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We studied the effects of CO2 elevation on leaf and canopy photosynthesis and optimal leaf area index (LAI) for stands of the annual species Abutilon theophrasti and Ambrosia artemisiifolia. Leaf photosynthesis was modeled as a function of photosynthetic photon flux density (PPFD) and nitrogen content per unit leaf area (NL). There was a curvilinear relationship between the light-saturated rates of leaf photosynthesis (Pmax) and NL. CO2 elevation significantly increased Pmax as a function of NL in both species. Dark respiration (Rd) was linearly correlated with NL. CO2 elevation slightly but significantly increased Rd in Abutilon, while it had no significant effect on Rd in Ambrosia. The initial slope of a light-response curve was determined from quantum yield (ϕabs) multiplied by leaf absorptance and then calibrated against NL. Daily canopy photosynthesis, calculated by integration of leaf photosynthesis with the actual distribution of leaf area, leaf N, and PPFD within a canopy, showed fairly good agree...

We examined how CO2 concentrations and temperature interacted to affect growth, resource acquisition, and resource allocation of two annual plants that were supplied with a single pulse of nutrients. Physiological and growth measurements were made on individuals of Abutilon theophrasti (C3) and Amaranthus retroflexus (C4) grown in envi- ronments with atmospheric CO2 levels of 400 or 700 gtL/L and with light/dark temperatures of 28?/22? or 380/3 10C. Elevated CO2 and temperature treatments had significant indepen- dent and interactive effects on plant growth, resource allocation, and resource acquisition (i.e., photosynthesis and nitrogen uptake), and the strength and direction of these effects were often dependent on plant species. For example, final biomass of Amaranthus was enhanced by elevated CO2 at 280 but was depressed at 380. For Abutilon, elevated CO2 increased initial plant relative growth rates at 280 but not at 380, and had no significant effects on final biomass at either temperature. These results are interpreted in light of the interactive effects of CO2 and temperature on the rates of net leaf area production and loss, and on net whole-plant nitrogen retention. At 280C, elevated CO2 stimulated the initial production of leaf area in both species, which led to an initial stimulation of biomass accumulation at the higher CO2 level. However, in elevated CO2 at 280, the rate of net leaf area loss for Abutilon increased while that of Amaranthus decreased. Furthermore, high CO2 apparently enhanced the ability of Amaran- thus to retain nitrogen at this temperature, which may have helped to enhance photosyn- thesis, whereas nitrogen retention was unaffected in Abutilon. Thus, at 280, final biomass of Abutilon was not stimulated in a high CO2 environment whereas the final biomass of Amaranthus was. At 380, Abutilon had slightly reduced peak leaf areas under elevated CO2 in comparison to ambient CO2 grown plants, but increased rates of photosynthesis per unit leaf area early in the experiment apparently compensated for reduced leaf area. For Amaran- thus at 380, peak leaf area production was not affected by CO2 treatment, but the rate of net leaf area loss hastened under elevated CO2 conditions and was accompanied by sub- stantial reductions of whole-plant nitrogen content and leaf photosynthesis. This may have led to the reduced biomass accumulation of high CO2 grown plants that we observed during the last 30 d of growth. Plants of both species grown in elevated CO2 exhibited reduced tissue-specific rates of nitrogen absorption, increased plant photosynthetic rate per unit of conductance, and increased initial allocation of biomass to roots, irrespective of temperature. Plants of both species grown under an elevated temperature regime had substantially decreased repro- ductive allocation, increased allocation to stem biomass, and increased plant water flux at both CO2 treatments. The age of plants also affected our interpretations of plant responses to CO2 and temperature treatments. For example, significant effects of CO2 treatment on the growth of Abutilon were evident early, prior to the initiation of flowering, when nitrogen availability would have been highest and pot space would not have been limited. Never- theless, the opposite was true for Amaranthus, in which significant effects of CO2 treatment on plant growth were not detectable until the final 30 d of the experiment. Elevated CO2 interacted with temperature to affect plant productivity in different ways than would have been predicted from plant responses to elevated CO2 alone. Furthermore, a majority of the interactive effects of CO2 concentration and temperature on plant growth could be interpreted in light of their effects on the rates of net leaf area production and loss, nitrogen retention, and, to a lesser degree, photosynthesis and resource partitioning.

