Biology Report on Duckweed

Investigation into plant growth - Lemna minor L. - Common Duckweed

Abstract
Lemna minor L. is an aquatic plant that is commonly found throughout the waterways of Australia. L.minor is “composed
 of a simple floating disc of photosynthetic tissue” (Environmental Leverage Inc. 2006) and can frequently be observed “floating in still or slow-moving fresh water” (Cross, J. 2001) in the wild. This investigation into the growth pattern of L. minor was undertaken in order to determine how the concentration of nutrients in a medium affected the growth of this plant. Throughout the duration of this experiment the L. minor was cultivated in petri dishes filled with a range of differing nutrient concentrated solutions. After weeks of observations it was found that the plants grown in a nutrient concentrated solution of 6.5mL/L displayed the most growth. Meanwhile the L. minorcultivated in a solution of nutrients that was either much more or less concentrated displayed significantly less favourable amounts of growth. By understanding the intricacies of this plant’s growth pattern, these results can be applied outside the science lab for the purpose of utilizing this rapidly growing organic resource. For ever expanding fields such agriculture and aquaculture have yet to fully realize the potential of this tiny plant and how it could revolutionize the treatment of water and conventional farming techniques.

Introduction

“Identifiable… by its diminutive size and free floating habit,” (Rook, E. 2004) Lemna minor L. or duckweed, as it is commonly known; “is found almost everywhere except permanently frozen poles and the driest deserts.” (Tan, R. 2001) Specifically, L. minor “belong to the monocotyledon family Lemnaceae, [which are] a family of floating aquatic plants.” (Landesman, L. 2005) These plants were the subject of an extended experimental investigation which aimed to determine whether the concentration of nutrients in the environment affected the growth of L. minor.

Throughout the investigation, it will be hypothesised that if the concentration of nutrients is increased, then the population of Lemna minor L. will also increase. The reason why this plant species was involved in the investigation was because L. minor “are among the smallest and simplest flowering plants.” (Landesman, L. 2005) The motivation behind choosing a simple plant was because of the time constraints of the school term.

The anatomy of the plant was also studied throughout the investigation, and was sketched after a sample had been stained and magnified. For the full experimental write-up and diagram see Appendix A.

Lemna minor L.

Whole Mount

LP. x 40

The diagram in figure 1 formed a vital aspect of the investigation because it provided a means of explaining the population growth of this plant. This is because L. minor “normally reproduces by budding (asexually).” (Fatula, D .2001) In other words, these aquatic plants “reproduce vegetatively… [with] buds from which more fronds grow.” (Cross, J. 2001) However, each frond can only reproduce a “limited number of times before… they turn yellow as they lose their chlorophyll” (Tan, R. 2001) and eventually die. As Andrews (2007) remarked; “this process is known as chlorosis.” The reason why chlorophyll are so crucial to the survival of the plant is because these “light-absorbing molecules” (Science Desk Reference. 1995) allow it to convert sunlight into energy.    

During this form of asexual reproduction, the buds grow, and “new fronds emerge from slits in the side of their parent fronds… [and] until they mature, [these] daughter fronds… remain attached to their parent fronds.” (Cross, J. 2001) This is labelled in figure 1.

L. minor is “also very capable of sexual reproduction (Fatula, D. 2001) and produces the smallest flowers of any plant. (The New Book of Knowledge. 2001)

The magnified photograph of L. minor in figure 2 shows “contact between adjacent plants with pollen-bearing stamens and receptive stigmas [which] could result in pollination.” (Cross, J. 2001) “This pollination [would] produce a seed, from which a new plant could grow.” (Science Encyclopedia. 2005)

It is because of these methods of reproduction that L. minor is so successful in the wild, “reproducing at twice the rate of other vascular plants.” (Rook, E. 2004) In fact; the growth pattern of L.minor can be said to be almost exponential. This form of growth is defined as “growth where the number of individuals doubles on average in a given time.” (LemnaTec. Inc. 2003) “It is because of the exponential growth rate [of] Lemnaceae [that] herbicides must be used repeatedly”(Environmental Leverage Inc. 2006) to control it in the wild.

