Induction of Photosynthetic Oxygen Evolution in Spinach Leaves

When leaves are illuminated, the rate of oxygen evolution does not immediately reach its maximum steady state value. The time between the onset of illumination and the attainment of this maximum steady state rate is called the induction period. Induction in chloroplasts has been extensively studied (for review see Walker,1981) but this knowledge has not been applied systematically to define induction in leaves. A (Hansatech Ltd. Paxman Rd, Hardwick Industrial Estate, King’s Lynn, Norfolk; Delieu, Walker 1983) leaf disc oxygen electrode chamber, which measures O2 evolution in the gas phase and chlorophyll fluorescence simultaneously, was used to characterise the induction period in spinach leaf discs. High CO2 (5%) ensured that the stomatawere closed, and allowed detection of induction without the complication of a changing gas exchange barrier. A low light intensity of 5Wm−2 (unless otherwise indicated) was used in most of the experiments. The rate of oxygen evolution during the induction phase was measured by differentiating the voltage signal from the O2 electrode indicating the amount of oxygen in the chamber. This provided a more precise point of when the period of increasing rate of O2 evolution ended and the steady-state rate was achieved than did simple examination of the concentration of oxygen present in the chamber.