Lemna gibba G3, a long-day duckweed, has been extensively used in various fundamental and applied researches, including those aimed towards elucidation of the control of flowering. Under in vitro conditions, it flowers when cultured on E medium (a Hoagland-type medium) under inductive photoperiods. However, it fails to do so on Bonner-Devirian medium, which in composition is qualitatively similar to E medium, but contains all the constituents at much lower concentrations than those in E medium. The flowering on E medium could not have been because of the osmotic stress, as supplementation of BD medium with sucrose higher than normally used 30 mM or with sorbitol or mannitol, at equimolar concentrations, alone or along with 30 mM sucrose, failed to elicit flowering. However, the reciprocal replacement of macronutrients, micronutrients or iron-chelate components of one medium with the other revealed that BD medium, enriched with macronutrients of E medium, supports flowering. In a further experiment, involving replacement of constituents of macronutrients, individually or in combinations, modified BD medium that contained potassium nitrate and potassium dihydrogen phosphate at the concentrations used in E medium elicited flowering. However, BD medium in which concentration of only one of these two potassium salts was raised to provide potassium equal to that made available by both salts provided together did not induce flowering. Thus, it seems that flowering in L. gibba, along with other physical and chemical factors, is critically dependent on the threshold concentrations of potassium, nitrate and phosphate, provided together.
The research work presented in the paper was supported by Research and Development and DST-Purse grants provided to SBB by the University of Delhi. SC gratefully acknowledges the award of Junior and Senior Research Fellowships from the University Grants Commission, New Delhi.