We studied the effect of changes in net photosynthesis (A(net)) on respiration, soluble sugars and carbohydrates in Populus deltoides Bartr. ex Marsh. saplings under controlled environmental conditions by making daily measurements of leaf respiration (R(d)), stem CO(2) efflux and root CO(2) efflux at a constant temperature in growth chambers. After a pretreatment period, one of three treatments was applied for 5 to 7 days: (1) increased atmospheric CO(2) concentration; (2) decreased photoperiod and photosynthetic photon flux (PPF); or (3) continuous darkness. Increased A(net) in response to elevated CO(2) concentration resulted in a sustained increase in whole-plant respiration, with R(d) increasing 46%, stem CO(2) efflux increasing 130% and root CO(2) efflux increasing 16%. Elevated CO(2) concentration also caused a significant increase in leaf soluble sugars. Decreasing photoperiod and PPF or complete darkness caused a rapid decrease in respiration throughout the saplings. In the low light treatment, R(d) decreased 40%, stem CO(2) efflux decreased 78%, root CO(2) efflux decreased 74% and significant decreases in leaf and root soluble sugar and leaf nonstructural carbohydrate concentrations were observed. Continuous darkness resulted in a 70% decrease in R(d), a 65% decrease in stem CO(2) efflux, a 73% decrease in root CO(2) efflux and significant decreases in leaf and root soluble sugar and root carbohydrate concentrations. In all treatments, changes in respiration rates in all tissues occurred within hours of treatment application. In addition, a diurnal pattern in root CO(2) efflux was observed throughout the experiment under constant environmental conditions. The observed rapid changes in whole-plant respiration following treatment application and the diurnal patterns in root CO(2) efflux suggest that growth and maintenance respiration in the saplings was strongly dependent on newly acquired carbohydrates.