Effects of wind on stomatal conductance and transpiration rate of Betula papyrifera

Previous studies on transpiration rates of plants suggest that in the presence of wind, transpiration rates can increase as the boundary layer is removed. A branch of Betula papyrifera was set up as a potometer and exposed to windy conditions and its transpiration rate measured, as well as the stomatal conductance. Although the average rate of transpiration and stomatal conductance were lower for the branch exposed to wind, there was no significant difference between the treatment and control group data sets to suggest a direct relationship.

Introduction Plants are able to take in water and move it throughout its systems through the process of transpiration, in which moisture is carried from the roots to the leaves to be evaporated out through the stomata. Water loss through transpiration is crucial for plant survival as it allows them to cool their temperature, as well as increase their nutrient intake as additional water is absorbed to compensate for the loss. Although most of the water absorbed from the soil is lost through transpiration, plants utilize a small percentage of water to undergo photosynthesis. The energy required to carry out both transpiration and photosynthesis is obtained from sunlight, which can have a direct effect the transpiration rate.
Plants are at a higher risk of wilting in hot, sunny weather due to the increase in rate of transpiration and lack of water abundance (Ku et al., 1977). Plants accommodate the temperature increase in warm climates by opening their stoma to release water into the atmosphere. While sunlight and temperature have well documented effects on transpiration rates, wind also has an effect on the rate of transpiration. Caldwell, in his 1970 study, found that the stomata of evergreen shrubs immediately closed when exposed to high winds, decreasing the transpiration rate. However, in plants whose stoma do not actively react to the wind, it can cause the transpiration rate to increase since the presence of wind can remove the boundary layer of water vapor on the leaf surface, causing water to be lost at a faster rate (Bovich, 2015).
In order to observe how wind can affect the rate of water loss on Betula papyrifera, a branch of the deciduous birch tree was exposed to windy conditions and the water loss was measured in comparison to a branch kept at standard conditions. It was hypothesized that the branches exposed to wind would have greater transpiration rates than the control plants, as the loss of the boundary layer would result in faster loss of water.
Materials and methods Two healthy branches of Betula papyrifera were cut from the quad area between buildings 8 and 3, and were attached to rubber tubing beneath the water surface in a basin to prevent air bubbles from entering the xylem water stream. One branch was used as the potometer for the control group, and the other was used as the experimental group to be exposed to wind. Adjacent lab students responsible for the experimental group held a household hair dryer to the branch after the initial amount of water in the pipette was measured, and continued to measure the movement of water every five minutes for 60 minutes. The initial pipette measurements for the control group was also measured, as well as at every five minute intervals for the next hour. After the measurement was taken at five minutes, the stomatal conductance was measured three times using a leaf porometer on three separate leaves, and the average was calculated. The stomatal conductance measured how open the stomata were on the leaf, taking into account the difference in the concentration of water vapor outside the leaf and inside the leaf. Twelve individual leaves were also measured to find the surface area required to calculate the average transpiration rate.
Results
Table 1 shows the calculated transpiration rates of the control and experimental group obtained from experimental measurements. The experimental group that was exposed to wind had a slower average transpiration rate than the control group (0.001832 mol m-2 s-1 vs 0.00369 mol m-2 s-1). The variance of the transpiration rate, which is the average of the squared differences from the mean, was smaller for the experimental group. The p-value was larger than 0.05, at 0.431576641.

| Control | Wind | Mean | 0.003686059 | 0.001831937 | Variance | 7.84088 x 10-05 | 1.85828 x 10-05 | P(T<=t) two-tail | 0.431576641 |
Table 1: Transpiration Rate in mol m-2 s-1

Fig 1. Graphical depiction of the average transpiration rate of Betula papyrifera, which was lower in the experimental group exposed to windy conditions.

The stomatal conductance was smaller for the wind group, at 105.5 mmol m⁻² s⁻¹ compared to the control group’s 127.4 mmol m⁻² s⁻¹. The variance was larger for the treatment group at 7831, compared to the control group’s 7076. The p-value was larger than 0.05, at 0.2448. Table 2 compares the mean and variance values of the two groups. | Control | Wind | Mean | 127.3994444 | 105.4596111 | Variance | 7075.709194 | 7830.851843 | P(T<=t) two-tail | 0.244765892 |
Table 2: Stomatal Conductance in mmol m⁻² s⁻¹.

Fig 2. Graphical depiction of the average stomatal conductance of Betula papyrifera, which was lower in the experimental group exposed to windy conditions.

Discussion At an initial glance, the collected data supports previous research performed by Caldwell on wind’s effect on evergreen shrubs, as the experimental group saw a decrease in the average transpiration rate, as well as showed a decrease in average stomatal conductance in comparison to the control group. This would suggests that the B. papyrifera compensated for the presence of wind by closing its stomata, evidenced by the decreased stomatal conductance, which resulted in a lower transpiration rate in order to control the loss of water. Stomatal conductance has been shown to be sensitive to the humidity of the air and leaf temperature (Collatz et al, 1991). However, the P-value was 0.432, greater than 0.05, which indicates that there is no statistically significant difference between the two groups, despite the recorded average transpiration and conductance levels being lover for the treatment group. Stomata of B. papyrifera have been shown to be more open at high humidity than at low humidity in previous studies (Norby and Kozlowski, 1982). However, the force of the wind expelled by a hand-held dryer in this experiment may not have been significant enough to warrant a stomatal response. It is possible that more significance between the data sets would have been obtained had the treatment group been exposed to the windy conditions for a longer duration of time. Nevertheless, under the conditions of the experiment, we were not able to provide a direct correlation between wind and the transpiration rates or stomatal conductance of B. papyrifera. In conclusion, there is no significant data to suggest that Betula papyrifera experiences a change in the rate of transpiration in windy conditions, nor is there evidence to suggest that stomatal conductance of the plant decreases in with wind. Although the average rate of water loss and stomatal conductance was lower in the experimental groups, the result of the p-values from the paired-sample t-test suggests that the two groups did not display any significance to warrant a direct cause-and-effect of B. papyrifera and its retention of water in windy conditions.

Literature cited
Bobich, E.B. 2015. Form and Function in Plants Laboratory (Bot 201L). California State Polytechnic University, Pomona, CA.

Caldwell, M. M. 1977. Plant Gas Exchange at High Wind Speeds. Plant Physiology 46:535-537.

Collatz, G. J., J. T. Ball, C. Grivet, and J. A. Berry. 1991. Physiological and environmental regulation of stomatal conductance, photosynthesis and transpiration: a model that includes a laminar boundary layer. Agricultural and Forest Meteorology 54:107-136.

Ku, S., G. E. Edwards, and C. B. Tanner. 1977. Effects of Light, Carbon Dioxide, and Temperature on Photosynthesis, Oxygen Inhibition of Photosynthesis, and Transpiration in Solanum tuberosum. Plant Physiology 59:868-872.

Norby, R. J., and T. T. Kozlowski. 1982. The role of stomata in sensitivity of Betula papyrifera seedlings to SO2 at different humidities. Oecologia 53:34-39.