Kinetics of leaf oxygen uptake represent in planta activities of respiratory electron transport and terminal oxidases

We present, for the first time, the oxygen response kinetics of mitochondrial respiration measured in intact leaves (sunflower and aspen). Low O-2 concentrations in N-2 (9-1500 ppm) were preset in a flow-through gas exchange measurement system, and the decrease in O-2 concentration and the increase in CO2 concentration as result of leaf respiration were measured by a zirconium cell O-2 analyser and infrared-absorption CO2 analyser, respectively. The low O-2 concentrations little influenced the rate Of CO2 evolution during the 60-s exposure. The initial slope of the O-2 uptake curve on the dissolved 02 concentration basis was relatively constant in leaves of a single species, 1.5 mm s(-1) in sunflower and 1.8 mm s-1 in aspen. The apparent K-0.5(O-2) values ranged from 0.33 to 0.67 mu M in sunflower and from 0.33 to 1.1 mu M in aspen, mainly because of the variation of the maximum rate, V,,x (leaf temperature 22 degrees C. The initial slope of the 02 response of respiration characterizes the catalytic efficiency of terminal oxidases, an important parameter of the respiratory machinery in leaves. The plateau of the response characterizes the activity of the mitochondrial electron transport chain and is subject to regulations in accordance with the necessity for ATP production. The relatively low oxygen conductivity of terminal oxidases means that in leaves, less than 10% of the photosynthetic oxygen can be reassimilated by mitochondria.