674 LIGHT AND LIFE 



Review of Flowering in the Lemnaceae 



The flowering of most Lemnaceae species in nature appears to be 

 infrequent, although the fact that the flowers are unHkely to be 

 noticed unless deliberately looked for makes the true frequency diffi- 

 cult to assess. Saeger (14) collected field evidence that flowering may 

 be at least partly controlled by the composition of the pond water. 

 Reports of flowering induced by ultraviolet radiation and high pH 

 values under poorly controlled conditions (4) have not been con- 

 firmed. In a survey of the growth of a large number of strains 

 (clones) of various species under aseptic, controlled conditions, Lan- 

 dolt (11) reported flowering in several strains, usually in old, 

 crowded cultures, but did not investigate further. 



The first demonstration of photoperiodism in the Lemnaceae was 

 that of Kandeler (10, 11), who found that certain strains of Lermm 

 gibba, grown under aseptic conditions at about 30°C, flowered as 

 long-day plants with a critical daylength of 12-14 hours. Two points 

 of particular interest emerged from this work. If fluorescent light 

 alone was used as a light-source, flowering took place rapidly only in 

 "old" medium — medium in which the plants had grown for some 

 time. In light containing a higher proportion of far-red radiation 

 (e.g., supplemented with incandescent light) flowering occurred 

 rapidly even with frequent transfers to fresh medium. A crude ac- 

 tion spectrum suggested that the far-red also promoted flowering 

 when given as an interruption of the dark period. 



A conditional short-day response in a strain of Lemno perpusilla 

 grown under aseptic conditions has been reported by the writer (6, 

 7, 8) . The term "conditional" is used here to indicate that flowering 

 in that plant is daylength-indifferent under certain conditions but 

 typically short-day in others. The major points can be briefly pre- 

 sented. 



When grown in a modified Hoagland's medium, with or without 

 sucrose and supplemented with only low (about 10-^ M) levels of 

 tartrate to maintain iron availability, L. perpusilla strain 6746 flowers 

 rapidly irrespective of daylength at temperatures of 25°-27°C. In 

 media su])ijlemcnted with sufficient levels of certain chelating agents 

 (e.g., ethylenediaminetetraacetic acid (EDTA) , at least K)-" M; 

 citrate or tartrate, at least 1 ()-•'' M) it responds as a typical short- 

 day i)laiu. Maxiimmi flowering then takes place only under photo- 

 periods of 10 hours or less, and none with photoperiods of 1(5 hours 

 or more. Bi icf iiucrruption of a long, flowcr-jnomoting dark period 



