Rising atmospheric CO2 concentration has been shown to affect photosynthesis and dark respiration of plants. However, it is unclear how CO2 enrichment will simultaneously affect rates of photosynthesis and respiration and the abundance of organelles in which they take place. In a study with Nicotiana sylvestris, we examined the relationships between chloroplast number and photosynthesis and between mitochondrion number and dark respiration of plants grown at 365 or 730 mumol mol(-1) CO2 in environmentally controlled chambers. Elevated CO2 significantly increased the width of chloroplasts due to the large size of starch grains present in CO2-enriched leaves. Higher CO2 concentration, however, had no effect on the length of chloroplasts. The dimension of mitochondria and cross-section area of leaf cells were also unaffected by CO2 concentration. Elevated CO2 increased chloroplast number per unit cell area by 71% and net photosynthesis (A) per unit leaf area by a similar magnitude (67%). Mitochondrion number per unit cell area increased by 130%, whereas dark respiration (Rd) per unit leaf area was only 36% higher during daytime and 48% higher during nighttime periods at elevated CO2. Daytime and nighttime Rd decreased over time in both CO2 treatments, but A remained relatively unchanged throughout the growth period. As a result, photosynthetic efficiency increased over time. Our results demonstrate the agreement between chloroplast number and photosynthetic responses to CO2 enrichment and highlight the contrast between the large increase in mitochondrion numbers and the moderate increase in respiration at elevated CO2 in N. sylvestris. (C) 2003 Elsevier B.V. All rights reserved.
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