Environmental Factors for Plant Growth

Abstract

Different environmental conditions contribute to the limitations of plant growth. Salts are common and a natural constituent of all soils. Normally, salts are present in low amounts in top soil and plant growth is not affected. However, accumulation of salts, through natural means or man’s activities, can cause plant growth problems and result in poor growth or death of plants.

Considering that Bermuda is such a small island, farmers’ crops are constantly being exposed to salt spray. Therefore, there must be a saline threshold for the various crops. Bermuda is famous for The Bermuda Yellow Onion, therefore I felt it was appropriate to investigate The Salinity Tolerance of the Bermuda Yellow Onion.

The objective of this study is to find out if the Bermuda Yellow Onion has a saline threshold, and if so what is this saline limit. To examine this question, I have constructed several small experiments, including the Saline Affect on Biomass, Germination Rate, Onion Growth, and the Water Content in Soil after Plant Growth.

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Throughout this investigation, I found that salinity effects both germination and overall growth of the Bermuda Yellow Onion, decreasing the yield percentage produced. The appearance of all plants grown in saline solutions are recognized as an effect of salt stress; poor germination and establishment, reduced plant vigour and stunted growth, smaller than normal leaves, slightly yellowing leaves, and a burnt appearance on tips of leaves.

All experiments carried out show that salinity has an affect on the osmotic ability of both seed and plant, which has an effect on other plant functions such as photosynthesis and transpiration. Without the plant able to function effectively nor efficiently, the plant will produce fewer yields or eventually die, depending on the salinity strength.

This discovery suggests that there is a definite saline threshold for the Bermuda Yellow Onion.

Introduction

Originally, Bermuda grew and exported tobacco, but during the 1870s and 1880s Bermudians approached the production of crops with a more serious attitude, and agriculture became a thriving export business.

Every onion was hand picked, wrapped and packed for the market in New York. Every member of the family was involved in preparing the garden produce for export for the steamers that sailed twice a month from Hamilton. Bermuda did not only export onions to the New York market but also other garden produce such as arrowroot, celery, tomatoes, and potatoes.

Because Bermuda’s onions were so delicious they were widely sought and enjoyed not only in the U.S. but all over, so production increased to meet the growing demand. Bermuda was often referred to as The Onion Patch and Bermudians were obviously nicknamed Onions because of this. The export of Bermuda onions came to an abrupt end when U.S. tariff walls were raised against such imports.

Bermuda is quite a small island, only 21 square miles, and the crops are often exposed to salt spray from the surrounding ocean, which affects the soil that will in the long run affect plant growth.

Soil is a multicomponent system, which means that soil is made up of solid, liquid and gaseous components. This system consists of inorganic and organic solid components, soil solutions and as well as a gaseous phase. These three states are in a constant flux, maintaining a state of equilibrium. This is maintained by a chain reaction, one phase will manipulate the following phases until the equilibrium state is achieved. One way of achieving equilibrium is cation exchange; this involves cations interchanging between the solid phase and the liquid phase. This exchange reaction occurs from the negative charge of soil colloids.

Soil contains a mixture of different types of clays; soil salinity causes particles to bind together into aggregates, this process is called flocculation. Flocculation is good for soil aeration, root penetration, and root growth. Increasing soil salinity may have a positive effect on soil aggregation and stabilization, but high salinity also has a negative effect on plants.

High sodium concentration in soil, gives an opposite affect than soil salinity. Sodium causes soil dispersion and the clay platelets and aggregate swelling. When too many sodium ions come between the clay particles, the bonded clay particles are disrupted. This separation of clay particles causes them to swell and soil dispersion occurs. Soil dispersion causes clay particles to clog soil pores; this reduces the permeability of the soil. When soil is wetted and dispersion occurs repeatedly, it then becomes solidified to a cement-like soil with little or no structure.

The reason why other salts such as calcium and magnesium do not have the same affect as sodium is because they are smaller so they can collect closer to the clay particles. As shown in figure 1. Soil dispersion also has an affect on infiltration and hydraulic conductivity of the soil.

A Lack of Water

Salinity also affects the Evapotranspiration (ET), as salinity in the soil increases the ET decreases. Because of the saline soil having an osmotic pressure greater that the plant cell sap, there is a link between the effect of salinity on ET and the yield of plants.