We studied leaf area production in two annual plant species, Abutilon theophrasti and Amaranthus retroflexus, under three day/night temperature regimes (18°/14°, 28°/22°, and 38°/31°C) and two concentrations of carbon dioxide (400 and 700 @mL/L). The production of whole—plant leaf area during the first 30 d of growth was analyzed in terms of the leaf initiation rate, leaf expansion, individual leaf area, and, in Amaranthus, production of branch leaves. Temperature and CO2 influenced leaf area production through effects on the rate of development, determined by the production of nodes on the main stem (the plastochron index), and through shifts in the relationship between whole—plant leaf area and the number of main stem nodes. In Abutilon, leaf initiation rate was highest at 38°, but area of individual leaves was greatest 28°. Total leaf area was greatly reduced at 18° due to slow leaf initiation rates. Elevated CO2 concentration increased leaf initiation rate at 28°, resulting in an increase in whole—plant leaf area. In Amaranthus, leaf initiation rate increased with temperature, and was increased by elevated CO2 at 28°. Individual leaf area was greatest at 28°, and was increased by elevated CO2 at 28° but decreased at 38°. Branch leaf area displayed a similar response to CO2, but was greater at 38°. Overall, wholeplant leaf area was slightly increased at 38° relative to 28°, and elevated CO2 levels resulted in increased leaf area at 28° but decreased leaf area at 38°. The effects on leaf area closely parallel rates of biomass accumulation in the same experiment, suggesting that responses of developmental processes to elevated CO2 and interacting factors may play an important role in mediating effects on plant growth.

In order to investigate the effects, without competition, of CO{sub 2} on germination, growth, physiological response, and reproduction, the authors focussed on co-occurring species that are prominent members of an annual community in Illinois. Five species of old field annual plants - Abutilon theophrasti (C{sub 3}), Amaranthus retroflexus (C{sub 4}), Ambrosia artemisiifolia (C{sub 3}), Chenopodium album (C{sub 3}), and Setaria faberii (C{sub 4}) - were grown for their entire life cycle as individuals at CO{sub 2} concentration of 350 {mu}L/O, 500 {mu}L/L, and 700 {mu}L/L. Emergence time, growth rate, shoot water status, photosynthesis, conductance, flowering time, nitrogen content, and biomass and reproductive biomass were measured. There was no detectable effect of enhanced CO{sub 2} on timing of emergency in any of the species. The three levels of carbon dioxide concentration were shown to produce varying effects on remaining quantities measured in the five different plants. Some of these differences were not statistically significant. The response of most characters had a significant species {times} CO{sub 2} interaction. However, this was not simply caused by the C{sub 3}/C{sub 4} dichotomy. Reproductive biomass (seed, fruits, and flowers) increased with increasing CO{sub 2} in Amaranthus (C{sub 4}) and in Chenopodium and Ambrosia (both C{submore » 3}), but there was no change in Setaria (C{sub 4}), and Abutilon (C{sub 3}) showed a peak at 500 {mu}L/L. Species of the same community differed in their response to CO{sub 2}, and these differences may help explain the outcome of competitive interactions among these species above ambient CO{sub 2} levels.« less

We studied the effects of CO2 elevation on leaf and canopy photosynthesis and optimal leaf area index (LAI) for stands of the annual species Abutilon theophrasti and Ambrosia artemisiifolia. Leaf photosynthesis was modeled as a function of photosyn- thetic photon flux density (PPFD) and nitrogen content per unit leaf area (NL). There was a curvilinear relationship between the light-saturated rates of leaf photosynthesis (Pmax) and NL. CO2 elevation significantly increased Pmax as a function of NL in both species. Dark respiration (Rd) was linearly correlated with NL. CO2 elevation slightly but significantly increased Rd in Abutilon, while it had no significant effect on Rd in Ambrosia. The initial slope of a light-response curve was determined from quantum yield (4gabs) multiplied by leaf absorptance and then calibrated against NL. Daily canopy photosynthesis, calculated by integration of leaf photosynthesis with the actual distribution of leaf area, leaf N, and PPFD within a canopy, showed fairly good agreement with the canopy photosynthesis estimated from growth analysis. CO2 elevation increased canopy photosynthesis by 30- 50%. Based on the leaf photosynthesis model for Abutilon, we calculated daily canopy photosynthesis for a given LAI and N availability, in which N was assumed to be distributed optimally within a leaf canopy to maximize daily canopy photosynthesis. An optimal LAI to maximize daily canopy photosynthesis was obtained for each level of N availability and this optimum increased with increasing N availability. Contrary to the often predicted increase in LAI with CO2 elevation, the optimum LAI did not increase at high CO2 when N availability was limited. Two factors were suggested to be involved in counteracting the increase in LAI in a high-CO2 world. One is the higher 4abs of plants grown in elevated C02, which makes leaves in the canopy more N limited, favors higher NL and thus lowers optimal LAI. The other is the higher Rd in elevated C02, which leads to higher light compensation points, and lowers optimal LAI.

