Effects of Low Concentrations of Salicylic Acid on Plant Growth

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Small amounts of salicylic acid are known to be present in plants.  Originally salicylic acid was extracted from the willow bark to make the well known pain relief medication Aspirin.  Salicylic acid is thought to promote disease resistance, increase flower life, inhibit seed germination, and promote ethylene synthesis.  My initial research indicated that large concentrations of salicylic acid actually killed plants so this study deals with plant growth attributed to the addition of low concentrations of salicylic acid.  The latest research into plant growth is explained and experimental evidence is provided that shows whether salicylic acid has an affect on plant growth.


seed tray 2 seed tray 2 close up seed tray 1 bean shoot seed tray 1 seed sizes measuring seeds

There is a lot of information about growing plants and about plant life ("botany").  One substance mentioned is acid, which is found in “pain relief” medication like Aspirin tablets.  Researchers think that this might be a new plant growth hormone but no evidence is given to prove it.  I was curious to see if there was a relationship between plant growth and the amount of aspirin fed to plants.  After my experiment I could show that some amount of Aspirin might actually increase plant growth compared to no Aspirin at all, while a lot of Aspirin harms the plant.  This research gives some background about plants, the plant growth process, plant growth hormones and Salicylic Acid.  

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Plant trivia quiz of the month... 

Click here to see Virtual Garden grow...  (Uses Marcomedia Flash 5)


The plant kingdom (plantae) is divided into 12 divisions called phyla and there are at least 4 classification systems that experts use to classify plants: by reproduction, by tissue-structure, by seed structure and by stature.

The bean plant is a member of the pea family (Fabaceae) and is a dicot which means it has two seed leaves.  Most trees, shrubs, vines, and flowers belong to this group of around 200,000 species. Most fruits, vegetables come from this class.

Bean Plants and How Beans Grow


wild common bean

string bean

bush bean

Wild variety of common
bean (Phaseolus aborigineus)

String bean

Bush bean


bean seed embryo


Germination is the sprouting of a seed.  When germination begins the seed needs a lot of water.  The water makes a chemical change that enables the embryo to store food and energy for growth.  The water also causes the baby seed (“embryo”) to enlarge and split the seed coat.  Germinating seeds require a large amount of oxygen and give off carbon dioxide.  One part of the embryo, called the radicle, comes out and grows making the roots.  Another part, called the plumule, turns into the shoot of the plant.  The endosperm is the food store that surrounds the baby plant.


bean seed growth

Common bean


After the shoot and roots become visible the first plant part you see is called the cotyledons.  They help feed the new seedling until it can make its own food.

Plant Food

All plants require food.  The food is also call nutrients.  Macronutrients are needed in large quantities (such as carbon, oxygen, nitrogen and phosphorus).  Micronutrients like iron, copper, manganese, boron and zinc are needed in small amounts.  Three important nutrients are nitrogen, phosphorus, and potassium.

Nitrogen is very important and has to be converted to nitrate by the bacteria that live on roots and in the soils.


nitrogen cycle


The Nitrogen Cycle is very important to plants. Nitrogen is a major component of amino acids, proteins, and chlorophyll present in plants and is needed for plants to live.


Fertilizers were made to supply plants with any nutrients that they are short of to stimulate growth and are made of nitrogen, phosphorus and potassium.


How to Grow Plants


1.     Allow room to grow so pick the right sized pot.

2.     Don't put too many plants in it. 

3.     Keep them warm.  Plants grow best if the temperature does not go below 12°C or above 24°C.

4.     Plants can grow in lots of different kinds of light.  Some need shade but most love sunlight the best.  Give those plants 7 to 12 hours of strong light each day.  Plants make their own food because they have chlorophyll.  Photosynthesis is the chemical change that produces food for the plant.  In photosynthesis, carbon dioxide, gas and water are mixed to make sugar and oxygen.  Sunlight is the energy it needs to make the chemical reaction.

5.     Water your plants only when they need it.  If roots are not given enough water, the plants will wilt and die.  If the roots are given too much water they will rot and the plant will also die. 

6.     Plants don't breathe like people, but they do take up air through their leaves and roots.

7.     Plants take up food from the soil (called nutrients) to help them grow strong.

8.     Care for your plants and be patient, plants need time to grow.


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Soil pH

Soil pH or soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH units.  The pH scale goes from 0 to 14 with pH 7 as the neutral point.  From pH 7 to 0 the soil is increasingly more acidic (eg. soda = 2 – 4) and from pH 7 to 14 the soil is increasingly more alkaline (eg. borax = 9).  Most minerals and nutrients are more soluble in acid soils than in slightly alkaline soils.

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Plant Cell Types

Plant cells help the plant grow.  Parenchyma tissue is made up of thin-walled cells that store water and salt.  Xylem tissue is made up of thick-walled cells that transport water and dissolved minerals from the roots up through the stem to the different parts of a plant.  Phloem tissue contains living cells that transport food produced throughout the plant.  Collenchyma tissue has thick walls for added strength and support.  Meristematic cells give rise to all three fundamental mature cell types. Their major function is cell division.

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Plant Hormones


Plant growth is controlled by growth hormones such as auxins, cytokinins, gibberellins (GA), and abscisic acid (ABA).  Water stress, the lack of water, decreases auxin and increases abscisic acid.  Auxins are involved in the growth of all parts of the plant, and abscisic acid is an inhibitor that may be involved in inducing dormancy.  Pruning is a stress because hormones made by buds are not present.  The result is the cytokinin to auxin ratio is high which produces unusual growth.  Sometimes plants grow better because of this.  Sometimes stress is helpful.  For example, at the end of summer, the changes in temperature, light and moisture change the hormone balance in plants causing the plant to enter dormancy long before the frost comes protecting plants from frost.  Sometimes stress is harmful and the plant does not thrive.


The stimulating hormones (auxin, cytokinin, and brassinosteroid) generally suppress the effect and production of the inhibiting hormones (ABA, ethylene, and GA) and vice versa.


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Salicylic Acid (SA)

Salicylic Acid naturally occurs in plants in very low amounts.  It is also manufactured and used to make Aspirin tablets.  Evidence has been found that shows Salicylic Acid can stimulate flowering.  Also, Salicylic Acid has been found to cause temperature increases of as much as 14ºC above the ambient temperature in the Arum group of plants (eg. duckweed, Skunk Cabbage).  Salicylic Acid is thought by some people to be a new plant growth hormone.  Also, some people use Aspirin in cut flowers to make the flowers last longer.

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(1)  Phyllis J. Perry, Science Fair Success with Plants, Enslow 1999

(2)  Robert W. Wood, Science for Kids, Tab 1991

(3)  Biological Sciences in Middle East Technical University


(5)  MadSci Network: Agricultural Sciences

(6)  Botany online University of Hamburg

(7)  Cornell University College of Agriculture and Life Sciences

(8)  Ellen Silva, Originally published as "Plant Stress Management,"  

Extension Technician, Department of Horticulture, in 

The Virginia Gardener Newsletter, Volume 10, Number 4.

(9)  Kimball's Biology Pages    http://www.ultranet.com/~jkimball/BiologyPages/  

(10) Santiago Ortiz - FLASH 5 - my modifications and use in Virtual Garden 


(11) Prof R. Koning, PhD Botany

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For problems or questions regarding this web contact Logan McLeod.
Last updated: May 23, 2003.