Using growth analysis to interpret competition between a C3 and a C4 annual under ambient and elevated CO2

Summary Detailed growth analysis in conjunction with information on leaf display and nitrogen uptake was used to interpret competition between Abutilon theophrasti, a C₃ annual, and Amaranthus retroflexus, a C₄ annual, under ambient (350 l l^-1) and two levels of elevated (500 and 700 l l^-1) CO₂. Plants were grown both individually and in competition with each other. Competition caused a reduction in growth in both species, but for different reasons. In Abutilon, decreases in leaf area ratio (LAR) were responsible, whereas decreased unit leaf rate (ULR) was involved in the case of Amaranthus. Mean canopy height was lower in Amaranthus than Abutilon which may explain the low ULR of Amaranthus in competition. The decrease in LAR of Abutilon was associated with an increase in root/shoot ratio implying that Abutilon was limited by competition for below ground resources. The root/shoot ratio of Amaranthus actually decreased with competition, and Amaranthus had a much higher rate of nitrogen uptake per unit of root than did Abutilon. These latter results suggest that Amaranthus was better able to compete for below ground resources than Abutilon. Although the growth of both species was reduced by competition, generally speaking, the growth of Amaranthus was reduced to a greater extent than that of Abutilon. Regression analysis suggests that the success of Abutilon in competition was due to its larger starting capital (seed size) which gave it an early advantage over Amaranthus. Elevated CO₂ had a positive effect upon biomass in Amaranthus, and to a lesser extent, Abutilon. These effects were limited to the early part of the experiment in the case of the individually grown plants, however. Only Amaranthus exhibited a significant increase in relative growth rate (RGR). In spite of the transitory effect of CO₂ upon size in individually grown plants, level of CO₂ did effect final biomass of competitively grown plants. Abutilon grown in competition with Amaranthus had a greater final biomass than Amaranthus at ambient CO₂ levels, but this difference disappeared to a large extent at elevated CO₂. The high RGR of Amaranthus at elevated CO₂ levels allowed it to overcome the difference in initial size between the two species.This study was supported by a grant from the US Department of Energy     

Clonal integration and effects of simulated herbivory in old-field perennials

Summary We compared the growth, phenology and leaf demography of partly defoliated, connected shoots with that of partly defoliated, severed shoots in four old-field perennials (Solidago canadensis, S. altissima, S. gigantea, Aster lanceolatus) with differing genet architectures (rhizome systems), in a common garden and in the field. Our main hypothesis was that defoliation would have fewer negative effects on shoot performance if shoots were connected than if their rhizomes were severed. Since degree of clonal integration is related to differences in genet architecture, our second hypothesis was that the effects of defoliation would be less pronounced in more integrated than in less integrated clones. Removing about 50% of the total leaf area from shoots had different effects depending on plant species, shoot density, and in particular whether rhizome connections between shoots were left intact or severed. In agreement with our prediction, experimentally isolated shoots in the field or in high density clumps in the garden suffered the most from defoliation, while shoots with intact connections or in low density clumps suffered the least. Our second prediction was neither confirmed nor falsified in the present study. Solidago altissima showed overcompensation in response to simulated herbivory in the common garden, i.e. defoliated shoots grew faster and were larger at harvest than their non-defoliated neighbours.     

Elevated CO2 and drought alter tissue water relations of birch (Betula populifolia Marsh.) seedlings

The effect of increasing atmospheric CO₂ concentrations on tissue water relations was examined in Betula populifolia, a common pioneer tree species of the northeastern U.S. deciduous forests. Components of tissue water relations were estimated from pressure volume curves of tree seedlings grown in either ambient (350 l l^–1) or elevated CO₂ (700 l l^–1), and both mesic and xeric water regimes. Both CO₂ and water treatment had significant effects on osmotic potential at full hydration, apoplasmic fractions, and tissue elastic moduli. Under xeric conditions and ambient CO₂ concentrations, plants showed a decrease in osmotic potentials of 0.15 MPa and an increase in tissue elastic moduli at full hydration of 1.5 MPa. The decrease in elasticity may enable plants to improve the soil-plant water potential gradient given a small change in water content, while lower osmotic potentials shift the zero turgor loss point to lower water potentials. Under elevated CO₂, plants in xeric conditions had osmotic potentials 0.2 MPa lower than mesic plants and decreased elastic moduli at full hydration. The increase in tissue elasticity at elevated CO₂ enabled the xeric plants to maintain positive turgor pressures at lower water potentials and tissue water contents. Surprisingly, the elevated CO₂ plants under mesic conditions had the most inelastic tissues. We propose that this inelasticity may enable plants to generate a favorable water potential gradient from the soil to the plant despite the low stomatal conductances observed under elevated CO₂ conditions.     

