Reach

The Works of Melissa Danko

Exploring the Interactions of Rye, Tomato, and Alfalfa Plants

Introduction and Background

The Plant Interaction Lab explored the idea of competition amongst plants.  By measuring how well rye, tomato, and alfalfa plants grew when planted in different combinations, the success of certain species over the others could be seen.  Understanding a plant’s interactions with other plant species is vital, as there are many types of interactions that have various effects.

Positive interactions provide benefits for one or more of the species involved.  These positive interactions include commensalism, where one species is benefited and the other is unaffected, and mutualism, where both species are benefited.  Parasitism is an interaction that is positive for one species and negative for the other; one species benefits while the other is harmed.  Negative interactions include amensalism, where one species is harmed and the other is unaffected, and competition, where both species are harmed.  Competition, because it is harmful to both species involved, reduces the number of organisms that can survive from both species. Competition is the interaction that was explored most heavily through the Plant Interactions Lab, and within this category of interactions are two sub-categories: intraspecific competition and interspecific competition.  Intraspecific competition refers to the competition that exists within a singular species.  In interspecific competition, the competition exists between different species.

Some of these essentials that plants compete over are light, water, root space, and nutrients such as nitrogen, phosphate, and oxygen.  Water is a particularly important resource because all plants need it for growth and survival.  The effect of water on competition is strongly determined by the plant species that are competing.  If one plant is much better at capturing water, more water will not necessarily mitigate competition.  However, an increase in water would typically lessen competition if that was the limiting resource.

Competition among plants reduces the number of plants in each species involved, and it can also make the surviving plants much weaker.  When plants are restricted from essential resources, their ability to reproduce is affected because the individual plants are lacking in development and growth.  While lessening competition benefits the species involved, it is not necessarily a benefit for the rest of the community.  Competition helps to maintain the balance of an ecosystem, and when it is disrupted, it has the potential to affect a large number of organisms.

Experimental Design

To carry out the Plant Interactions Lab, seeds of rye, tomato, and alfalfa were planted in small pots.  25 and 50 seeds of each species were planted in their own pots, as well as combinations of the species together.  The different seeds planted were as follows: 25 rye, 50 rye, 25 rye/25 tomato, 25 rye/25 alfalfa, 25 tomato, 50 tomato, 25 tomato/25 alfalfa, 25 alfalfa, and 50 alfalfa.  Pots with only 25 seeds of one kind of plant allowed for the examination of intraspecific competition when compared to the pots with 50 of that same plant.  The combinations of species (25 seeds of one and 25 of another) were planted so that interspecific competition could be studied; pots with 50 of one plant could be compared to the pots with 25 seeds from 2 different species (50 total seeds)  This design kept the number of seeds consistent, allowing for a more accurate comparison that eliminated space as a potential factor.  The pots were kept in the University of Michigan Botanical Gardens where half of the plants were watered every day and the other half were watered every other day.  By watering some plants more than others, the water’s effect on competition could be studied.  After approximately 6 weeks, the plants were counted, measured, and weighed, and the data was collected.

Results

For the purpose of assessing true differences in growth results, data deemed statistically insignificant (having a p value above 0.05) has been eliminated, but some exceptions will be mentioned when necessary (see Figure 4 for p-values).  The data shows that for all of the plant species watered every day, the pots with 25 plants had taller and heavier plants than the pots with 50 plants (see Figures 1 and 2 for plant weight and length data).  Although a few of these results are technically insignificant, the demonstration of intraspecific competition is still clear.  For the plants watered every other day, this same correlation holds true except for the comparison of weight between the 25 alfalfa pot and the 50 alfalfa pot.  In this particular case, the pots with 50 alfalfa had heavier plants than the pots with 25 alfalfa, although this difference was deemed to be statistically insignificant.  The pots with 25 rye grew taller and heavier when planted with 25 alfalfa than when 50 rye were planted together; these results apply to both plants watered daily and plants watered every other day.  Tomato planted with alfalfa also grew taller and heavier than did the 50 tomato plants.  Going along with that, alfalfa—watered daily or every other day—grew much worse in terms of height and weight when paired with both rye and tomato as compared to when alfalfa was planted alone.  These results suggest a parasitic relationship between alfalfa and the other two plants; the interaction was positive for both rye and tomato plants yet negative for the alfalfa.  When tomato and rye were paired, tomato did not grow as tall or as heavy as tomato planted by itself, and this remains true for both sets of watering regimes.  Rye, however, grew taller and heavier when paired with tomato, suggesting that rye is the stronger competitor for whatever resource was limiting.

