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Basic Plant and Soil Process Measurements for Range Ecosystem Modeling and Management—Updates for 2005


Leaf physiology: Three forage species from the CRP (Conservation Reserve Program) study site



For two years, our research has emphasized leaf gas exchange rate in a grazing and pasture system. This year we used additional data to test a hypothesis raised from our study in 2003. We observed that tall wheatgrass had less control of stomatal conductance under changing light conditions than did alfalfa (Medicago sativa). Tall wheatgrass tends to establish dense stands in moist soils. We hypothesized that this less control of water loss may contribute to the limited drought tolerance of tall wheatgrass in dry soils. Although our study in 2003 was based on comprehensive field measurements, the light-response curves were constructed using a rapid method that did not wait for the leaves to fully equilibrate with the chamber environment. This method is potentially problematic because stomates need about 15 to 30 minutes to equilibrate under the changed conditions of, say, light, humidity or CO2 concentration. To address this concern, this year we used a “slow” method to allow leaves sufficient time (15 to 40 minutes) to equilibrate after a chamber condition was altered (by altering chamber conditions we would have a better chance of understanding what the plants are doing and why). We chose August 23, 2005, a very windy and mostly clear day, to construct stomatal-light response curves. We thought that if tall wheatgrass really tended to keep its stomates open even under conditions that are not beneficial, then it might do so on this kind of day. Our expectation was confirmed (see result in Figure1).

 

Figure 1. Stomatal-light relationship for alfalfa and tall wheatgrass measured in a very windy (wind speed 25 to 30 mph) afternoon (1:00 to 3:00 pm), August 23, 2005.

 


As stated elsewhere (Campbell and Norman, 1998. p243), leaf stomatal conductance is also sensitive to increased CO2 concentration. We constructed 30 photosynthesis-CO2 response curves on clear days from mid-August to mid-September, 2005. During the process of taking these measurements, the chamber CO2 concentration underwent a series of changes from the ambient level of 350 ppm to about 1200 to 1500 ppm. We hypothesized that stomatal conductance of tall wheatgrass leaves might decrease less as the CO2 concentration rose. This hypothesis was also confirmed (see Table 1). Intermediate wheatgrass (Agropyron intermedium) also had a lower percentage of reduction, which we believe may be related more to leaf senescence, since this species matures earlier in the growing season.


Table 1. Stomatal response to increased CO2 concentration for leaves of three forage plant species grown in the Conservation Reserve Program (CRP) study site near the CGREC. The response is expressed as percent reduction in stomatal conductance between an exposure to the CO2 concentrations of 350 ppm and 1200 or 1500 ppm. The measurements were made on fair to clear days (from about 10:00 am to 4:00 pm) from August 14 to September 20, 2005.

Plant species

Number of measurements

Mean % reduction of stomatal conductance

tall wheatgrass

9

44.5a

intermediate wheatgrass

11

48.1a

alfalfa

10

66.8b

Significant differences (p=0.05differences at family error rate of =0.05 () to 40 pmin stomatal conductance between an exposure to the CO2 concenrtations of 3) are indicated with different letters.


Stomates provide the major pathway through which water vapor inside leaves evaporates into the drying air. The physiological evidence we have gathered for tall wheatgrass positively supports the notion we made earlier about this species. The tendency of this species to maintain open stomates would likely make it a big user of soil moisture, which poses a problem in semi-arid areas where water resources are scarce. However, this species can be used to maintain the water level in areas where soil salinity is a problem.

 


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