1. Effect of Salinity Concentration on Water Diffusion Rates by Allium fistulosum
Ean Tucker1
, Sam Holberg1
, Dr. Caye Drapcho1
BE 4120
1. Clemson University Department of Environmental Engineering and Earth Science
Abstract Results Model
Introduction
Materials and Methods
Materials
● Tap water
● Table salt
● Food coloring
● Beaker (3)
● LED grow light
● Allium fistulosum (3)
● Scale
● Parafilm
Methods
1. The water vessels are first prepared. One beaker will contain no salt, only
tap water. The next beaker will contain a 1.5 grams of salt per 150 mL of
water. The last beaker will contain the higher 3.0 grams of salt.
2. Food dye is added to the water in each beaker.
3. The original weight of the water in the beaker is recorded.
4. Allium fistulosum is cut down to 3 inches, and added to each beaker
5. Parafilm is added to the top of each beaker to minimize the evaporation
and transpiration of water
6. A grow light is turned on for 24 hour light.
7. Every day a reading is taken by weighing the remaining water in the
beaker, and separately weighing the Allium fistulosum
8. The data is recorded and analyzed in Excel
At the end of the 120 hour research period, it was clear that the salt solute negatively affected the
growth rate of the Allium fistulosum. It was concluded that the weight of the onions grew as the
weight of the water decreased regardless of the salt concentration in the beginning of the period. The
salt concentration affected the growth rate shown by the vast differences in the weights and length of
growth of the onions. Researchers credit this varying in weights over the research period to
asbrotion of water and growth of the onion. Errors in regards to the lowering water weight were
considered in the form of evaporation in the beaker and transpiration out of the plants stem. After
graphing all three of the growth curves, it was found that the onion in freshwater was in growth
phase, the onion in the 10 ppt beaker was in stationary phase, and the onion in the 20 ppt beaker was
in the decay phase.
Due to mass diffusion coefficient (DAB) being time dependence in relation to capillary action, a
stella model was used to predict the values as they changed with respect to time. The DAB of the
onion in the 20 ppt solution had the lowest coefficient value of 0.00313 m2
/s. As the salinity
decreased, the DAB increased due to the plant’s better ability to pull up water from the soil. Higher
diffusion rates across the root membrane were found with lower density water. The DAB of the onion
in freshwater with no salt concentration was found to be 0.0101 m2
/s.
Conclusion
Acknowledgements: We would like to thank Dr. Caye Drapcho and the Clemson University
Department of Environmental Engineering and Earth Sciences for making this project possible
References
Figure 1: Experimental Design
Figure 3 shows the growth of the three Allium
fistulosum after 6 days of constant light. Onion 1
continued to grow throughout the experiment. Onion 2
(10 ppt), hit a stationary phase after 4 days. Onion 3 (20
ppt) stopped growing in mass after 3 days, and has
begun losing mass as time continues. The red food
coloring in the water was successful in visualizing some
of the water uptake by the plants roots. All three Allium
fistulosum started at the same length of plant and length
of roots. We can conclude that Onion 1 grew the most
over the time interval recorded.
A STELLA model was developed to predict how the diffusion coefficient will change with
time. For capillary action, the diffusion coefficient is time dependent; the pressure gradient
changes as concentration of water changes. A pressure gradient is the driving force in
capillary action. The STELLA model also predicts the mass of each onion and the mass of
water over time. Theoretical values for the mass of the onions and the water are produced,
predicting the weight if the onions experience continuous growth. Figure 4 shows the
developed STELLA model and Figure 5 shows the graphs produced by the STELLA model.
Figure 4: STELLA Model Developed for Predicting Diffusion Coefficient
Figure 5: Graphs for Theoretical Mass of Onion and Water and Theoretical Diffusion Coefficient
Plants absorb water and nutrients through the xylem: a tissue made up of thin tubes
located just below the surface of the plant’s stems. The molecules in this tissue attract
water molecules from the soil, so that the water is pulled upwards. This process is
called capillary action (Ludanov, 2018). Capillary action, a type of mass transfer, in
plants is accomplished through adhesion and cohesion. Adhesion, the attractive
tendency of one molecule to a different type of molecule, allows for the water from the
roots to stick to the organic tissue in the plant. Cohesion, the attractive force between
like molecules, allows for the water to stick together as it moves through the organic
tissue.
