Effect of pH on the action of Gibberellin in the growth of shoot height of green chilli

To learn from what I observe has always been my passion. Being passionate about gardening, some basic knowledge about choosing the appropriate soil for my plants and using the correct fertilizers to grow them has been my forte. The journey started from a challenge that I faced while growing a tomato plant in my roof top garden. Despite adding Gibberillin, the plant did not show sufficient symptoms of growth. To inquire the reason behind it, I tried to find the factors that could have opposed the growth. As also studied during my IGCSE Biology course, soil pH is an important factor for growth. If the soil is too acidic or too basic the growth of the plant is hindered. Thus, I tested the pH of the soil using a pH paper and found that the soil pH was too acidic to promote the growth of the plant. From my knowledge of DP-Biology, I know about the various hormones which are essential to promote the growth of plant like-auxin, cytokinnins, absicissic acid, Gibberillin and so on. It must be noted that the type and composition of soil where plants are grown will differ and thus the pH will differ too. Thus, I was intrigued to know if the pH of the soil at which the soil is grown will have an impact on the role or the mechanism of the hormone action on plants or not. Will the change in pH alter the pathway hormone follows to trigger cell growth and thus the growth of plant? Thus, I arrived at the research question stated below.

How does the effect of the concentration of Gibberellin (in ppm) on the growth (in terms of vertical shoot height) of a green chilli (Capsicum annum) varies with the pH – acidic (pH = 4.00), basic (pH = 9.00) and neutral (pH = 7.00)?

Gibberellin is a class of plant hormone that regulates various developmental stages of the plant like – ‘developmental processes, including stem elongation, germination, dormancy, flowering, flower development, and leaf and fruit senescence’. They are named as GA-1, GA-2, GA-3 and so on based on the time they are identified. This hormone is prepared in the plastid along a terpenoid pathway. Following this, they undergo certain structural modifications in endoplasmic reticulum and finally in the cytosol. In the germination stage, GA acts as a precursor of the enzyme alpha-amylase which catalyses the hydrolysis of starch into sucrose and provides nutrients to the seedlings soon after they are contact to water. Cell elongation is one of the main role performed by this hormone which aids the plant in multiple stages of development. The hormone changes certain ‘rheological properties of the cell wall’. Consequently, the cell wall becomes less rigid and the osmotic pressure in it decreases due to a decrease in the water potential. This allows the cell to inflow of water into the cell wall and thus increases its volume which finally result in elongation of the cells.

Gibberellin, being a hormone, it is highly sensitive to pH with a tolerance range that lies within 3 to 10. Changes in pH alters the hydrogen ion concentration of the endoplasm and thus affect the mechanism in which they secrete enzymes for starch hydrolysis. Lowering the pH increases the response to GA.

This is because as the pH is decreased, the dissociation of GA is inhibited. Thus, more amount of undissociated GA molecules which can penetrate into the cell increases. Moreover, with lowering of pH, there is excess of hydrogen ions in the medium which interferes with various proteins in the cell wall and degrades or decomposes them; an effect known as ‘acid effect’. This makes the cell wall more permeable and facilitates the passage of more GA molecules into the cell. Both of these together will eventually increase more penetration of GA into the cells and thus promote the elongation of cell walls via the process of water uptake. GA also triggers the production of the enzyme hydrolase by endosperm which aids in the breaking down of dormancy.

Chilli is a small shrub branched fruit bearing plants. It has two major types of fruits-red and green. The major constituent of the fruit is capsaicin which plays major role as a taste enhancer and has other nutritional benefits as well. This plant was originally grown in Mexico and then became to the cultivators across South America, India, Thailand because of its high rate of vegetative reproduction. It acts as a major anti-oxidant and contains huge amount of Vitamin-A and Vitamin-C. Because of the fact that this plant can be grown easily without the need of any major fertilizer or special conditions, this has become so popular amongst both farmers and amateur gardeners. It needs roughly an exposure to 5-6 hours of sunlight everyday and adequate amount of water. It grows well in pots with the use of requisite amounts of organic fertilizers.

In a research paper on” The effect of external pH on the gibberellic acid response of barley aleurone effect” it was observed that “a reduction in medium pH from 6.0 to 3.7 enhanced the GA3-response of isolated barley (Hordeum vulgare L.) aleurone layers.” To be specific, there are quite a few researches that reports the positive impact of responses towards GA by the endosperm in germination stage of various seeds. However, the exact mechanism behind this has not been investigated and researched much. Another research on the effect of pH on the activation of molecules towards GA was done by measuring the % germination of barley seeds and the result of that is shown below.