The tolerance of plants to salinity is linked to the salinity of the soil, which is known as the total amount of soluble salt in soil. The relative growth of plants in saline soils will determine their salt tolerance.

You measure salinity by the range of electrical conductivity levels throughout the soil. Electrical Conductivity (EC) is the ability of a solution to transmit an electrical current. In order to determine soil salinity EC, an electrical current is imposed in a glass cell using two electrodes in a soil extract solution taken from the soil being measured. The units are usually given in deciSiemens per meter (dS/m).

The salinity of seawater is usually 35 parts per thousand (also written as ppt) in most marine areas. This salinity measurement is a total of all the salts that are dissolved in the water. Although 35 parts per thousand is not very concentrated (the same as 3.5 parts per hundred, pph, or percent) the water in the oceans tastes very salty. The interesting thing about this dissolved salt is that it is always made up of the same types of salts and they are always in the same proportion to each other (even if the salinity is different than average). The majority of the salt is the same as table salt (sodium chloride) but there are other salts as well. The table below shows these proportions:

Salinity has many consequences to the growth of plants. Considering the small size of Bermuda, salinity can contribute to hindering plant growth of farmer’s crops. Since Bermuda has been recognized for growing onions, I felt that it is rational to investigate the salinity tolerance of the Bermuda Yellow Onion. From the knowledge I have attained, I expect throughout my investigation that the yield produced by the onion will decrease with increasing salinity.

Method and Materials

I will concentrate on germination and the growth of onion seedlings to investigate the salinity tolerance of the Bermuda Yellow Onion. I will investigate the affect of various salinities on the germination rate and the affect of various salinities of the growth of the onions by measuring height and the biomass and dry biomass of the onion samples. Salinity concentration used will be constant in both investigations, consisting of a constant of 0ppt concentration, as well as concentrations of 0.5ppt, 1.0ppt, 1.5ppt, 2.5ppt and 3.5ppt.

All experiments will be carried out in my biology classroom under controlled conditions. Experiments will be set up along side the windows so exposure to light is the same. Temperature may fluctuate because of air conditioning in the classroom, but since each sample is exposed to the same conditions, there will be no contamination to the final results. Hence, if the temperature should affect the growth in any way, each sample will be affected in the same way.

Watering of samples will be daily and in precisely equal amounts, no matter what saline level the water is the samples will be supplied with the same amount of water.

However there is one variable I can not have control over, human contamination. Since I will not be in the room supervising at all times I am not able to account for other students interfering with the samples, but I trust no one will cause disorder.

Biomass

Initially I will investigate the average dry biomass of random onion seedlings so I will be able to compare water content at a later stage. Dry bio mass will be found by firstly recording 10 random onion seedling samples masses then leaving them in a drying oven until a constant mass is obtained. Drying oven will be set at a temperature of 100oC; because of course 100oC is the temperature at which water boils. Therefore I will be able to evaluate the average percentage water mass loss of the seedlings grown in the

Investigation of saline affect on Germination

Using Petri dishes I will investigate the affect of various salinities on the germination rate. It takes 7-14 days for an onion seed to germinate, so I will carry out the experiment over a 2 week period. Each Petri dish will be lined with three layers of paper towel and watered daily with 5 ml of water to ensure accuracy (show in the figure below). For each saline concentration there will be five Petri dishes containing 20 seeds each.

Investigation of saline affect on Onion Growth

Pot trials will be carried out in 4 gallon pots over a 7 week period; in each pot I will grow 5 onion seedlings 10-15 cm apart in -3/4 inch of soil deep. Each seedling will have an initial height of 6.75 cm above soil, and I will be weekly their progress or even retreat.

Seedlings require 1 inch of water per week, so each sample set will be watered 1 inch relevant to the 4 gallon pots, which is approximately 1.5L of water. Therefore Plants will be watered daily with 200ml of water. Deionized water will be used to eliminate other ions as a variable. To make the saline solutions I will add 1 kg of water approximately for every ppt amount in grams of dissolved salts, for each specified saline percentage Consider 5ppt: when 5g/1kg=5g/1000g and 1g=1ml, then 1000g=1liter, therefore 5g of NaCl per liter of water(5g/1l)=5ppt

To avoid phototropism pots will be rotated daily because sunlight thorough the window is the only source of light plant samples will receive. Following the seven week period seedlings will be extracted from the soil measured and weighed and then placed into the drying oven until a constant dry bio mass is found.

