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Physiological Characteristics of Photosynthesis and Respiration in Stems of Populus tremuloides Michx. ¹

^a Department of Botany and Forest Pathology, State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210

The physiological responses of 6- to 8-year-old aspen (Populus tremuloides Michx.) stems to temperature, light, and CO₂ concentration were investigated in the field throughout the year using infrared CO₂ analysis. Light response studies showed that the rate of gross photosynthesis was linear from 0 to 400 ft-c (0 to 1.6 mw/cm² of 400-700 nm) with light saturation being reached between 800 to 1400 ft-c (3.2 to 5.6 mw/cm² of 400-700 nm). At this light intensity, the respiratory CO₂ loss was reduced to 10 to 15% of dark rates. Net photosynthetic CO₂ uptake was not observed even at intensities as high as 3400 ft-c (13.6 mw/cm² of 400-700 nm). The light response curve was similar for both winter and summer stems.

During summer months, the respiratory and photosynthetic rates of the aspen stem increased with temperature at a near constant rate between 5 and 35 C. For winter stems, the gross photosynthetic rate increased in a pattern similar to the dark respiratory rate as the temperature rose from 3 to 17 C. Below 0 C and above 17 C, however, the gross photosynthetic rate fell off in relation to the respiratory rate so that the per cent of CO₂ reassimilated decreased from 75% to less than 50%. Measurable bark photosynthetic activity was not observed below —3 C.

The gross photosynthetic rate of stems was not affected when the gas passing through the cuvette contained concentrations of CO₂ ranging from 0 to 580 µl CO₂/l air.

³ To whom reprint requests should be addressed.

¹ This research is part of the dissertation submitted by K. C. F. in partial fulfillment of the Ph.D. requirements.