Abstract: Short-day(SD)treatments caused a decrease in the in vitro activities of the nitratereductase in two strains of Lemna minor that differ in photoperiodic behaviours and in L.paucicostata 6746. SD treatment resuIted in a steady increase in peroxidase activity andascorbic acid content in two strains of L. minor Gl and L. paucicostata 6746 with short-dayresponses. The content of ascorbic acid increases in parallel to the rise in the activity level ofperoxidase. Under SD, the photoperiodically insensitive strain L. minor G2 exhibited intensefluctuations and a subsequent decline in peroxidase activity; the content of ascorbic acid inthis strain showed a continuous decrease during the same period. Comparison of peroxidaseisoenzyme between the two L. minor strains indicated that the fast-moving anionic band wasabsent in the insensitive strain. The same enzyme band in the strain with a short-day response displayed an induced decrease in activity