Floral Induction in a Photoperiodically Insensitive Duckweed, Lemna paucicostata LP61 ROLE OF GLUTAMATE, ASPARTATE, AND OTHER AMINO ACIDS AND AMIDES

The effects of 20 amino acids and two amides were studied on the flowering of a photoperiodically insensitive duckweed, Lemnapaucicostata LP6. Alanine, asparagine, aspartate, cystine, glutamate, glutamine, glycine, lysine, methionine, proline, serine, and threonine induced flowering under a photoperiodic regime of 16 hours light and 8 hours darkness. Among these, glutamate and aspartate were found to be the most effective for flower induction. These acids could initiate flowering even at 5 x 10-7 molar level, though maximal flowering (about 80%) was obtained at 10-5 molar. Change in the photoperiodic schedule or the pH of the nutrient medium did not influence glutamateor aspartate-induced flowering. The low concentrations at which glutamate and aspartate are effective suggests that they may have a regulatory role rather than simply acting as metabolites. of a more direct effect of amino acids on flowering in L. paucicostata 6746 have come from several different laboratories (79, 13, 19, 20). L. paucicostata, strain LP6, is a photoperiodically insensitive duckweed which does not flower under any of the photoperiods tried when grown in the basal Bonner-Devirian (2) medium supplemented with 10-4 M EDTA (9, 12). Whether this strain lacks the photoperiodic sensitivity due to aberration at the genetic level or if there is some block in its metabolic pathway leading to flowering response remains to be investigated. However, it has been possible to induce flowering in this strain by chemicals such as ethylenediamine-di-o-hydroxyphenylacetic acid, 8-hydroxyquinoline, salicylic acid, and acetylsalicylic acid (10-12). In the present investigation, we demonstrate the effect of certain amino acids and amides on induction of flowering in L. paucicostata LP6. To our knowledge, this is the first report where such a profuse flowering could be obtained with amino acids. It is well established that both qualitative and quantitative changes in amino acid and protein content occur upon photoinduction of the flowering process. There are several studies of protein synthesis, some using inhibitors, where changes have been correlated with flowering (1). Although the role of amino acids in flowering of higher plants has not been investigated very extensively, there is some evidence for a regulatory role in the family Lemnaceae (6). Nakashima (14) reported that high concentrations of several amino acids inhibit flowering in long-day Lemna gibba G3, but the inhibitory effect of arginine could be partially relieved by lysine (which was itself inhibitory). A high endogenous ratio of arginine to lysine was correlated with inhibition of flowering (15). Maeng and Khudairi (13) found an increase in the endogenous level of serine in L. gibba G3 during early phase of the photoinductive cycle, followed by a later decline in the total amino acid pool, occurring before the floral primordia were visible. Amino acids were also shown to influence flowering of the shortday duckweed, Lemna paucicostata 6746. Posner (16, 17) reported that the inhibitory effect of sugars on flowering of strain 6746 in a low-strength medium was reversed by several amino acids including aspartate and glutamate. Subsequently, reports I Supported by the Department of Science and Technology of the Government of India. 2 Present address: MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824. 3 Supported by a fellowship of the University Grants Commission, New Delhi. MATERIALS AND METHODS Aseptic cultures of Lemna paucicostata LP6 were regularly maintained in a medium containing major salts of Bonner-Devirian (2) and minor salts of Heller (5), supplemented with 1% (w/v) sucrose and 10-4 M EDTA. The pH was adjusted to 5.5 before autoclaving the nutrient medium at 1.08 kg/cm2 for 15 min. The stock and experimental cultures were kept under a photoperiodic schedule of 16 h light and 8 h darkness. Light was provided from a mixed bank of fluorescent tubes (Philips TL 6580 W/54 RS, 68000K) and incandescent bulbs (Philips Argenta, 100 W). Fluence rate in the spectral range between 400 to 750 nm, measured with the help of the LiCor model 1800 portable spectroradiometer, varied from 10.0 to 10.5 W m-2. The temperature was maintained at 26 + 20C during the light period and at 22 + 10C in darkness. For experimental cultures, single 3-frond colony was inoculated in each 250 ml Erlenmeyer flask containing 100 ml nutrient medium. All amino acids and amides used for experiments were L-form (obtained from Sigma) and were filter-sterilized and supplied to 2-d-old cultures under aseptic conditions. Our earlier work on the kinetics of flowering in strain LP6, with respect to the effects of chemicals such as ethylenediamine-di-o-hydroxyphenylacetic acid, has shown that maximal flowering is usually obtained between 7 to 9 d after chemical-treatment (9, 12). Therefore, in the present investigation also multiplication rate and flowering were estimated 7 d after addition of the respective adjuvants. MR4 was calculated according to Clark (3) as MR = (log(, Fd log1o Fo) 1000/d, where F0 is the original frond number and Fd is the frond number on day d. Flowering per4Abbreviation: MR, multiplication rate. 904 www.plantphysiol.org on August 11, 2017 Published by Downloaded from Copyright © 1988 American Society of Plant Biologists. All rights reserved. FLORAL INDUCTION BY AMINO ACIDS IN LEMNA centage was calculated by dividing the number of flowering fronds by total number of fronds and multiplying by 100. All flowering stages were taken into consideration. For each replicate culture, percentage flowering was calculated from about 60 to 100 plants. The values so derived from three replicate cultures were averaged. Each experiment was repeated at least once and usually several times, but data of only single representative experiment are presented here.