Photocontrol of seed germination in arable land.

摘要

The overall objective of this thesis was to determine the role of light in triggering germination during soil disturbance, and to identify the mechanisms whereby buried seeds in arable soils detect the light signals.; In the first section it is shown that a short period of burial induces a dramatic, {dollar}approx{dollar}10.000-fold increase in light sensitivity in the seeds of the arable weed Datura ferox. This increase in sensitivity has been interpreted as a natural transition from the "low-fluence" (LF) to the "very-low-fluence" (VLF) mode of phytochrome action. Field experiments indicated that germination of buried seeds may be triggered by millisecond-exposures to sunlight. This observation suggests a key role for the process of sensitization in the mechanisms that allows seeds to detect soil cultivation events in arable lands.; Large scale field experiments using standard farm equipment further tested the above hypothesis. Results demonstrated that cultivating agricultural land during daytime can increase germination of buried seed populations between 70 and 400% above levels recorded following nighttime cultivations. Covering the tillage implements during daytime cultivation decreased the number of dicotyledonous seedlings emerged, while strong artificial illumination ({dollar}{lcub}>{rcub}300 murm mol msp{lcub}-2{rcub} ssp{lcub}-1{rcub}{dollar}; 400-800 nm) of the soil surface during nighttime tillage significantly increased 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 event. The detection by the seeds of the extremely short exposure to sunlight requires high photosensitivity, and provides an "adaptive purpose" for the evolution of the VLF response mechanism in phytochrome-controlled seed germination.; Seeds of Datura ferox were used to investigate the connection between the induction of sensitivity to VLFs and endogenous abscisic acid (ABA) levels. Results indicated that, in buried seeds, the decline in endogenous ABA levels coincided with or preceded the induction of the VLF germination response. Exogenous ABA applied to the incubation medium preferentially reduced the VLF response compared with its effect on the LF response. These results support the hypothesis that the reduction in endogenous ABA levels during seed burial plays a role in switching the seeds from the LF to the VLF mode of phytochrome action.