Investigation of water content in soil after plant growth

Along with the experiments I will also take samples of soil from each individual saline percentage pot at the end of the experiment to investigate the water content of the different salinity concentrated soils. (Soil samples will be taken from a dept if 10cm) I will take 3 samples of each concentrated soil and place them in small crucibles, in order to obtain the soils mass, I will have to weigh the crucible before and after placing the soil sample in the crucible.

To find the water content of the soil samples I will place the crucibles containing the soil samples into a drying oven at a temperature of 100oC, and then measure the mass of each sample after (samples will be left in drying oven for 3 days and measured each day to obtain a constant mass result).

Salinity effect on germination rate is very evident. At the highest salinity levels 2.5% and 3.5%, seeds are not able to germinate at all. While at the lower salinities, germination rate increase as salinity levels decrease. This is because the NaCl in the saline solution attracts the water molecules restricting the uptake of water. As a result the more saline the solution is, the greater the attraction between NaCl molecule and water molecules will be. This results in an impact on the osmotic ability of the seeds.

The germination of seeds is dependent on both internal and external conditions, with one of those conditions including the availability of water. Mature seeds are often extremely dry and need to absorb a significant amount of water, relative to the seeds dry biomass, before cellular metabolism and growth can resume. Most seeds respond best when there is enough water to moisten the seeds but not soak them.

The uptake of water by seeds is called imbibitions, which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store a food source, such as starch, proteins, or oils, to provide nourishment to the growing embryo inside the seed. When the seed imbibes enzymes are activated that break down the stored food into metabolically useful chemicals, allowing the cells of the embryo to divide and grow, so the seedling can emerge from the seed. For that reason, without the right supply of water, the cellular metabolic pathways will not take place.

There is an immediate effect on the growth of onions once salt is introduced to the environment. Plants grown in 0% salinity progressively grows over the seven week period, while salinities 0.5% and 1.0% decrease within the fifth week and begin to slowly progress again. Plants grown in salinities of 0.5% began to grow with high progression and had a very large decrease. Plants grown in salinities of 1.5% progress slightly and in the fourth week, growth declines and in the last week plants begins to progress again. Plants grown in salinities 2.5% and 3.5% had scarcely any progression, plants seem to stay at almost a constant height until plants die.

Although in the beginning, plants seem to make some sort of progression, in the end plants grown in saline solutions show an extreme case of drought stress. As plants are continually watered with saline solutions, the salt begins to accumulate in the soil. With increasing salt accumulating in to soil, the greater the inhabitation of uptake of water to the plant is. Explaining why plants grown at lower salinities appear in the beginning to progress then suddenly have a great decrease in growth.

As the experiment progressed unpredictably, seedlings began to disappear. I concluded a possible answer causing this problem could be that as water was being retained in the soil and roots begin to rot. There is an excess of water in plants grown in saline waters, because of the fact that salt inhibits water uptake, the excess of water makes it difficult for the roots to receive any oxygen, thus leading to roots rotting. As well, of course without roots there is no way in which a plant is able to transport material in and out leading to overall death of the plant. Explaining why plant began to disappear throughout the experiment.

It is apparent that salinity has an affect on plant yield. All plants grown in saline soils parentage yield has decreased in fresh biomass. That fact that only plants grown in saline soils of 1.5%-3.5% has a decrease shows that there may be a saline threshold for Bermuda Yellow Onion. Since water moves into the cells because they are full of salts and sugars shows that Bermuda Yellow Onions cell sap concentration of salt is lower than 1.5%.

Root cells receive sugars from the leaves and actively absorb salts from the soil. This concentration of salts and sugars causes water from the soil to move into the cell. But if the solution out side the root cells are more concentrated with salt than the solution inside the root cells than water will not diffuse into the cells.

Water is truly vital for growth. Plants grow in two ways, cell division and cell expansion. Cell division creates more cells and cell expansion is the increase in cell size. Cells grow by taking up water. If water is reduced during growth, final cell size and overall plant growth is reduced. Increasing salinity values causes drought stress for the plants resulting in smaller, weaker plants.

