Effects of carbon dioxide enrichment on growth, photosynthesis, and leaf senescence in bean (Phaseolus vulgaris L.) plants

Incremental increases in absolute growth rate with increasing carbon dioxide concentrations ranging from 340 to 3000- ul 1⁻¹ were observed in continuously carbon dioxide enriched bean plants over the course of development. However, relative growth rate of enriched plants was enhanced only early in development. Unit leaf rate was also increased by enrichment primarily in early development, explaining, in part, the trends in relative growth rate. Carbon dioxide enrichment also caused large increases in leaf dry weight, which constituted the major change in carbon partitioning among plant parts. Leaves of enriched plants showed a decrease in photosynthetic capacity which was not associated with changes in chlorophyll concentration, or photorespiration rate. The decrease .in capacity was less evident in older leaves, or in those maintained at low light intensity or with reduced chlorophyll levels, suggesting that the reduced photosynthetic capacity was due to the higher photosynthetic rate of those leaves during growth. Respiration rate in leaves of enriched plants was also increased, but only under conditions which caused a concurrent decrease in photosynthetic capacity. Enriched leaves consistently showed higher starch content with generally a lower photosynthetic capacity that control leaves. Furthermore, an increase in sink demand did not influence the photosynthetic capacity and starch content of enriched leaves to the extent of control leaves. A high leaf starch content and leaf dry weight correlated with accelerated senescence of plants. Conditions such as temperature, which facilitated resulted in the most rapid and the primary leaves of enriched high light intensity or reduced greater starch accumulation, also extensive senescence.