MOVEMENT OF RESPIRATORY CO2 IN STEMS OF LOBLOLLY-PINE (PINUS-TAEDA L)SEEDLINGS

Temperature-independent fluctuations in stem CO2 efflux were measured in Pinus taeda L. seedlings. Stem CO2 efflux was measured during high and low transpiration rates, high and low net photosynthesis rates, an d normal and interrupted substrate supply conditions. Stem CO2 efflux rates were an average of 6.7% lower during periods of high transpirati on compared to periods of low transpiration. This difference in stem C O2 efflux rates was not due to water stress. The most likely cause was movement of respiratory CO2 in the transpiration stream. Interruption of substrate supply to the stem by phloem girdling reduced stem CO2 e fflux rates. Increasing net photosynthesis rates from low to high had no effect on stem CO2 efflux, but decreasing net photosynthesis from h igh to low caused relatively small reductions in Stem CO2 efflux. Thes e results indicate that diurnal changes in net photosynthesis rate may play a small role in temperature-independent afternoon depressions of stem CO2 efflux. The transport of respiratory CO2 by the transpiratio n stream compromises measurements of woody tissue respiration obtained by commonly accepted gas exchange techniques. This phenomenon could a lso affect measurement of leaf net photosynthesis and branch woody tis sue respiration.