Dusk and Dawn Effect on Spruce Photosynthesis. Comparative Studies of Long Term IR-CO2-Analyses of Whole Plants and Short Term 14CO2-Incorporation in Detached Leaves

Abstract

Summary Two methods were used for measuring photosynthesis in spruce plants exposed to different light and twilight periods: (1) Long-term experiments were performed on whole plants in an open system, using an IR-CO 2 -gas analyser over a period of ten days. (2) Short-term experiments with 14 CO 2 -incorporation during 23 s. were carried out with detached pieces of leaves. With the long-term method, a plateau in photosynthetic rate was reached one hour after the onset of maximum light. With the short-term method, a decline in photosynthetic rate was attained shortly after exposure to maximum light level. Possible explanations for these differences are discussed. With both methods, a close relationship was observed between light levels and photosynthetic rates, independent of the duration of twilight.