Aust. J. Plant Physiol., 1980, 7, 473-85 Carbon Dioxide Exchange in response to Change of Environment and to Defoliation in a Tobacco Crop* D. M. WhitjieldAB, D. J. ConnorA and P. J. M. SaleC A School of Agriculture, La Trobe University, Bundoora, Vic. 3083. " Present address: Tobacco Research Station, Department of Agriculture, Victoria, Myrtleford, Vic. 3737. Division of Irrigation Research, CSIRO, Griffith, N.S.W. 2680. Abstvact Rates of carbon dioxide exchange of field-grown tobacco crops at early flowering and maturity were measured using a pair of large closed-system field chambers. Photosynthetic responses to irradiance and temperature were investigated on both occasions. Rate of dark respiration and its response to temperature were measured during the night. Defoliation treatments were employed to disrupt the correlation between leaf age and light environment in the canopy. In these experiments, the short-term photosynthetic response to irradi- ance was determined for crops that were progressively defoliated upwards or downwards. Long- term effects of varying intensities of downward defoliation were also investigated. Maximum photosynthetic rates of 3.7 g CO, rn-, h-l were achieved at early flowering. These had fallen to 1.9 g CO, rn-, h-l at maturity. Maximum rates occurred at an irradiance of approxi- mately 700 W m-,. Short-term shifts in temperature in the range 10-32°C had little effect during the day, but dark respiration was strongly dependent on temperature. Defoliation experiments demonstrated that lower leaves retained a significant potential for photosynthesis but their contri- bution to the total exchange of CO, of mature crops was only small. This was attributed in part to the poorer light regime in the lower canopy. Results are discussed in the context of the development of yield and quality in flue-cured tobacco. Introduction Assimilated CO, accounts for approximately 90% of the dry weight of plants, and measurements of short-term fluctuations in the net rate of CO, exchange ( P ) of single leaves, whole plants, or communities of plants (PC)have contributed sub- stantially to our understanding of factors influencing the rate of dry-matter accumu- lation by crops. Whilst yield is directly related to rates of dry matter accumulation, PC is also inferred to be an important determinant of quality of flue-cured tobacco. The labile carbon reserves of leaves are dependent on photosynthetic activity (Chan and Bird 1960; Austin 1972; Ho 1976), and must be adequate to satisfactorily cure tobacco (Johanson 1951). For example, 'trashy leaf', of no commercial value, has long been associated with 'overripe' lower leaves which have been permitted to remain on the plant for too long. However, Johanson also found that apparently normal leaves of tobacco from the lower half of the plant produced 'trashy' leaf when reserves were insufficient to meet the respiratory requirements of the leaf during curing. In this instance, he attributed low reserves to low PC during prolonged periods of overcast weather * Part V in the series 'Structure, light relations and photosynthesis of tobacco canopies'. Part IV, Aust. J. Plant Physiol., 1980, 7, 463-72.