photosynthesis would be expected to occur only at higher irradiances. This is because chloroplast shading can occur within the mesophyll layers just as "mutual shading" occurs among leaves within a canopy. The phenomenon of chloroplast shading was discussed by Lundegardh (12) and has been demonstrated recently by Oya and Laisk (15). The low irradiance-grown plants of both species had greater Chl and PSU contents/unit of mesophyll volume than plants grown at high irradiance. Since at low irradiance the light-harvesting capacity of the photosynthetic apparatus limits photosynthetic rate, the plants grown in low irradiance had higher photosynthetic rates/unit volume than the high irradiance plants (Table II). However, the increase in mesophyll volume/unit area in the high irradiance plants more than compensated for their lower contents of Chl and PSU/unit volume so that the photosynthetic rates/unit area were higher in the high irradiance plants at measurement irradiances above 320 ,E m2sec '. At lower measurement irradiances the higher dark respiration rates in the plants grown in high irradiance probably contributed to their lower net
photosynthetic rates/unit area.
405
Acknowledgements-We thank P. C. Quimby, Jr. and J. D. Hesketh for reviewing the manuscript and for helpful discussions, and C. R. Meyer. A. Lane, and R. N. Paul, Jr. for technical assistance. Special thanks are due J. M. Chandler for helpful discussions concerning the behavior of velvetleaf and cotton under field conditions and S. M. Buckner for her excellent typing of the manuscript.
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