In terms of germination, all but 4 of the plant combinations showed daily watering to be advantageous (see Figure 3 for germination numbers).  The plants with higher germination after less watering were 25 rye, 25 tomato planted with 25 rye, 25 alfalfa, and 25 alfalfa planted with 25 tomato.  Overall, tomato had the highest percentage of seeds that germinated, followed by rye, followed by alfalfa.

As for height and weight in relation to watering, almost all of the plant combinations that got watered daily were taller and heavier than their half-watered counterparts.  Exceptions are 25 tomato, which grew taller and heavier when it was watered half as much, as well as 25 tomato planted with 25 rye.  Despite this, watering appears to have benefited the plants.

Discussion

Through analysis of the results, a few conclusions can be reached.  The idea that tomato and rye engage in a parasitic relationship is supported by the idea that upon rye and tomato both being planted with alfalfa, alfalfa did not grow as well and rye and tomato grew much better.  According to GardenGuides, alfalfa contains a nitrogen and phosphate-containing chemical called triacontanol which aids in the growth of tomatoes.  Use of triacontanol has been proven to increase yield of crops and it is generally known as a growth regulator (Laughlin, 1983).

Also apparent from the results is the existence of intraspecific and interspecific competition.  In many cases, interspecific competition was not as fierce as intraspecific.  The data shows that many 25/25 plant combination pots outgrew the pots with 50 of one kind of plant, showing that a species had a harder time competing against itself than competing against a different species.  This could be explained by the fact that different plants use different resources in varied amounts.  Nitrogen is typically the limiting factor, and depending on a plants’ nitrogen transport system in its roots, it could either be very efficient at nitrogen uptake in low nitrogen areas, or very unsuccessful (Ruzicka, 2010).  When different plants compete, they may have different ways of obtaining required nutrients.

Rye grew the tallest and heaviest, followed by tomato and then alfalfa, so it would seem likely that rye is the best competitor.  However, if germination is the determining factor, tomato is the most successful.  This demonstrates a problem with basing success off of plant length and weight.  Some plants are naturally lighter than others, while some do not grow very tall despite being good competitors.  Length and weight also do not determine reproductive success, which is one of the most important factors in an organism’s biological fitness.

In an actual ecosystem setting, the best competitor does not always rule the community.  This is due to many factors such as weather, disease, or predation.  The best competitor might not always find the limiting resource first, which could also prevent their domination.  In the case of plants, predation is likely a big issue.  If a certain plant can out-compete the others but it gets eaten by some other organism, its ability to fight for resources no longer matters.  Understanding all of these extra factors—in addition to competition at its most basic level—is important for growing plants successfully.

In agricultural ecosystems where the planting of rye, tomatoes, and alfalfa is common, maximizing yield while maintaining a low cost is crucial.  Knowing how certain plants interact with each other makes it possible to manipulate the success of the plants’ growth and development.  Realizing that alfalfa improves the growth of rye and tomatoes would be important for people trying to grow those two species efficiently.  Competition is good for more than commercial use, however.  Competition maintains diversity in an ecosystem and preserves the delicate balance that exists between all of the species and their surroundings.

Figures

Figure 1: the average weights of rye (R), tomato (T), and alfalfa (A) combinations, also including differences in watering regime.

Figure 2: the average lengths of rye, tomato, and alfalfa combinations, also including differences in watering regime.

Figure 3: the average germinations of the three plants, also including differences in watering regime.

Figure 4: the p-values for comparing the data

Works Cited

Garden Guides. “How to Use Alfalfa Meal to Grow Tomatoes | Garden Guides.” Garden Guides, Your Guide to Everything Gardening. Web.

Laughlin, R.G., et al. “Growth enhancement of plants by femtomole doses of colloidally dispersed triacontanol.” Science 219 (1983): 1219+. General OneFile. Web.

Ruzicka, Daniel R., et al. “Tomato root transcriptome response to a nitrogen-enriched soil patch.” BMC Plant Biology 10 (2010): 75. Academic OneFile. Web.

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