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


Whole-plant physiology: Growing four grass species in a greenhouse


 

Different perennial plant species have different survival strategies, with some species having deep and/or horizontally extensive roots and other species relying on the upper soil layers for water and nutrients. This reality makes them difficult to monitor, both physically and physiologically, during field studies. By contrast, in a greenhouse, both the above-ground and below-ground parts of plants can be placed under complete control, allowing researchers to better obtain a complete picture of plant growth and gas exchange. In this year’s greenhouse experiment, we planned to estimate growth and nitrogen uptake in response to drought in frour grass species: western wheatgrass (Agropyron smithii), Kentucky bluegrass (Poa pratensis), blue grama (Bouteloua gracilis), and switchgrass (Panicum virgatum). Five replications were made. We planned to harvest plants a couple of times sequentially within several weeks. The drought treatment groups received water every five days, while the control groups were watered once every other day. The plants’ growth and nitrogen uptake rates can be estimated from biomass harvest and leaf nitrogen content analysis. The experiment progressed for about seven weeks, with three harvests (see Figure 2 for the experiment design). The biomass samples will be weighed and analyzed this winter. In this section of the report, we will show some diagnostic results of leaf water status and gas exchange rate.


The behavior of western wheatgrass was not what we had expected. Although it is believed to be well adapted to drought, water potential measurements (Table 2) show that it had the most sensitive response to drought among the four species studied. Also, about four weeks into the experiment, several plants of western wheatgrass in one or two drought treatment sections died and showed signs of “damping off” (Rick Bohn, personal communication, August 2005). This did not occur in the control group, nor did it happen in the other three species.

 

Table 2. Predawn leaf water potentials for leaves of the four grass species used for the greenhouse growth study. Data were measured in the early morning (2:00 am to 5:00 am) of September 6, 2005. The differences between the drought treated and the control samples for each species are indicated as “NS” (not significant) or “*” (significant at p=0.05).

Species

Replicate

Control (MPa)

Drought (MPa)

Difference

western wheatgrass

10

-0.67

-1.70

*

Kentucky bluegrass

10

-0.35

-0.44

NS

blue grama

10

-0.32

-0.54

NS

switchgrass

10

-0.16

-0.81

*


We constructed eight curves describing relationships between photosynthesis and leaf internal CO2 concentration and used a method by Sharkey (1985) to calculate whether stomates posed a significant limitation to photosynthesis (we especially expected this to happen for plants in the drought group). The equation used is:


ole.gif

where Ao is the photosynthesis observed when internal CO2 is manipulated to the same level of the ambient, and A is the photosynthesis under normal conditions. We did not observe any sign of stomatal limitation of photosynthesis, because the calculated Ls is less than 3%.

 

Next we measured midday leaf stomatal conductance before and after watering the pots following a five-day drought cycle. Western wheatgrass had the most sensitive response, with stomatal conductance increasing from a closure (of 0.02 mol m-2 s-1) to wide open ( 0.18 mol m-2 s-1) (Table 3). On the other hand, Kentucky bluegrass responded to some extent but not significantly.

 

Table 3. Changes in the midday leaf stomatal conductance for the drought treatment groups of the four grass species before watering (September 11) and after watering (September 12), following a five-day drought period without watering.

Before watering

After watering

Species

N

Conductance

(mol H2O m-2 s-1)

N

Conductance

(mol H2O m-2 s-1)

Difference

(p value)

western wheatgrass

5

0.02

3

0.18

0.17

(p<0.0001)

Kentucky bluegrass

5

0.09

3

0.17

0.08 (p=0.096)

blue grama

5

0.13

3

0.13

0.00 (p=1)

switchgrass

5

0.04

3

0.10

0.05 (p=0.55)


From our experience of measuring the leaf-water potential and gas exchange of western wheatgrass and bluegrass in the field, we expected a very favorable measure of both conductance and water potential in western wheatgrass, even when the soil was quite dry. It is possible that, under field conditions, the deep root system in western wheatgrass helped it absorb water even when the surface soil became dry. In the laboratory situation, when the western wheatgrass was grown from seeds, it showed a sensitive response to drought. What does this mean? Is the growth medium not favorable for this species? Or, perhaps, were all individual western wheatgrass plants affected by a fungus (damping off) that may not have been visible? Or does this grass intrinsically have a limited capacity to survive drought if grown from seeds? To answer these questions and better understand the behavior of this species in the greenhouse experiment, we will need to analyze biomass and nitrogen content.


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

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Next section   --   Soil microbial biomass: an estimate for the Missouri Coteau mollisols


NDSU Central Grasslands Research Extension Center

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