Plant responses to elevated CO ₂ concentration at different scales: leaf, whole plant, canopy, and population

Abstract

Abstract Elevated CO 2 enhances photosynthesis and growth of plants, but the enhancement is strongly influenced by the availability of nitrogen. In this article, we summarise our studies on plant responses to elevated CO 2 . The photosynthetic capacity of leaves depends not only on leaf nitrogen content but also on nitrogen partitioning within a leaf. In Polygonum cuspidatum , nitrogen partitioning among the photosynthetic components was not influenced by elevated CO 2 but changed between seasons. Since the alteration in nitrogen partitioning resulted in different CO 2 ‐dependence of photosynthetic rates, enhancement of photosynthesis by elevated CO 2 was greater in autumn than in summer. Leaf mass per unit area (LMA) increases in plants grown at elevated CO 2 . This increase was considered to have resulted from the accumulation of carbohydrates not used for plant growth. With a sensitive analysis of a growth model, however, we suggested that the increase in LMA is advantageous for growth at elevated CO 2 by compensating for the reduction in leaf nitrogen concentration per unit mass. Enhancement of reproductive yield by elevated CO 2 is often smaller than that expected from vegetative growth. In Xanthium canadense , elevated CO 2 did not increase seed production, though the vegetative growth increased by 53%. As nitrogen concentration of seeds remained constant at different CO 2 levels, we suggest that the availability of nitrogen limited seed production at elevated CO 2 levels. We found that leaf area development of plant canopy was strongly constrained by the availability of nitrogen rather than by CO 2 . In a rice field cultivated at free‐air CO 2 enrichment, the leaf area index (LAI) increased with an increase in nitrogen availability but did not change with CO 2 elevation. We determined optimal LAI to maximise canopy photosynthesis and demonstrated that enhancement of canopy photosynthesis by elevated CO 2 was larger at high than at low nitrogen availability. We also studied competitive asymmetry among individuals in an even‐aged, monospecific stand at elevated CO 2 . Light acquisition (acquired light per unit aboveground mass) and utilisation (photosynthesis per unit acquired light) were calculated for each individual in the stand. Elevated CO 2 enhanced photosynthesis and growth of tall dominants, which reduced the light availability for shorter subordinates and consequently increased size inequality in the stand.