Disappearance of rhythmicity in growth response to dark- and light-breaks in Lemna gibba G3 due to iron deficiency

Inhibition of flower initiation in light culture of a long-dayduckweed, Lemna gibba G3 by an inserted period of brief darknesschanges its magnitude with the application time of the dark-break.‘Response vs. time of the dark-break’ curve consistsof superimposed thermo-insensitive oscillatory and thermo-sensitivehourglass components. Oscillation is diurnal and damps in afew days. The situation is very similar to what has previouslybeen revealed concerning the non-photosynthetic light-requirementof the same strain of duckweed for frond multiplication. Increasingphotophily for flower production is likely accompanied by decreasingphotophily for frond production, and vice versa. The presenceof iron chelators such as o-phenanthroline and ,'-dipyridylor reduced iron concentration in culture medium abolishes theoscillatory change, without modifying the hourglass change,in either vegetative or reproductive photophily. Ferrous, butnot ferric, ions likely mediate between the basic clock andthe photophily. Critical daylength for flower induction is notaffected by exogenous chelators. (Received November 17, 1970; )