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The effects of increasing air temperature and changing daily temperature regime under conditions of elevated CO2 on the physiology, biomass, and reproduction of a C3 plant species were investigated. Phytolacca americana L. (Phytolaccaceae) was grown under either ambient (370 μmol mol-1) or elevated (700 μmol mol-1) CO2 at three air temperature regimes (day/night temperatures of 26°/20°C, T1; 30°/24°C, T2; and 28°/24°C, T3). Length of day/night temperature exposure was adjusted so that average daily temperature was 22°C in T1 and 26°C in T2 and T3. Daily temperature regime was different for T2 and T3: plants in T2 experienced a higher maximum daily temperature but for a shorter daily duration than plants in T3. Elevated CO2 increased photosynthetic rate, total biomass, and root-to-shoot ratio (RSR) and decreased stomatal conductance and transpiration as well as allocation to reproduction. In contrast, elevated temperatures had no effect on photosynthetic rate, stomatal conductance, or total biomass, but they decreased RSR and increased transpiration, reproductive biomass, and allocation. Both elevated CO2 and increased temperatures advanced timing of flowering. The plant-level transpiration rate exhibited a unique response to each of the daily temperature regime treatments. These results indicate that elevated CO2 and increased temperatures elicit different responses at the physiological and whole-plant levels in P. americana, with little interaction between the CO2 and temperature effects. Furthermore, some evidence indicates that a changing daily temperature regime may be an important factor determining plant responses to warming temperatures and should be incorporated into predictions of plant and ecosystem responses to future climate change. [ABSTRACT FROM AUTHOR]

One basic assumption of the various theories proposed to explain differences in resource-allocation patterns among plants is that different structures or activities are alternatives: a gain in one results in a proportional loss in another. We attempted to test this assumption in Agropyron repens by determining whether the resources allocated to reproduction reflect the effect of reproduction on vegetative growth. This was done by comparing the growth of reproductive and vegetative plants grown under conditions differing only in photoperiod. Reproduction in this species was much less costly than the resources allocated to reproduction would indicate. Despite the large proportion of resources allocated to reproduction in most treatments, reproduction had little detrimental effect on total growth; in a few cases, it actually enhanced vegetative growth. There were substantial differences among genotypes and environments in the relationship between the resources allocated to reproduction and its effect on vegetative activity. This suggests that even in comparisons of plants of the same species, resource allocation to reproduction is not necessarily a reliable indicator of the cost of reproduction. Furthermore, since plants can experience different costs per unit of resource allocated to reproduction, it is conceivable that differences among plants in the resources allocated to reproduction may, in part, reflect these differences in cost. A plant with a low cost of reproduction, for example, may be able to allocate a larger proportion of its resources to reproduction. This suggestion represents an alternative to the traditional explanations of life history theory for differences in reproductive effort among plants. In our experiment, the relative differences among different plants in the effect of reproduction on vegetative activity remained the same whether the effect was measured in terms of lost biomass, lost carbon (biomass plus respiration), or lost nitrogen. There were differences, however, between these measures of the cost of reproduction and the cost measured in terms of lost vegetative phosphorus, suggesting that it is important to use the appropriate currency to evaluate the cost of reproduction. [ABSTRACT FROM AUTHOR]

The assessment of reproductive effort (RE) in plants has been hampered by the uncertainty about which resource should be used as the currency to evaluate resource-allocation patterns. In this paper, we argue that carbon, because of its important energetic role in plants, tends to integrate the allocation patterns of other resources and can therefore be used as a common currency to assess allocation patterns. We show that the respiratory cost of plant growth in Agropyron repens increases as the concentration of nitrogen and phosphorus in the tissue increases. Furthermore, the respiratory cost per unit of nutrient generally increases as these nutrients become less available. This means not only that carbon allocation tends to reflect the distribution of other nutrients, but also that the allocation of carbon tends to be biased toward that of the most limiting resources. The second point is particularly important because the concept of resource allocation is relevant in life history studies only if resources are limiting growth. This bias in the allocation of carbon is reflected in the convergence at low levels of applied nitrogen of RE calculated in terms of total carbon (biomass plus respiratory carbon) and that calculated in terms of nitrogen. In our experiments, the overall rank correlation between RE calculated in terms of carbon and nitrogen across various genotypes and resource treatments was 0.804. Convergence was not evident between reproductive efforts calculated in terms of carbon and phosphorus, probably because phosphorus was not a seriously limiting factor in these experiments. However, the overall rank correlation between carbon- and phosphorus-based measures of RE was still high (0.900), suggesting that even when phosphorus is not limiting, carbon is an adequate measure of phosphorus allocation. [ABSTRACT FROM AUTHOR]

