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Effects of Air Velocity, Humidity, and Ambient Light on Transpiration

By using a manometer with water, we measured plant transpiration with respect to wind, humidity, and light. Light exposure creates a higher rate of transpiration (0.556 ). The control had 0.141 while the wind exposed sample had 0.097 and the higher humidity sample had 0.088 .


Plants require water for many metabolic functions. The transport of water involves transpiration, adhesion, cohesion, tension, and root pressure. Transpiration is the loss of water from evaporation at the leaves. Adhesion is the tendency for water to be attracted to other substances. Cohesion is the tendency for water to be attracted to itself. Tension is the stress placed on a rope. Similarly, water can also be put under tension. Root pressure is the pressurization of water at the roots which could drive the water up higher.

Part of the transport of water in plants also involves water potential, symbolized by the Greek letter psi (). The water potential is normally negative for solutions. Water tends to move to lower water potentials. Water potentials are lowered as water evaporates from a solution - concentration of solute increases.

Our experiment measures the water lost through transpiration. Evaporation of water at the leaf is replaced by water from the stem. Because the leaf lost water, its water potential is lower, and thus the stem's water wants to go to the leaf. Our experiment was designed to measure how much water would be lost during a given amount of time. We would investigate the differing rates of transpiration in windy, sunny, and humid environments.

Materials and Methods

The plants were "planted" into plastic tubing with water in it. The tubing was connected to a graduated pipette with 0.1 mL graduations. Readings were taken roughly every hour. Afterward, the leaf area of each plant was measured by tracing it on graph paper and counting the squares.

Plants were exposed to lamps, with large beakers serving as heat shields, a fume hood, a plastic bag which would contain the humidity, and the science room. Thus the plants would have the same CO2, same temperature, same humidity, and roughly the same lighting except when the variables said otherwise. Lighting in the fume hood was different than the control plant.

The lab may have had some safety considerations with insertion of a glass pipette into plastic tubing and handling of any knives for the cutting of the plant stem and leaves.


                 Area           Cumulative Water Loss (mL/m^2) 

Control       0.00763      0  14.43  22.30     30.16  38.03   47.21 

     Time:              0:00   1:00   2:00      3:05   4:20    5:35 

Floodlight      0.004      0     10     55       100                

     Time:              0:00   0:30   1:45      3:00                

Fan            0.0135      0   7.40  13.32     19.99  27.39   35.53 

     Time:              0:00   1:30   2:30      3:35   4:50    6:05 

Mist          0.00675      0   7.41  14.81     22.22  29.63   29.63 

     Time:              0:00   1:00   2:00      3:05   4:20    5:35 

Table 1: Water Loss

The time are written below each reading and lists hours : minutes elapsed. The data was collected by measuring the level of the water in a pipette with 0.1 mL markings. After the change in volume was calculated from the difference between the reading and the initial volume reading, the difference was divided by the area, in square meters, of the leaves on the plant.

The area of the leaves was determined by tracing leaves on graph paper and counting the squares that the leaves covered. The graph paper was marked in 0.5 cm increments. Most uncertainty in the quantitative results stems from errors in counting half squares.

Graph 1: Water Loss

The graph above has inconvenient markings on the x-axis because of the way Excel 3.0 handles times.

  1. The rate of water loss can be calculated by . By using the last reading, we average the water loss during the whole experiment. The average water loss for the control plant is 0.141 . For the light, fan, and mist samples, the water loss is 0.556 , 0.097 , and 0.088 respectively.
  2. Light causes an increase in transpiration because the stomata open to admit CO2 for photosynthesis. In order to admit CO2, the plant also loses water. The mist allows the plant to conserve water because the air is already holding water and cannot easily hold anymore. Thus the plant is spared from the air's tendency to desiccate the leaves. The wind causes the conservation of water because the plant notices the wind and promptly closes the stomata. Also, it could be said that the guard cells are quickly dried up, causing the stomata to close. Thus the plants in a breezy environment save water.
  3. The conditions do not alter the water potential gradient from stem to leaf of the plant. Instead, the conditions alter the gradient between the leaf and the atmosphere. By altering this gradient, the stimulus changes the other gradients. The leaf's high transpiration rate lowers its own water potential which consequently is quelched by the petiole or stem. The transport of water also reduces the water potential of the stem which gathers water from manometer.
  4. Plants reduce water loss from their leaves through a waxy cuticle and the use of guard cells around the stomata. The guard cells are able to open and close the stomata thus preventing the insides of the leaf, the spongy mesophyll, from drying out and from requiring more water.


Our data reflected the expected outcome in all cases but the wind exposed data. We had successfully predicted that the lamp exposed plant would photosynthesize and require more CO2 and consequently release more water. The humid plant also reflected our expectations because it would seem that a humid environment would prevent as much water from evaporation due to the initial presence of water in the leaves. The wind exposed sample did not reflect our expectations which said that it would lose water because of the amount of new dry air passing over its leaves. Possibly, the wind caused its stomata to close more tightly causing a low loss of water.

Established literature echoes our initial hypotheses and qualifies their analysis of the wind sample. The AP Biology Laboratory Manual for Teachers explains that the stomata may close at high wind velocities to prevent extreme water loss.

Because this experiment was performed only on one plant per group, it is hard to say how significant this data is. Furthermore, the lack of active temperature, lighting, humidity, wind velocity, water pressure requires more qualifiers to our data than would seem necessary. The high temperature of the water which served as a heat sink for our light exposed plant makes it hard to justify the added variables between the light exposed plant and the control. Similarly, the wind exposed plant had different lighting sources than the control. Furthermore, the plants were spread so far across the room that temperature equality could not be assured. Even the measuring device offers some possibility for error. Because the stems were not at the same levels relative to the water level in the pipette, the pressures from the water column on the pipette could cause additional water to be pushed into the stem. In all, the experiment cannot produce significant results without additional controls and simply more data.