Methodology:

21 petri dishes were each filled with 200mL of varying nutrient solutions. There were 7 different solutions containing nutrient concentrations ranging from: 0.0, 3.25, 6.5, 9.75, 13.0, 16.25 to19.5mL/L. After the petri dishes had been filled, they were then labelled so that the differently concentrated solutions could be distinguished.

Next 10 Lemna minor L. were transferred to each petri dish using the inoculating loop. Under Protocol One only plants over 2mm in size were considered to be plants during this process and throughout the investigation. The petri dishes were then sealed and randomly positioned under the 5 hydroponic light banks and stands: see figure 3.

These hydroponic lights were suspended 320mm above the surface by the stands and had a diameter of 26mm. Each of the 5 stands held a bank (two) of hydroponic or Growlux lights, which each emitted 36 watts of power and were on constantly throughout the experimental investigation. As the L. minor grew under these lights, their population growth was carefully monitored and the number of fronds recorded using Protocol One.

Throughout the investigation all of the equipment was positioned so that the plants were sheltered from the wind and extra light, which would have introduced unwanted variables into the investigation and potentially jeopardised any results. Nevertheless it was recognised that variables would occur during the investigation and some of these; such as the differing nutrient solution concentrations were actually intended. This variable was deemed to be the independent variable, while the number of plants growing in the solution at any given time was determined to be the dependent variable. 

Numerous other variables were foreseen and deliberately controlled to ensure that accurate results were recorded. These controlled variables included:

1.      The amount of sunlight the plants received: The petri dishes were under the hydroponic lights 24/7 and other sunlight was prevented from reaching the plants because the windows in the immediate vicinity of the experiment were blacked out.

2.      The amount of time they were allowed to grow: The experiment was planned to have duration of 28 days.

3.      Species of plant: Lemna minor L. was the only plant studied during the experiment so the growth patterns recorded were just for this species.

4.      Amount of nutrient solution: 200mL was the exact amount of solution that was poured into each petri dish.

5.      Distribution of plates under lights: The dishes were placed at random intervals under the 5 hydroponic light banks in case one of the lights broke or was switched off.

6.      Petri Dish volume: The area available for the plant to flourish was controlled by ensuring all the petri dishes were the same volume.

7.      Temperature and Humidity: The experiment was conducted in a controlled environment so it can be inferred that the temperature and humidity stayed the same. Although a little breeze or change in temperature or humidity could have affected the circumstances of the experiment, this would have been the same for all the petri dishes as they were positioned in the same vicinity during the same period of time.

After recognising these factors and ensuring that they did not occur, the results attained from this investigation can be confidently expressed as accurate representations of the appropriate data.

Results

The following results were collected over 28 days at approximately the same time each day:

Lemna minor L. Population At Various Treatments
Date	0mL/L	3.25mL/L	6.5mL/L	9.75mL/L	13mL/L	16.25mL/L	19.5mL/L	Average
28/II	10	10	10	11	10	11	11	10
2/III	10	13	29	22	20	17	14	18
5/III	12	35	38	36	28	17	8	25
7/III	12	81	107	50	40	18	14	46
9/III	13	102	152	114	60	21	7	67
12/III	13	270	262	230	99	30	9	130
14/III	14	293	337	384	135	46	11	174
16/III	41	362	563	548	216	62	11	258
19/III	42	914	947	866	371	97	10	464
21/III	44	931	1064	1134	495	128	10	544
23/III	46	976	1508	1247	639	185	5	658

Lemna minor L. Growth Pattern

Discussion

The data that was collected on the growth patterns of Lemna minor L. was compiled into a table of averages in figure 4. For analytical purposes this data was then converted into a graph for in figure 5. For the raw data gathered throughout the investigation on the growth pattern of L. minor see appendix C.

Careful analysis of the graphed data identified the occurrence of numerous trends and patterns relating to the theory of the population growth of L. minor. Central to this theory are the growth curves predominantly displayed on the graphed results.

The graph shown in figure 6 displays the growth curves of a theoretically perfect growth of L. minorthrough its various stages of population growth. In theory these growth curves are an ‘S’ shape; reflecting the plant’s numerous stages of growth. These stages can be divided into 4 different phases which have been labelled in figure 6.