CO2-induced growth enhancements of co-occurring tree species decline at different rates

To elucidate how enriched CO₂ atmospheres, soil fertility, and light availability interact to influence the long-term growth of tree seedlings, six co-occurring members of temperate forest communities including ash (Fraxinus americana L.), gray birch (Betula populifolia), red maple (Acer rubrum), yellow birch (Betula alleghaniensis), striped maple (Acer pensylvanicum), and red oak (Quercus rubra L.) were raised in a glasshouse for three years in a complete factorial design. After three years of growth, plants growing in elevated CO₂ atmospheres were generally larger than those in ambient CO₂ atmospheres, however, magnitudes of CO₂-induced growth enhancements were contingent on the availability of nitrogen and light, as well as species identity. For all species, magnitudes of CO₂-induced growth enhancements after one year of growth were greater than after three years of growth, though species' growth enhancements over the three years declined at different rates. These results suggest that CO₂-induced enhancements in forest productivity may not be sustained for long periods of time. Additionally, species' differential growth responses to elevated CO₂ may indirectly influence forest productivity via long-term species compositional changes in forests.     

Elevated CO2 and plant nitrogen-use: is reduced tissue nitrogen concentration size-dependent?

Plants often respond to elevated atmospheric CO₂ levels with reduced tissue nitrogen concentrations relative to ambient CO₂-grown plants when comparisons are made at a common time. Another common response to enriched CO₂ atmospheres is an acceleration in plant growth rates. Because plant nitrogen concentrations are often highest in seedlings and subsequently decrease during growth, comparisons between ambient and elevated CO₂-grown plants made at a common time may not demonstrate CO₂-induced reductions in plant nitrogen concentration per se. Rather, this comparison may be highlighting differences in nitrogen concentration between bigger, more developed plants and smaller, less developed plants. In this study, we directly examined whether elevated CO₂ environments reduce plant nitrogen concentrations independent of changes in plant growth rates. We grew two annual plant species. Abutilon theophrasti (C₃ photosynthetic pathway) and Amaranthus retroflexus (C₄ photosynthetic pathway), from seed in glass-sided growth chambers with atmospheric CO₂ levels of 350 mol·mol^–1 or 700 mol·mol^–1 and with high or low fertilizer applications. Individual plants were harvested every 2 days starting 3 days after germination to determine plant biomass and nitrogen concentration. We found: 1. High CO₂-grown plants had reduced nitrogen concentrations and increased biomass relative to ambient CO₂-grown plants when compared at a common time; 2. Tissue nitrogen concentrations did not vary as a function of CO₂ level when plants were compared at a common size; and 3. The rate of biomass accumulation per rate of increase in plant nitrogen was unaffected by CO₂ availability, but was altered by nutrient availability. These results indicate that a CO₂-induced reduction in plant nitrogen concentration may not be due to physiological changes in plant nitrogen use efficiency, but is probably a size-dependent phenomenon resulting from accelerated plant growth.     

Allocation,within and between organs,and the dynamics of root length changes in two birch species

Spatial and temporal dynamics of biomass allocation within and between organs were investigated in seedlings of two birch species of contrasting successional status. Seedlings of Betula alleghaniensis Britt (yellow birch) and B. populifolia Marsh (gray birch) were grown for 6 weeks at two nutrient levels in rectangular plexiglass containers to allow non-destructive estimates of root growth, production and loss. Leaf area and production were simultaneously monitored. Yellow birch responded more to nutrient level than gray birch in terms of total biomass, shoot biomass, leaf area and root length. Yellow birch also flexibly altered within-organ allocation (specific leaf area, specific root length and specific soil amount). In contrast, gray birch altered between-organ allocation patterns (root length:leaf area and soil amount:leaf area ratios) more than yellow birch in response to nutrient level. Yellow birch showed greater overall root density changes within a very compact root system, while gray birch showed localized root density changes as concentric bands of new root production spread through the soil. Species differ critically in their responses of standing root length and root production and loss rates to nutrient supply. Early successional species such as gray birch are hypothesized to exhibit higher plasticity in varied environments than later successional species such as yellow birch. Our results suggest that different patterns of allocation, within and between plant organs, do not necessarily follow the same trajectories. To characterize thoroughly the nature of functional flexibility through ontogeny, within- and between-organ patterns of allocation must be accounted for.     