As the density of the capillary liquid increases, the more difficult the liquid suction
becomes for the plant. While there are natural salts in soil water that plants require to
grow, there are harmful levels of soil salt concentrations. As the soil salt concentration
nears that of the root concentration, there is less pull by the root tissues (Boyer, 1985).
This experiment demonstrated the effects of varying levels of soil salinity on the
growth and water absorption by Allium fistulosum. This species of bulb onion was
chosen based on its accelerated growth pattern compared to other easily accessible
plants. Three salinity levels of 0, 10 and 20 ppt were chosen based on real world soil
concentrations. The desired level of salinity in soil water is usually 6-8 ppt, so levels of
10 and 20 ppt were chosen to demonstrate the negative effect of salinity on the capillary
action of soil based plants (Steudle, 2001).
The mass of each onion and the mass of water was measured over time, as shown in Figure
2. The mass transfer rates were determined by the slope of a linear regression line. Some mass
of water was lost due to evaporation and transpiration, determined by taking the transfer rate
of water and onion mass transfer rate and subtracting them. By 120 hours, the mass of the
onion with 1.5 grams of salt was in stationary growth and the mass of onion with 3 grams of salt
was already decaying. The slope between hour 0 and hour 24 was drastic compared to the other
slopes between the other times, most likely due to a big concentration difference between the
water and the onion originally.
Figure 2: Experimental Data for Mass of Onion and Mass of Water over Time
For water transport in plants, the main mode of mass
transport is by use of capillary action. Capillary action is the
movement of liquid through thin cylindrical tubes using
cohesive and adhesive forces. The main driver of capillary
action is a pressure gradient. Equation 1 shows the diffusion
coefficient calculation for mass transfer by capillary action.
For our experiment, we estimated the theoretical diffusion
coefficient from experimental values, but we can rearrange
to determine the pressure differential. For Equation 1, K is
the absolute permeability, krl
is the relative permeability for
liquid flow, v is viscosity of the liquid, ds is saturation
difference and dpc
is the difference in capillary pressure.
Equation 1: Diffusion Coefficient for Capillary
Action
Equation 2: Jurin’s Law
1. Boyer, J. S. (1985). Water transport. Annual Review of Plant Physiology, 36(1), 473–516. https://doi.org/10.1146/annurev.pp.36.060185.002353
2. Choudhary, M. K., Karki, K. C., & Patankar, S. V. (2004). Mathematical modeling of heat transfer, condensation, and capillary flow in porous insulation on a cold pipe.
International Journal of Heat and Mass Transfer, 47(26), 5629–5638. https://doi.org/10.1016/j.ijheatmasstransfer.2004.07.016
3. Drapcho, C. 2019. Unpublished Laboratory Notes, BE 4101, Clemson University
4. Ludanov, K. I. (2018). Transpiration Mechanism of Capillary Transport the Xylem of Plants. Ukranian Journal of Physics, 59(8).
https://doi.org/https://doi.org/10.15407/ujpe59.08.0781
5. Steudle, E. (2001). Water uptake by plant roots: An integration of views. Recent Advances of Plant Root Structure and Function, 71–82.
https://doi.org/10.1007/978-94-017-2858-4_9
Equation 3: Modified Jurin’s
Law
The capillary rise can be measured by
using Jurin’s Law. Equation 2 shows Jurin’s
Law equation, but since the liquid is water,
Equation 3 can be used. For α, a constant 3.8
mm is used. Assuming the pressure is
steady-state, θ is zero. An average xylem
radius of 40 micrometers is used. It is
determined by Equation 3 that the capillary
rise at steady-state pressure is 95mm.
Figure 3: Final Growth of each Onion
1 2 3
Increasing soil salinity is a major agricultural and ecological issue that is becoming
increasingly prevalent around the world. The objective of this experiment was to
determine the effect varying levels of salinity had on the mass transfer of water into
the tissue of Allium fistulosum.
Three beakers were prepared; each with water, food coloring and their respective
amount of table salt. The plants were cut to the same size and placed in a beaker. After
allowing the covered beakers to sit under a LED grow light for six days, much higher
levels of growth were observed in the beakers with lower salt concentrations when
compared to their higher counterparts. The control beaker had the most transfer of
water into the roots with the highest end mass diffusion value of 0.0101 m2
/s. The plant
placed in the saltiest water had the lowest end mass diffusion rate of 0.00313 m2
/s.
While the growth patterns held true with the predicted outcome, researchers
acknowledge error in the form of evaporation and slight differences in initial weight
and water content.