However due to the differing nutrient concentrations in the solutions that the L. minor were cultivated in throughout this investigation, the graphed results in figure 5 vary  greatly from the theoretical model graph of L. minor growth shown in figure 6. For instance the L. minor grown in a nutrient concentrated solution of 19.5mL/L displayed minimal growth and the graphed results looked nothing like figure 6. Yet when the plant was grown in a solution that had a 6.5mL/L concentration of nutrients, the growth resembled that of the model in figure 6 except for the last phrase of growth where it did not begin to slow down. See appendix D for the graphed average results on the L. minorcultivated in the different nutrient solutions.

With such contrast between the various amounts of growth the L. minor exhibited; in correspondence to the varying nutrient concentrations they were cultivated in, only a graph that displayed the average growth of the plant in all the nutrient concentrations could be used for analysis. This was the case because a medium had to be found between the numerous sets of data gathered throughout this investigation.

The average growth of Lemna minor L. which was observed and recorded is shown in figure 7 and is remarkably similar to the theoretically ‘perfect’ model of the L. minor pattern of growth shown in figure 6. The 4 different phases of the growth curves are also apparent in figure 7, except for the final phase when the growth of the plant is theoretically supposed to slow to a stop. However it can be speculated that had the experiment continued for a longer period of time then the L. minor would have reached that phase of growth.

Nevertheless the growth curves of the average population of L. minor mirror those of the model graph in figure 6 very closely. The differently labelled phases in this figure can even be applied to figure 7 where they can be used to explain the different stages of this plant’s growth. The first stage, phase A is the lag phase which occurs when “plants are transferred to a fresh medium with abundant nutrients.” (LemnaTec. Inc. 2003) The growth throughout this phase is minimal as the L.minor undergoes biochemical changes in preparation for growth.

The next phase of the growth curves is the period of explosive growth where the Lemna minor L. “can double its biomass.” (Duckweed. 2001) This phase is represented as section B of figure 6 and is referred to as the exponential growth phase.  See appendix E for details on the equation for ideal exponential growth.

The graphed results of the L. minor growth shown in figure 7 display the beginnings of phase C in the growth curves. This is known as the transitional phrase and occurs when the population growth begins to slow down. As the nutrients in a medium are gradually used up, the plant growth becomes very limited which eventually leads to phase D. The final phase detailed in figure 6 is the equilibrium phase; and during this final stage of growth the birth of new plants is directly related to the number of deaths of older L. minor. 

Although on the whole the experiment was conducted in a satisfyingly thorough manner, there was one major flaw with its design: the hydroponic lights. While in nature the sun is only present for roughly half the day, the hydroponic lights were on 24/7 for the duration of the experiment. This is not at all realistic as the plants were being unnaturally stimulated. This could potentially place any results gathered in the science lab out of sync with the growth pattern of L. minor grown naturally in the wild. This could be rectified by simply turning the lights on and off at dawn and dusk.

Another mistake that could be remedied was the presence of insects in the solutions where the L.minor was being cultivated. As they died, they would release extra nutrients into the solution. While in the higher concentrated nutrient solutions this might not pose such a problem, this could potentially hamper results at the other end of the scale where the L. minor was deliberately deprived of nutrients. This could be remedied by simply taping the petri dishes shut in future experiments.

The final error in the investigation was in the timeframe that was incorrectly represented on the results graph in figure 5. This was inconsistent as the growth rate of the L. minor was recorded every two or three days depending upon the weekend. This lapse in time is not represented in the graph as an extra day so it appears that the data was collected over the same period of time. This problem could be solved by the inclusion of an extra day in the graph to show that a day had been skipped.

Conclusion

“Many studies have indicated that duckweed growth is a function of nutrient levels and not pH and/or temperature.” (Environmental Leverage Inc. 2006) This would explain why the population growth of the Lemna minor L. cultivated in differing nutrient concentrations were so drastically different. From these results it could be inferred that L. minor grows best in a nutrient rich medium, yet that could hardly explain why this aquatic plant displayed minimal and even negative amounts of growth whengrown in a more concentrated nutrient solution as figure 8 shows.

As a nutrient concentration of 6.5mL/L is the recommended dosage for optimal growth in aquatic plants, that could explain this anomaly amongst the data. For as Andrews; (2007) a renowned biology teacher suggested: “A nutrient concentration of 19.5 may be toxic.” This logic is at odds with the hypothesis of this investigation, which proposed that if the concentration of nutrients is increased, then the population of Lemna minor L. would also increase. Instead based on the results of this report it could be concluded in a revised hypothesis that if the concentration of nutrients is increased before the point that it becomes toxic, then the population of Lemna minor L. will also increase.

