SEED-GERMINATION AFTER SHORT-DURATION LIGHT EXPOSURE - IMPLICATIONS FOR THE PHOTO-CONTROL OF WEEDS

1. A soil disturbance treatment often results in more annual weed seed lings emerging when conducted in light than when conducted in darkness , suggesting that seed germination in some species can be induced by s hort-duration light exposure (SDLE). We tested 70 species, most of the m agricultural weeds, for their response to SDLE. Seeds were wet and c old stratified in darkness for 18 weeks before being subjected to ligh t (12 hours per day), darkness, or 5 s light exposure (PAR: 210 mol m( -2) s(-1)) followed by darkness. 2. Of the 44 species for which meanin gful 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 per ennial species. There was no tendency for small-seeded species to be m ore light sensitive than large-seeded ones. 3. Most species were repre sented by seeds from three populations and in many cases, large differ ences in percentage germination were found. Statements about a species ' germination biology based on studies of a single population must the refore be interpreted with caution. 4. The results suggest that a larg e number of weed species could potentially be 'photo-controlled'. Howe ver, the large variations in dormancy level between populations and th e relatively small reduction in germination percentage in many cases i mply that the effects of a strategy to photo-control, e.g. harrowing i n darkness, can be unpredictable and sometimes small.