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Water RelationsName________________________

Water plays an important role in plants. For many, water comprises 90% or more of their weight. It serves to dissolve and bring in minerals from the soil, it is the solvent for sugars from photosynthesis being carried throughout the plant, it maintains the turgor pressure in the cells, and it cools the leaves as it evaporates. Today you will investigate some of the roles of water in plants.

Hydroponics

Observation. Plants are usually seen with their roots in soil. Soil probably provides the nutrients needed for plant growth.

Question. When plants have no soil available with the usual minerals bound to clay or humus particles, how much mineral nutrition is needed?

Hypothesis #1: Pure water is ideal for plants.
Hypothesis #2: A certain amount of mineral nutrients is ideal for plants.
Hypothesis #3: Plants can take as much mineral nutrient as you can give.

Prediction. Plants grown under various nutrient concentrations should show varying degrees of plant growth and development.

If H1 is true, then plants will grow best, when given distilled water.
If H2 is true, then plants will grow best, when given some intermediate concentration of nutrients.
If H3 is true, then plants will grow best, when given the highest concentration of nutrients.

Experiment: Earlier in the semester, you planted radish seeds in perlite in plastic cups and added fertilizer at different concentrations until the cup was almost full. Now it is time to examine the plants to see the effect of different concentrations of nutrients in water on plants.

Locate the four cups that belong to you. They should be labeled 10x, 1x, 0.1x, and 0x. These numbers represent the concentrations of 20-20-20 fertilizer relative to the normal (1x) 12 grams per liter. Each solution was also given 1mL/L of MS-micronutrient Stock Solution.

Relative
Concentration
Grams per
Liter 20-20-20
Percent
10x12012
1x121.2
0.1x1.20.12
0x00

If there are more than five plants in a cup, thin them down to the five most typical. Examine the plants and record the following information. Please note, in filling in the charts, do not put a 0 when there is no plant to measure! Use an X instead!

 Concentration
 10x1x0.1x0x
# Germinated Plants        
Height of Plants (cm)        
round to nearest whole cm        
(Record for Each Plant)        
         
         
Average Height of Plants Plants (cm)        
Number of True Leaves        
(Record for Each Plant)        
         
         
         
Average Number of True Leaves        
Hypocotyl Color        
Cotyledon Color        
True Leaf Color        

Analysis: Calculate the averages requested in the table above. Circle the best colors!

Decision: Use the data from the experiment and the analysis above to decide about each hypothesis. Explain the basis of your decision.

HYPOTHESIS #1: Pure water is ideal for plants

  is rejected       cannot be rejected   because plants in 0x concentration:

were   taller       shorter   than plants in other concentrations.

had   more       fewer   leaves than plants in other concentrations.

HYPOTHESIS #2: A certain amount of mineral nutrients is ideal for plants

  is rejected       cannot be rejected   because plants in an intermediate concentration:

were   taller       shorter   than plants in other concentrations.

had   more       fewer   leaves than plants in other concentrations.

HYPOTHESIS #3: Plants can take as much mineral nutrient as you can give.

  is rejected       cannot be rejected   because plants in 10x concentration:

were   taller       shorter   than plants in other concentrations.

had   more       fewer   leaves than plants in other concentrations.

-   /36

Effect of nutrients on plant tissue

Observation. You have observed that plants do not grow well in high concentrations of nutrients. It might be easy to understand why some nutrients are needed and why distilled water would not support optimal growth.

Question. Why did high concentrations of nutrients inhibit growth? From your study of water relations with cells, you might know that distilled water is hypotonic to cells and leads to turgor pressure. Perhaps the high nutrient concentration inhibited growth by destroying turgor pressure. Was the 10x solution strongly hypertonic?

Hypothesis. The 10x fertilizer solution was strongly hypertonic.

Prediction. If the 10x fertilizer solution is hypertonic, then the plant tissue will lose water and therefore lose weight, when placed in the solution.

Experiment. Rinse the four cups from Hydroponics and refill them 2/3 full with the four fertilizer solutions: 10x, 1x, 0.1x, and 0x. Be sure to label the cups clearly!

Cut four slices of equal thickness (3-5mm) from a potato. Stack the slices and trim them so that all peel is removed. Continue to trim the stack of slices until the slices are of equal dimensions and will fit in the bottom of the cups.

Blot each slice dry with a paper towel. Weigh each slice individually keeping track of which slice is which. Put one slice in each cup of fertilizer solution.

 Fertilizer Concentration
 10x1x0.1x0x
Initial Weight of Slice (g)        

Wait one hour but swirl the cups periodically during the hour.

Blot each slice dry with a paper towel. Weigh each slice again and record as before:

 Fertilizer Concentration
 10x1x0.1x0x
Final Weight of Slice (g)        

Other Notes:
In trying to assess crispness, attempt to break each slice in half. Crispy slices break cleanly but with some force required, flaccid slices bend easily but do not break cleanly.

 10x1x0.1x0x
Crispness: crispy
moderate
flaccid
crispy
moderate
flaccid
crispy
moderate
flaccid
crispy
moderate
flaccid
Color: mod blue
lt blue
white
mod blue
lt blue
white
mod blue
lt blue
white
mod blue
lt blue
white

Discard the potato slices, peels, and perlite into the compost area. Discard the cups in the trash.

-   /16

Analysis: Calculate the percent weight change for each slice using the formula:

(final weight - initial weight) x 100
initial weight

Fertilizer Concentration
10x1x0.1x0x
Percent
Weight Change
______% ______% ______% ______% Round to
whole percent!

The concentration that is most hypotonic is _____x.

The concentration that is most hypertonic is _____x.

The most nearly isotonic concentration is _____x.

Decision:

The hypothesis: "The 10x fertilizer solution was strongly hypertonic"
is:   rejected       not rejected  

The effect of water level

Observation. Plants need water to survive.

Question. In a typical soil, how much water is needed?

Hypothesis. The more water is available, the better it is for the plant.

Prediction. Plants given unlimited water supply will grow more strongly than plants given a more restricted water supply.

Experiment. Earlier in the semester you planted radish seeds in pots. The pots were placed inside other empty pots to elevate them, and the stacks of pots were placed in a flat filled with water. Locate your pots in the greenhouse. If there are more than 5 plants in a pot, thin to the five most typical. Examine the plants and record the following information (remember when to use X!):

 Number of Outside Pots
 18120
# Germinated Plants      
Height of Plants (cm)      
round to nearest whole cm      
(Record for Each Plant)      
       
       
Average Height of Plants Plants (cm)      
-   /18

 Number of Outside Pots
 18120
Number of True Leaves      
(Record for Each Plant)      
       
       
       
Average Number of Leaves      
Widest True Leaf (cm)      
round to nearest whole cm      
(Record for Each Plant)      
       
       
Average Widest True Leaf (cm)      
Hypocotyl Color      
Cotyledon Color      
True Leaf Color      
Soil Moisture soggy
moist
dry
soggy
moist
dry
soggy
moist
dry

Analysis. Calculate the averages requested in the tables above. Circle the best colors.

The tallest plants were found in:   soggy       moist       dry   soil.

The plants with the most leaves were found in:   soggy       moist       dry   soil.

The plants with the widest leaves were found in:   soggy       moist       dry   soil.

The plants with the best colors were found in:   soggy       moist       dry   soil.

Decision.

The hypothesis: "The more water is available, the better it is for the plant"

is:   rejected       not rejected  

Discard the plants and soil in the compost container. Rinse out the pots and leave them in the designated area.

-   /26

 

This page © Ross E. Koning 1994.

 

 

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Send comments and bug reports to Ross Koning at koningre∂gmail⋅com.