Another factor that supports these findings is the length of time the L. minor cultivated in the nutrient concentrated solution of 6.5mL/L stayed in the exponential phrase of growth. This is the phrase of growth where the L. minor multiply rapidly until the nutrients at the plant’s disposal are consumed. This results in the growth of the plant slowing down as it relies on photosynthesis to create energy. This is evident in figure 9 where the L.minor was cultivated in a nutrient solution of 6.5mL/L and yet its growth had not yet slowed down by the time the experiment was concluded.

It can be inferred that had the experiment continued over a longer period of time for the L. minorcultivated in this nutrient solution, the nutrients would eventually have been consumed and the L.minor would entered the transitional phrase as its growth slowed. The L. minor grown in this solution maintained its rapid growth because “if the growth medium is rich, plants will remain in the exponential growth phase for a longer period.” (LemnaTec. Inc. 2003) The L. minor grown in a solution with a nutrient concentration of 6.5mL/L displayed the highest amount of growth because it stayed in the exponential phrase of growth the longest.

Another factor that contributed to the results attained from this investigation was the chemical makeup of the nutrient solution used to cultivate the L. minor. This would have had a huge effect upon the final results gathered because L. minor requires “high levels of phosphorus and/or ammonia” (Environmental Leverage Inc. 2006) to thrive. The nutrient solution employed throughout this investigation contained both these chemicals which would explain the plants rapid growth when the solution was not concentrated in toxic proportions. See appendix F for the formula of this nutrient solution.

The rapid growth of L. minor and its ability to absorb the nutrients in the surrounding environment have given this plant some valuable properties when applied to the real world. For instance, L. minor“has some desirable properties for water purification… [it] treats waste by breaking it down and converting it into two components: biomass (duckweed leaves and roots) and treated water.” (Environmental Leverage Inc. 2006) Zhang Jinping, director of the Biology Monitoring Office supports this theory when he commented that the presence of L. minor in water “shows that the water quality has improved.” (Tian, J. 2001)

L. minor has also been utilized as a “fast sustainable harvest… for livestock fodder, aquaculture feeds, waterfowl grazing, compost materials [and] biofuel carbon” (Duckweed. 2001) because of its rapid growth pattern. In terms of fodder, L. minor can be used “as part of a system for recycling cattle wastes from feedlots.” (Landesman, L. 2005) The logic behind this is that thousands of cattle produce huge amounts of manure, the runoff of which is collected in ponds of water. By introducing L. minor to these ecosystems, the plant can be used to treat the water and then be recycled as fodder because “their high fat and protein content makes them a source of food for animals and poultry.” (Rook, E. 2004) This concept can also be applied to aquaculture, where certain species of fish “were observed to feed readily on fresh duckweed [while the plant removed] as much as 99% of the nutrients and dissolved solids in wastewater.” (Duckweed. 2001)

L. minor also have the ability to rejuvenate life in a waterway, because their “colonies provide [a] habitat for micro invertebrates” (Texas Cooperative Extension. 2007) to survive. This then forms the basis from which other forms of life can thrive, creating a healthy ecosystem. Furthermore the presence of L. minor in a waterway “suppresses nitrifying bacteria” (Duckweed. 2001) which can help make the water safe for human consumption. For many nations where clean drinking water would be a godsend, the introduction of L. minor into their waterways could have solve their problem. In addition, since L. minor “have a high protein content (10 to 40% protein on a dry weight basis).” (Landesman, L. 2005) Utilizing this resource for the purpose of human consumption could aid many developing countries where famine is rife.

For such a seemingly simple plant, the possibilities of utilizing it as a resource seem endless. Its rapid growth coupled with its ability to “remove unwanted nutrients and waste products from… [a] system” (Duckweed. 2001) make it very valuable for “agriculture and wastewater treatment.” (Landesman, L. 2005) Perhaps someday if Lemna minor L. is utilized correctly it will revolutionize water treatment and the production of food. Eventually this “entire system… [will offer] a natural and sustainable approach” (Duckweed. 2001) to these age old problems.

Bibliography

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