Plant responses to elevated CO_2 concentration at different scales: leaf, whole plant, canopy, and population

摘要

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 photo-synthetic 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.