With the lack of water, there is a of soluble salts and sugars therefore photosynthesis cannot occur efficiently, explaining the less biomass produced in the higher salinity levels. Plants need water for photosynthesis. Photosynthesis produces biomass in which energy from sunlight converts carbon dioxide and water to carbohydrates and oxygen. If they lack it, they wilt. When they have a deficiency of water, the stomata close and CO2 cannot diffuse into the leaves. Without CO2, photosynthesis would not occur, as it should.

(Refer to Appendix 1, Table 6)

The water content if saline soils are very similar on either end of the saline scale. Low salinities of 0-1 are similar and higher salinities of 1.5-3.5 are similar. Although there is a generous jump between the two set groups. This jump further enhances the possibility of a salinity threshold of the Bermuda yellow Onion.

As salinity inhibits the osmotic affect less water is taken up by plants roots, so daily watering will not help the plant to absorb any more water. Therefore, more water is left in the soil. Plants grown in salinities 2.5% and 3.5%, demonstrate this because towards the end of the experiment water began to retain in the bottom on the trays.

A saline soil is defined as having a high concentration of soluble salts, high enough to affect plant growth. In saline soils water is held tighter to the soil, the presence of salt in the water causes plants to exert more energy extracting water from the soil. The main point is that excess salinity in soil water can decrease plant available water and cause plant stress.

There are several factors that hinder water flow from the soil to the roots. One is the soil-root contact, and as the root dries out it shrinks away for the soil particles. Therefore, soils of higher salinities retain a greater amount of water, thus providing more evidence that salinity inhibits plants osmotic ability.

High sodium concentration in soil causes soil dispersion and the clay platelets and aggregate swell. When too many sodium ions come between the clay particles, the bonds between the clay particles are disrupted. This separation of clay particles causes them to swell and soil dispersion occurs. Soil dispersion causes clay particles to clog soil pores; reducing the permeability of the soil.

The ideal soil is one that holds moisture and at the same time allows a constant flow of air through the soil. Soil cannot be over-saturated with water or air would be excluded. The reason why soil is so important to a plant’s survival is this need to maintain a balance between moisture and air.

The quality of the soil, as determined by physical characteristics, can help or hinder a plant. In this case the quality of saline soils hinder a plants growth, with soils retaining water, it prevents airflow getting to the roots. Airflow brings oxygen to the roots and to micro-organisms, and removes carbon dioxide from the soil. With a lack of oxygen to the roots respiration and ATP production via oxidative phosphorylation, a process that essentially requires the presence of oxygen will not occur.

Evaluation

During the investigation of Salinity effect on Onion growth experiment, I ran into some problems with collecting the data. Since Onion seedlings grow with multiple leaves I was not completely sure how to measure the height, hence do I measure only the tallest leaf. Then again, do I measure each individual leaf? I decided to measure each individual leaf, but then I ran into a problem with my decision, as the investigation progressed, leaves began to die and new shoots appeared. With this I could not keep track of what leaves had died and which leaves are the new ones, with that I decided to jus keep the measurements of the tallest leaf, and continue taking only the measurement of the tallest leaf.

I also could have grown the plants in a more controlled area, because I am not able to account for anyone tampering with the experiment.

Conclusion

The Bermuda Yellow Onion can tolerate salinity levels up to 1.0% – 1.5%, but only for a short period. If plants are continually watered with the same saline solution, the salt will eventually accumulate in the soil and show severe signs of drought stress.

This tolerance level is present because the cell sap has a certain concentration level and once the saline soils exceeds the cell sap concentration, the plants can no longer grow, (tolerate) the saline soils. This is because once the solution outside the soil is higher than the solution inside the cells, naturally osmosis occurs and the cells become flaccid.

Water is essential in the plant environment for a number of reasons. Water transports minerals through the soil to the roots where they are absorbed by the plant. Water is also the principal medium for the chemical and biochemical processes that support plant metabolism. It also acts as a solvent for dissolved sugars and minerals transported throughout the plant.

In addition, evaporation within intercellular spaces provides the cooling mechanism that allows plants to maintain the favorable temperatures necessary for metabolic processes. Without the essential amount of water, the essential minerals would not be transported throughout the plant leading to growth deficiency.

As this is a problem in Bermuda because the island is so small farmers land is constantly exposed to salt spray. There is no way to reverse the effect of salinity, but there are ways in which farmers could successfully grow plants in high saline soils. By providing adequate drainage, maintain adequate soil moisture, and simply grow salt tolerant plants.