PHOTOSTIMULATION OF SEED-GERMINATION DURING SOIL TILLAGE

We conducted intensively replicated field experiments in the Willamette Valley, Oregon, USA, in order to study the role of light in triggering seed germination during soil tillage. We found that the normal practice of cultivating agricultural land during daytime can increase germination of buried seed populations between 70 and 400% above the levels recorded following nighttime cultivations. Experimental reduction of the irradiance under the tillage implements during daytime cultivation decreases the number of dicotyledonous seedlings emerged, while strong artificial illumination (> 300 mu mol m(-2) s(-1); 400-800 nm) of the soil surface under the implements during nighttime tillage significantly increases seedling densities. These results suggest that the enhancement of seed germination caused by daytime tillage, compared with nighttime tillage, is due to light that penetrates into the soil during the actual disturbance. The detection by the seeds of this very short exposure to sunlight requires a high photosensitivity, which provides an adaptive 'purpose' for the evolution of the very-low-fluence response mechanism in phytochrome-controlled seed germination. Seedling emergence induced by nighttime control tillage was considerable in some experiments, suggesting that light perceived by seeds after cultivation or other microenvironmental factors affected by tillage may be important in triggering germination in disturbed soil.