My favorite organism for plant physiology experiments is the common duckweed, Lemna minor L. Lemna minor is an aquatic plant that grows on the surface of fresh water lakes and ponds. It can often produce large colonies in natural habitats (see image below of a duckweed colony on the surface of the Celery Bog, West Lafayette, IN, U.S.A.):
Lemna minor belongs to the angiosperm (flowering plants) family Lemnaceae, although it only rarely produces flowers (Lemna - Discover Life: http://www.discoverlife.org/mp/20q?search=Lemna).
The plant grows primarily vegetatively and, as described in more detail below, has a rapid growth rate and is highly nutritious. It is recognized as one of the fastest growing angiosperms and is rich in omega-3 fatty acids, minerals, vitamins, and protein with an excellent amino acid profile. Shown below is a close-up image of Lemna minor placed on a flat bed scanner (roots down) and then scanned:
Each parent frond reaches a maximum size of about 3 mm in length and develops a single root that can reach several cm in length. Each parent frond produces daughter fronds alternately from 2 apical pockets. As the daughter fronds develop they each produce a single root, break off from the parent, and then themselves begin producing daughters (Cross, J.W. The Charms of Duckweed (2002): http://www.mobot.org/jwcross/duckweed/duckweed.htm).
Plants collected from the wild can be surface sterilized by dipping in bleach for a few seconds, rinsing with sterilized distilled water (i.e. water that has been boiled or autoclaved and then allowed to cool to room temperature), and transferred to sterile growth medium. I have had good success maintaining a sterile culture of Lemna minor with the following inorganic growth medium:
The bleach treatment tends to kill off the parent fronds and roots, leaving the daughters located in the sterile parent frond apical pockets. These eventually develop to maturity and produce a colony (vegetative clone) that is genetically uniform:
The fronds have air pockets that give the plants bouyancy. The roots help anchor the fronds (Cross, J.W. The Charms of Duckweed (2002): http://www.mobot.org/jwcross/duckweed/duckweed.htm).
One approach to measure growth is to subculture fronds to a pond (e.g. a white greenhouse seedling tray) and photograph the pond daily, and use photographs to estimate % pond coverage (to the nearest 5%) by the colony. Shown below are images used to estimate % pond coverage for fronds grown in full sunlight (12 h dark, 12 h light) in a greenhouse. In this case plants were exposed to the atmosphere under non-sterile conditions with non-sterilized growth medium and this resulted in some algal growth:
When the natural logarithm (Ln) of % pond coverage is plotted against time (days) this yields a straight line, the slope of which can be used to calculate doubling time (i.e. the time it takes for the fronds to double in number and mass). Doubling time = Ln(2) divided by the slope of the regression line:
When a portion of this culture was sub-cultured into fresh medium shaded with shade cloth to reduce daytime sunlight exposure by 50%, this resulted in decreased growth (i.e. increased doubling time):
Alternatively, on a smaller scale, a limited number of fronds can be inoculated into beakers containing nutrient medium and fronds counted daily. Time-lapse photography is also a convenient way of monitoring growth of Lemna (not shown).
Lemna and its relatives have some of the highest growth rates (i.e. smallest doubling times) of all angiosperms (Ziegler, P., Adelmann, K., Zimmer, S., Schmidt, C., Appenroth, K.-J. Relative in vitro growth rates of duckweeds (Lemnaceae) – the most rapidly growing higher plants. Plant Biology (Special Issue: Duckweed - Research and Application). Volume 17, Issue Supplement s1, p. 33-41 (2015): http://onlinelibrary.wiley.com/doi/10.1111/plb.12184/full).
Using the growth medium described above individual nutrient levels can be systematically altered (e.g. nitrogen source and concentration can be adjusted, or individual nutrients can be omitted) to investigate plant growth response to nutrient supply. I have used Lemna minor to study nitrogen assimilation and amino acid biosynthesis in plants, employing the stable isotope nitrogen-15 (15N) and mass spectrometry. Plants can be easily transferred to a nutrient medium in which the natural nitrogen-14 salts are replaced with nitrogen-15 salts and plants sampled at intervals to determine the time-course of labeling of each amino acid (see for example: Rhodes, D., Rich, P.J, Brunk, D.G. Amino acid metabolism of Lemna minor L. IV. 15N-Labeling kinetics of the amide and amino groups of glutamine and asparagine. Plant Physiol. 89: 1161-1171 (1989): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1055991/).
Because Lemna is edible, contains no known toxic molecules, has a high protein content composed of a well-balanced mixture of essential amino acids (especially lysine, tryptophan, and methionine), is rich in vitamins, minerals and omega-3 fatty acids, and compares favorably with spinach and kale as nutrient sources (Edelman, M., Colt, M. Nutrient value of leaf vs. seed. Front. Chem. 21; 4:32 (2016): https://www.frontiersin.org/articles/10.3389/fchem.2016.00032/full) it is often used as a source of food for fish (Yilmaz,E., Akyurt, I., Günal, G. Use of duckweed, Lemna minor, as a protein feedstuff in practical diets for common carp, Cyprinus carpio, fry. Turkish Journal of Fisheries and Aquatic Sciences 4: 105-109 (2004): http://www.trjfas.org/abstract.php?lang=en&id=238). Its close relative, Lemna gibba, has been proposed as a candidate for providing food for astronauts in space missions using Controlled Ecological Life Support Systems (CELSS)(Gale, J., Smernoff, D.T., Macler, B.A., MacElroy, R.D. Carbon balance and productivity of Lemna gibba, a candidate plant for CELSS. Adv. Space Res. 9(8): 43-52 (1989): https://www.ncbi.nlm.nih.gov/pubmed/11537389).
I look forward to the time when Lemna is used for missions to Mars and beyond. Please also consider it as a potential food source if you are concerned about the next Grand Solar Minimum and next potential Little Ice Age (Mörner N.-A. The approaching new Grand Solar Minimum and Little Ice Age climate conditions. Natural Science 7: 510-518 (2015): https://www.scirp.org/journal/PaperInformation.aspx?PaperID=61284)!