Belowground positive and negative feedbacks on CO2 growth enhancement

In this paper we present a conceptual model of integrated plant-soil interactions which illustrates the importance of identifying the primary belowground feedbacks, both positive and negative, which can simultaneously affect plant growth responses to elevated CO₂. The primary negative feedbacks share the common feature of reducing the amount of nutrients available to plants. These negative feedbacks include increased litter C/N ratios, and therefore reduced mineralization rates, increased immobilization of available nutrients by a larger soil microbial pool, and increased storage of nutrients in plant biomass and detritus due to increases in net primary productivity (NPP). Most of the primary positive feedbacks share the common feature of being plant mediated feedbacks, the only exception being Zak et al.'s hypothesis that increased microbial biomass will be accompanied by increased mineralization rates. Plant nutrient uptake may be increased through alterations in root architecture, physiology, or mycorrhizal symbioses. Further, the increased C/N ratios of plant tissue mean that a given level of NPP can be achieved with a smaller supply of nitrogen.Identification of the net plant-soil feedbacks to enhanced productivity with elevated CO₂ are a critical first step for any ecosystem. It is necessary, however, that we first identify how universally applicable the results are from one study of one ecosystem before ecosystem models incorporate this information. The effect of elevated CO₂ on plant growth (including NPP, tissue quality, root architecture, mycorrhizal symbioses) can vary greatly for different species and environmental conditions. Therefore it is reasonable to expect that different ecosystems will show different patterns of interacting positive and negative feedbacks within the plant-soil system. This inter-ecosystem variability in the potential for long-term growth responses to rising CO₂ levels implies that we need to parameterize mechanistic models of the impact of elevated CO₂ on ecosystem productivity using a detailed understanding of each ecosystem of interest.     

Chloroplast culture: The chlorophyll repair potential of mature chloroplasts incubated in a simple medium

The chlorophyll repair potential of mature Cucumis chloroplasts incubated in a simple Tris-HCI/sucrose medium is described. The chloroplasts were isolated from green, fully expanded Cucumis cotyledons which were capable of chlorophyll repair. This was evidenced by a functional chlorophyll biosynthetic pathway in the mature tissue. The biosynthesis of protochlorophyllide from exogenous δ-aminolevulinic acid was used as a marker for the operation of the chlorophyll biosynthetic chain between δ-aminolevulinic acid and protochlorophyllide. The conversion of exogenous protochlorophyllide into chlorophyll a was used as a marker for the operation of the chlorophyll pathway beyond protochlorophyllide. It appeared from these studies that contrary to published reports, unfortified fully developed Cucumis chloroplasts incubated in Tris-HCl/sucrose without the addition of cofactors exhibited a partial and limited chlorophyll repair capability. Their net tetrapyrrole biosynthetic competence from δ-aminolevulinic acid was confined to the accumulation of coproporphyrin. No net tetrapyrrole biosynthesis beyond coproporphyrin was observed. However, the plastids were capable of incorporating small amounts of δ-amino-[4-¹⁴C]levulinic acid into [¹⁴C] protochlorophyllide but were incapable of converting exogenous protochlorophyllide into chlorophyll. After prolonged incubation of the unfortified chloroplasts in the dark, a fluorescent protochlorophyllide-like compound accumulated. This compound [Cp (E₄₃₀-F₆₃₁)] exhibited a soret excitation maximum at 430 nm (E₄₃₀) and a fluorescence emission maximum at 631 nm (F₆₃₁) in methanol/acetone (4 : 1, v/v). Cp (E₄₃₀-F₆₃₁) was shown to be neither protochlorophyllide nor zinc-protochlorophyllide but an enzymatic degradation product of chlorophyll. The exact chemical identity of this compound has not yet been determined.     

Remote Sensing of Solar-excited Plant Fluorescence as a Measure of Photosynthetic Rate

Leaf level net photosynthetic rates (P _N) of laurel oak (Quercus hemispherica) juveniles grown under contrasting nutrient and CO₂ regimes were negatively correlated with red to far-red ratios, R/FR (690/760 nm), steady-state, solar-excited fluorescence ratios (r ² = 0.66, n = 12) measured across 12 plant canopies. Laurel oak juveniles that had been subjected to nitrogen stress over a period of a year demonstrated higher R/FR than their counterparts that had been provided with sufficient nitrogen. Plants that had been grown at elevated CO₂ concentrations, EC [700 mol (CO₂) mol^-1] also exhibited significantly higher R/FR when subjected to normal ambient carbon dioxide concentrations than their counterparts grown under ambient concentrations, AC [380 mol (CO₂) mol^-1]. All fluorescence measurements were obtained by observing a multi-plant canopy using a unique solar-blind passive sensor. This sensor, which utilizes Fraunhofer-line discrimination techniques, detects radiation at the cores of the lines comprising the atmospheric oxygen A- and B-bands, centered at 762 and 688 nm, respectively. These results support the use of solar-excited steady-state plant fluorescence as a potential tool for remote measurement of canopy radiation use efficiency.