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

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Abstract
The physiological responses of 6 to 8 yr old aspen (P. tremuloides Michx.) stems to temperature, light, and CO2 concentration were investigated in the field throughout the year using IR CO2 analysis. Light response studies showed that the rate of gross photosynthesis was linear from 0-400 ft-c (0-1.6 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 10-15% 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.degree.-35.degree. C. For winter stems, the gross photosynthetic rate increased in a pattern similar to the dark respiratory rate as the temperature rose from 3.degree.-17.degree. C. Below 0.degree. C and above 17.degree. 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.degree. C. The gross photosynthetic rate of stems was not affected when the gas passing through the cuvette contained concentrations of CO2 ranging from 0-580 .mu.l CO2/l air.