Effects of CO_2 and Temperature on Growth and Resource Use of Co‐Occurring C_3 and C_4 Annuals

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 environments with atmospheric CO₂ levels of 400 or 700 @mL/L and with light/dark temperatures of 28°/22° or 38°/31°C. Elevated CO₂ and temperature treatments had significant independent and interactive effects on plant growth, resource allocation, and resource acquisition (i.e., photosynthesis and nitrogen uptake), and the strength and direction of these affects were often dependent on plant species. For example, final biomass of Amaranthus was enhanced by elevated CO₂ at 28° but was depressed at 38°. For Abutilon, elevated CO₂ increased initial plant relative growth rates at 28° but not at 38°, 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 28°, 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 28°, the rate of net leaf area loss for Abutilon increased while that of Amaranthus decreased. Furthermore, high CO₂ apparently enhanced the ability of Amaranthus to retain nitrogen at this temperature, which may have helped to enhance photosynthesis, whereas nitrogen retention was unaffected in Abutilon. Thus, at 28°, final biomass of Abutilon was not simulated in a high CO₂ environment whereas the final biomass of Amaranthus was. At 38°, 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 Amaranthus at 38°, 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 substantial 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 reproductive 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. Nevertheless, 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.