Polygonum pensylvanicum and Abutilon theophrasti, components of old—field communities in central Illinois were grown on a controlled soil moisture gradient to determine whether they exhibit different response centers. Germination response of a constant number of seeds per six moisture resource states, height, biomass, and reproductive response of initially constant numbers of plants per state were monitored. States, ranging from saturation down to 13.61 ± 5.53% moisture, are numbered in order of increasing drought. Maximum height of Polygonum occurred in states 4 and 5, and of Abutilon in 4, 5, 6. Vegetative and reproductive biomass maxima of Polygonum and Abutilon occurred in states 5 and 6, respectively. Abutilon reproduced only in states 5 and 6 where it overtopped the Polygonum canopy, while Polygonum reproduced in all states. Polygonum performance declined sharply in state 6. Thus, Polygonum and Abutilon response centers are displaced toward the wetter and drier ranges of the gradient, respectively. On a within—community level, Abutilon reproduces in less mesic resource states where Polygonum canopy height is reduced. In general, Abutilon reproduction may occur throughout the gradient when the Polygonum canopy opens. The performance of these species is strongly influenced by both physical and biotic conditions. Previously investigated adaptations, including drought resistance and rooting patterns in space and time, appear to underlie divergence along the moisture gradient. The response patterns found here are broad and exhibit considerable overlap as expected in fugitive strategies. The patterns of divergence shown on point and gradient scales in this and our previous work reflect a related complex of adaptations and strongly suggest that competitive divergence occurs in early successional communities.

Four members of an annual community were used to investigate the effects of changing neighborhood complexity and increased CO2 concentration on competitive outcome. Plants were grown in monoculture and in all possible combinations of two, three, and four species in C02-controlled growth chambers at CO2 concentrations of 350, 500, and 700 ,4L/L with ample moisture and high light. Species responded differently to enhanced CO2 level. Some species (e.g., Abutilon theo- phrasti) had increased biomass with increasing C02, while others (e.g., Amaranthus retro- flexus) had decreased biomass with increasing CO2 concentration. In mixtures, species tended to interact strongly, and, in some cases, the interaction canceled out the effects of CO2. Furthermore, there were clear differences in species behavior in different competitive mixtures as assessed by total biomass and seed biomass, and by an index of response to neighbors. In general, competitive arrays that had C3 species depressed the response of C4 species, especially Amaranthus. Ambrosia artemisi iWlia was the strongest competitor in this assemblage. Strong statistical interactions between CO2 and the identity of the com- peting species in mixtures were found to be primarily due to the as yet unexplained response of plants with CO2 at 500 ,L/L. The potential effects of CO2 on community structure could be profound, particularly at the intermediate levels of CO2 that are predicted to be reached during the first half of the next century.

Potentially adaptive effects of leaf thickness on photosynthesis and transpiration were studied for Abutilon theophrasti grown under high (600 μmol°m—2°s—1) and low (200 μmol°m—2°s—1) levels of photosynthetically active radiation. We determined the relationship of both surface area and volume of mesophyll cells of leaf thickness. Net photosynthesis and transpiration rates were measured on the third leaf of each seedling, under sunny (1200 μmol°m—2°s—1) and shaded (250 μmol°m—2°s—1) conditions. Plants grown under high light had greater mesophyll surface area and total mesophyll cell volume per unit leaf area than did plants grown under low light, and showed higher net photosynthesis under sunny conditions. There was little difference in transpiration rate between thick and thin leaves. Thick leaves and thin leaves showed similar net photosynthesis under shaded conditions. Consequently, the water use efficiency for thick leaves was greater than that for thin leaves under sunny conditions. Increased productivity without increased water use is potentially of adaptive significance in plants where the mesophyll of sun—grown leaves thickens.

This study examines the effects of short-term selection along resource gradients on variation within plant populations. Populations of Amaranthus retroflexus and Abutilon theophrasti were subjected to four generations of selection in the following treatments: soil moisture (low, moderate, or high), diversity of competitor species (low, moderate, or high), and density (low or high). In several cases such selection resulted in significant divergence of the population from its original configuration. The changes occurred in such characters as photosynthetic rate, reproductive allocation, flowering time, root/shoot allocation, and shoot biomass. In all cases where significant changes in overall level of variation occurred, these were in the direction of reduced variation. The rapidity and nature of these changes differed according to the level and type of treatment. Reductions in variation occurred almost uniformly in the density treatments. In contrast, changes in variation were most unpredictable in the diversity treatments, owing to a high level of variability in the competitive background that was apparently associated with generation-to-generation variability in light and temperature. In some generations Abutilon theophrasti was greatly suppressed by heterospecific competitors. This resulted in a significant reduction in variation in spite of high levels of environmental variability. Thus en- vironmental variation in itself may not be sufficient to maintain variation in the population if extreme fluctuation in population size results in genetic bottlenecks. Levels of morphological and physiological variation were maintained in all but a low- and a moderate-moisture line of Amaranthus retroflexus, but the diversity of electrophoretically marked genotypes declined in all moisture treatments. The loss of variation at moderate soil moistures may be due to an overlying gradient of nonspecific competition, a situation which cannot be avoided in even the simplest of gradients. Despite significant changes in several characters and in levels of overall variation, technological and analytical tools were found to be inadequate to determine if these changes led to significant and consistent differences in the response patterns of the species along these gradients.