Photosynthesis and Respiration by Developing Leaves of Cottonwood (Populus deltoides Bartr.)

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.