Humidity and Photosynthesis in a C4 Plant


Samantha Knapik, Daniel Foster


CU Boulder, Fall 2006


         Our experiment intended to test how humidity affects the rate of photosynthesis in C4 plants. In plants, water is lost and C02 is gained through stomata. Water loss is not an issue in humid environments because water is readily available. C4 plants have higher rates of respiration because they use CO2 more effectively. Humidity, in theory, facilitates a higher rate of photosynthesis because it aids CO2 availability to the Calvin Cycle. The presence of humidity allows plants to open more of their stomata at one time since the risk of H2O loss (dehydration) is lower in humid environments. Given that more stomata are open more CO2 can enter the plant and more CO2 is accessible for use by the Calvin Cycle Increased levels of humidity cause more of the plantÕs stomata to open making a higher quantity of CO2 to the plant for use in photosynthesis.

         To test our hypothesis, we measured CO2 levels in a chamber that contained a massed C4 plant and ran six CO2 consumption trials. Measurements were taken from a CO2 probe connected to a computer that measured the concentration in particles per million (ppm). Three chambers had environments with equivalently elevated levels of humidity in the form of moist paper towels. The remaining contained the control (dry) environments that had no additional humidity added. We hypothesize that the rate of photosynthesis in C4 plants will increase with an increase in the level of humidity in the surrounding environment.

         Our results are a comparison of photosynthesis and respiration between the environments. The mean rate of photosynthesis for the dry or control environment was -45.42 ppm/min/g compared to -38.60 ppm/min/g for the humid environment. The mean rate of respiration for the dry environment was 6.18 ppm/min/g compared to 4.97 ppm/min/g for the humid environment. Statistically, there were no significant differences shown for rates of photosynthesis or respiration between the dry and humid environments (p=. 26 and ____ respectively, p>.05).

         We expected to see the rate of photosynthesis increased in the humid environment because stomata openings. Our results are inconsistent with this, as we saw no significant differences between the two environments. Rabinowitz et al. 2004 conducted a similar experiment and found no difference in the rate of photosynthesis due to temperature change.  One potential problem is that not enough time had elapsed in the humid environment to considerably affect the opening of the stomata. which is based on osmoregulation of the stomataÕs guard cells. Time may not have been the sole factor in our inconsistencies; rather the H2O concentration of the environment may have played a role. There is a possibility that our environments werenÕt extreme enough to considerably affect the opening of the stomata.

         There are three logical extensions for this lab. First, you could increase the time of each trial to experimentally test if longer exposure time to humidity causes more stomata to open. If you increased humidity levels present in the three humid environments, results may differ. Finally, you could devise another procedure to test the rate of photosynthesis in the two environments.