Reproductive effort, or the proportion of an organism's resources allocated to reproduction, is a crucial aspect of an organism's life history; the optimal allocation of resources to reproduction in different environments has been the subject of much theorizing. Adequate tests of these theories have been hampered by the difficulties involved in assessing reproductive effort. In this paper, we address the problem of determining which structures and activities should be considered part of reproduction, using Agropyron repens as the experimental material. We approached the problem by first determining the structures and activities necessary for vegetative growth and then determining reproductive structures and activities by subtraction. Using carbon as the currency of allocation, we defined vegetative growth as those structures directly involved in the capture of carbon (i.e., leaves) plus all necessary support structures and activities. The necessary support structures and activities were determined by comparison with vegetative plants grown under similar conditions. Reproduction in A. repens involves not only the production of flowering and fruiting structures, but also the production of a substantial amount of stem material, the loss of carbon through respiration, and carbon gain through photosynthesis. Reproductive photosynthesis includes both direct photosynthesis by the reproductive structures and reproductive enhancement of leaf photosynthesis. Simple measures of reproductive effort based on biomass allocation to flowering and fruiting structures do not adequately reflect carbon allocation to reproduction since there is both genotypic and environmentally induced variation among plants in the amount of reproductive stem material and in the ability to compensate for the cost of reproduction through reproductive photosynthesis. [ABSTRACT FROM AUTHOR]

Accurately quantifying and interpreting the processes and outcomes of competition among plants is essential for evaluating theories of plant community organization and evolution. We argue that many current experimental approaches to quantifying competitive interactions introduce size bias, which may significantly impact the quantitative and qualitative conclusions drawn from studies. Size bias generally arises when estimates of competitive ability are erroneously influenced by the initial size of competing individuals. We employ a series of quantitative thought experiments to demonstrate the potential for size bias in analysis of four traditional experimental designs (pairwise, replacement series, additive series, and response surfaces) either when only final measurements are available or when both initial and final measurements are collected. We distinguish three questions relevant to describing competitive interactions: Which species dominates? Which species gains? and How do species affect each other? The choice of experimental design and measurements greatly influences the scope of inference permitted. Conditions under which the latter two questions can give biased information are tabulated. We outline a new approach to characterizing competition that avoids size bias and that improves the concordance between research question and experimental design. The implications of the choice of size metrics used to quantify both the initial state and the responses of elements in interspecific mixtures are discussed. The relevance of size bias in competition studies with organisms other than plants is also discussed.

In a test of the carbon/nutrient (C/N) balance hypothesis, we grew the perennial herb Plantago lanceolata in different CO2 and nutrient environments and then (1) measured the total allocation to shoots, roots, and reproductive parts and (2) quantified aucubin, catalpol, and verbascoside contents of replicate plants of six genotypes. Plants grown under low-nutrient conditions do have higher concentrations of carbon-based allelochemicals than plants grown under high-nutrient conditions. However, in contrast to the C/N balance hypothesis, plants grown in elevated (700 microL.L(-1)) CO2 conditions had similar, or lower, concentrations of carbon-based allelochemicals than plants grown in ambient (350 microL.L(-1) CO2 conditions. We suggest that augmented substrate concentrations (i.e., excess carbohydrates) are a necessary but insufficient trigger for increased secondary metabolism; instead, hormonal and/or direct physical cues (such as light) may be essential to synthesize or activate the appropriate enzyme systems. Moreover, although plant genotype significantly affected plant growth, reproduction, and chemistry, we never observed significant genotype-by-CO2 interactions for these factors, which suggests that changing CO2 environments may not improve the fitness of certain genotypes over others