Abstract
Rates of photosynthesis and respiration in developing cottonwood leaves in the seventh, tenth, and thirteenth positions from the stem base were followed over their 17-day maturation period (plastochron ages 0-7) by monitoring CO2 concentrations with an infrared gas analyzer Newly formed leaves of plastochron age 0 evolved CO2 at a high rate in the light but showed net uptake of CO2 by plastochron age 1 Maximum photosynthetic rates were attained just before leaf extension growth ceased and were maintained until the experiment was terminated. In another experiment, successive leaves from 7-, 10-, and 13-leaf plants were monitored for CO2 exchange Leaves between plastochron ages 1 and 2 from 7-leaf plants showed net uptake of CO2, but maximum photosynthetic rates were not maintained by leaves older than plastochron age 4 In contrast, only leaves of plastochron age 2 or older from 13-leaf plants showed net photosynthesis; but maximum rates were maintained by leaves until at least plastochron age 8 Dark respiration rates were high for newly formed leaves in both experiments but declined rapidly as the leaves matured.
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