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

Abstract

Detailed growth analysis in conjunction with information on leaf display and nitrogen uptake was used to interpret competition between Abutilon theophrasti, a C annual, and Amaranthus retroflexus, a C annual, under ambient (350 μl l) and two levels of elevated (500 and 700 μl l) CO. 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 CO 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 CO upon size in individually grown plants, level of CO did effect final biomass of competitively grown plants. Abutilon grown in competition with Amaranthus had a greater final biomass than Amaranthus at ambient CO levels, but this difference disappeared to a large extent at elevated CO. The high RGR of Amaranthus at elevated CO levels allowed it to overcome the difference in initial size between the two species.

Effects of Elevated Co2 on Aboveground Growth in Seedlings of Four Dominant Quercus Species

“…Generally, plants, including trees, respond to elevated atmospheric CO 2 by enhanced net photosynthesis, growth, and dry mass production (Lemon, 1983;Strain and Cure, 1985;Jarvis et al, 1989), because CO 2 is used as a source for photosynthesis. Moreover, elevated CO 2 enhances growth rates to a greater extent in young seedlings than in mature trees (Tolley and Strain, 1985;Bazzaz et al, 1989). However, in our study, plant responses to elevated CO 2 were species-specific, and in some cases, there was no impact, or even a negative impact on growth.…”

The nutritional control of root development

“…In such cases it may be necessary to use plant size or developmental stage as the basis for comparisons (Coleman et al, 1994;Evans, 1972). This can be particularly relevant in the case of the shoot:root ratio, which in some herbaceous plants increases markedly over the first few weeks of growth (Bazzaz et al, 1989). While ontogenetic drift is unlikely to be a major factor in any of the other morphogenetic effects discussed below, it will always be important to keep the phenomenon in mind when assessing any apparent examples of phenotypic plasticity in plants.…”

Nitrogen supply affects root:shoot ratio in corn and velvetleaf (Abutilon theophrasti)

“…If optimal partitioning theory holds true, then plants grown in limiting soil nutrient environments will partition a larger percentage of new biomass to roots as opposed to shoots, and they will have a higher root:shoot ratio. However, early in ontogeny plants will have a higher root:shoot ratio because of normal processes of early root growth and establishment (Bazzaz et al 1989). A key issue, therefore, is whether a higher root:shoot ratio observed in plants grown in low nutrient environments results from plasticity predicted by optimal partitioning theory or by slowed growth rates and stress-induced differences in stage of development.…”