Seed germination after short-duration light exposure: implications for the photo-control of weeds.
A soil disturbance treatment often results in more annual weed seedlings emerging when conducted in light than when conducted in darkness, suggesting that seed germination in some species can be induced by short-duration light exposure (SDLE). Seventy species, most of them agricultural weeds, were tested for their response to SDLE. Seeds were wet and cold stratified in darkness for 18 weeks before being subjected to light (12 h/day), darkness or 5 sec light exposure (PAR: 210 mol m-2 s-1) followed by darkness. Of the 44 species for which meaningful comparison was possible, germination was stimulated by SDLE in 24, while the other 20 showed no or inconsistent responses. Eight of 11 plant families contained species that responded to SDLE. Extreme light sensitivity was shown equally by summer annual, winter annual and perennial species. There was no tendency for small-seeded species to be more light sensitive than large-seeded ones. Most species were represented by seeds from three populations and, in many cases, large differences in percentage germination were found. Statements about a species' germination biology based on studies of a single population must therefore be interpreted with caution. The results suggest that a large number of weed species could potentially be 'photo-controlled'. However, the large variations in dormancy level between populations and the relatively small reduction in germination percentage in many cases imply that the effects of a strategy to photo-control, e.g. harrowing in darkness, can be unpredictable and sometimes small.