The result clearly indicates that at lower pH, the molecules are more activated towards GA.

Null hypotheses

• The vertical shoot height of green chilli plant does not depend on the concentration of GA added to it.
• At all pH, the response of the green chilli plant towards GA molecules is same.

Alternate hypotheses

• The vertical shoot height of green chilli plant depends on the concentration of GA added to it.
• At all pH, the response of the green chilli plant towards GA molecules is not same.

Concentration of GA (Gibberellic acid) in ppm

The concentration of GA used in this investigation was varied in the range of 100.00 ppm to 600.00 ppm; 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm and 600 ppm. A solution of 100 ppm means 100 mg (0.10 g) of GA in 1000 cc of distilled water. All these solutions were made by weighing the required amount of GA using a digital mass balance and adding them to 1000 cc of water added using a graduated measuring cylinder. As per the biological interval, the mean level of GA in a nutritious healthy chilli plant lies within the range of 200 ppm to 400 ppm. Thus, this range is chosen to have values within the biological interval as well as above it and below it.

pH at which the plant is grown

The external pH at which the plant is grown was varied using buffer solutions. Both acidic and alkaline pH values, pH = 4.00 and pH = 9.00 were used. As a control, the neutral pH (=7.00) was also used.

To measure the growth of the plant, the vertical shoot height was measured. A ruler and a thread was used for this. As one of the major functions of GA is to promote stem elongation through increases in cell volume, the vertical shoot height was chosen as the dependent variable.

• A face mask and a laboratory coat were used.
• Any edible materials were not brought inside the laboratory.Any edible materials were not brought inside the laboratory.
• All the solutions were prepared under the careful guidance of an expert.
• Ethanol was not inhaled or exposed to skin.
• The scissor was used carefully.
• Hair was always tied up.

• The buffer solutions were given to the chemistry laboratory for re-use.

• The unused solutions of GA were re-used in the garden.
• The plants were taken back by the researcher and re-planted in the balcony garden.

• Take a 1000 cc glass beaker.
• Weigh 0.10 ± 0.01 g of GA powder using a watch glass and a spatula on a digital mass balance.
• Transfer the weighed solid into the glass beaker.
• Transfer 200 cc of ethanol into the same beaker using a graduated measuring cylinder.
• Transfer 800 cc of tap water into the same beaker using a graduated measuring cylinder.
• Stir the solution using a glass rod.
• Repeat steps 1-6 with 0.20 ± 0.01 g of GA (for 200 ppm), 0.30 ± 0.01 g of GA (for 300 ppm), 0.40 ± 0.01 g of GA (for 400 ppm), 0.50 ± 0.01 g of GA (for 500 ppm), 0.60 ± 0.01 g of GA (for 600 ppm) solutions.

• Take 35 plastic cups.
• Weigh 100.00 ± 0.01 g of loamy soil and put in each cup.
• Weigh 1.00 ± 0.01 g of NPK-20:20:20 fertilizer and mix it well with the soil in each of the cup.
• Label all the cups. Label five of them as – 100 ppm, another five as 200 ppm, another five as 300 ppm, another five as 400 ppm, another five as 500 ppm, another five as 600 ppm.
• Keep all the 35 cups in a place near a window so that all of them can receive the same amount of sunlight.
• Take a thread and a ruler to measure the initial shoot height of the plant in Day-1.
• Take 100 cc of the 100 ppm GA solution.
• Take one buffer tablet of pH = 9.00, crush it using a mortar and pestle and add it to the GA solution.
• Take a 10.00 cc graduated pipette and add 2.00 ± 0.05 cc of the 100 ppm GA solution (pH = 9.00) to the first five plastic cups labelled as 100 ppm each day for a period of 10 days.
• After 10 days, take a ruler and a thread to record the shoot height.
• Repeat steps 5-8 for other GA solutions – 200 ppm, 300 ppm, 400 ppm, 500 ppm and 600 ppm. Use 100 cc tap water instead of GA solution for control (0.00 ppm).
• Repeat Steps 1-9 using the other two buffer tablets – pH = 7.00 and pH = 4.00.

For control, distilled water – 0.00 ppm of GA solution was used. The same procedure was followed.

Key

ISH = Initial shoot height in ± 0.05 cm (Shoot height measured in Day-1)

FSH = Final shoot height in ± 0.05 cm (Shoot height measured in Day-10)

DSH = Difference in shoot height (± 0.10 cm) = Final shoot height (± 0.05 cm) – Initial shoot height (±0.05 cm)

PI = Percentage increase in shoot height = \(\frac{DSH}{ISH}\) × 100