Xuejun Dong, NDSU Central Grasslands Research Extension Center
Table of Contents
Leaf water
potential-water content relationship
Leaf
photosynthesis-light relationship (CRP site)
Leaf photosynthesis-light relationship (grazing intensity site)
In the CGREC 2002 Annual Report, we introduced the proposal, Using an ecosystem model for range management in the Missouri Coteau of North Dakota and some related fieldwork we conducted under this theme. This spring the proposal was approved as a Center project. This past summer we completed physiological measurements in order to understand several essential components of the plant sub-model. The purpose of this report is to explain the field results in relation to range ecosystem modeling and management.
The main contention we had when organizing this research is that knowledge of ecosystem dynamics can aid the management of the rangeland. There are different starting points available in terms of ecosystem modeling, however, a focus in plant eco-physiology is beneficial because research in this area has made great progress in recent decades worldwide. Major achievements in plant physiology were made in the 1970s, and the progress of computer technology is expediting the integration and application of the biological knowledge. This is to say we have a relatively good theory about the functioning of the plants at the individual or organ level (the level addressed mainly by eco-physiology). One of the successful formulations of the current knowledge of plant and crop eco-physiology is found in the Hurley Pasture Model, on which our research project is primarily based.
One may ask, as man is now potentially able to manipulate genes of living organisms that are the ultimate control of various biological traits, including traits that are important to agriculture, are studies on the individual and organ level still necessary? I think the answer is yes. Although genes in plants have been identified to be specifically responsible for particular diseases and other traits, many production related traits are more likely to be controlled not by a single gene but by an array of genes, or may be not directly controlled at the gene level at all. For example, under a high light and high temperature environment, cool-season grasses usually don’t perform as well as warm-season grasses. Physiologically, this is mainly due to the fact that the cool-seasons tend to have a much higher rate of photorespiration than the warm-seasons, which greatly reduces net photosynthetic efficiency. This part of respiration happens in association with photosynthesis itself; it is in addition to the so-called dark respiration that is the basic and necessary energy expenditure to support plant life. Although it is theoretically possible to increase the cool-seasons’ photosynthetic efficiency by locating and manipulating genes that are directly responsible for photorespiration, the possibility seems slim in the near future. If some success is possible from the laboratory gene “operation” in the future, for production agriculture, more individual and organ level studies concerning the eco-physiological consequences of the genetic modifications will be necessary because the individual level is more directly involved with the management activities of the grasslands or croplands.
A successful application of the framework of Hurley Pasture Model requires accurate experimental and observational input. The model has over 60 ordinary differential equations. To overcome the complexity of the model, approximations have to be made. For example, coefficients of some physiological processes are assumed to follow simple mathematical relationships, which may be estimated from experimental data. Although not all the model parameters are estimable experimentally, good information on important processes such as photosynthesis, nitrogen uptake, and water relations are necessary for the application of the model. In fact, from a mathematical point of view, the inclusion of many differential equations into one ecosystem model must require a strong empirical (experimental) input, because the behavior of a single theoretical equation (for example, in some bioeconomical or population biology models) can be extremely complex. This year’s fieldwork was focused on characterizing major parameters in leaf water relations and single leaf photosynthesis. Though much work in these areas has been done in the past, the development of new equipment enables us to obtain higher quality field data (Figure 1).
 |
|
Figure 1. Li-Cor LI-6400 portable photosynthesis system |
Leaf water potential-water content relationship
Everyone has some sort of experience with the importance of water to plant life. We know our lawn needs water when it looks wilty and less green, we also notice how turgid grasses are after a good rain. For an ecosystem model to describe the dynamics of water, quantitative physiological data is required. We collected data on the relationship between leaf water potential and water content for the widest possible range obtainable experimentally. From these relationships we deduce important water relations parameters. Here, leaf water potential is measured as a unit of pressure, similar to measuring the tire pressure on our cars. It is more convenient to measure tire pressure using a gauge than directly measuring mass or volume of air in the tire; the same is true for measuring plant water in the field. At any moment, total water potential of a grass plant consists of two parts: osmotic and pressure. A fully turgid leaf has a total water potential of near zero because the osmotic component (negative) and pressure component (positive) cancel each other. When the leaf starts to lose water, which is inevitable under high sunlight, both components decrease but the pressure potential drops at a faster speed. As water loss continues, pressure potential becomes too low to support the turgidity of leaf cells and the leaf becomes wilted. The leaf water potential under this situation is the point of turgor loss. Near this point, not only do leaf cells experience a hydrologic difficulty, but other physiological and morphological syndromes may occur as well. In 2002, we noted Kentucky bluegrass leaf folding and concomitant stomatal closure at a leaf water potential of about –23 to –25 bar (1 bar ≈ 14.7 psi) with the progress of drought stress (see 2002 CGREC Annual Report Page 19). This year’s results suggest that the turgor loss point in this plant is at about this water potential range. The point for the more drought-adapted western wheatgrass is more negative, however.
Of the two components of leaf water potential, the osmotic part is widely described by a linear equation, which is called the osmotic line. One study in Hungary on semi-arid grasslands details the osmotic lines for range plants. However, reports on the pressure potential for range plants with narrow and small leaves, especially in the Northern Great Plains, are rare (Jim Romo, personal communications, University of Saskatchewan). Our data provides an estimate for the pressure potential function. Although accuracy is limited due to the difficulty of measuring water while plant leaves are at a higher water content range, this will to some extent ameliorate the reliance on leaf water relations data from other cultivated crops, most of which have different leaf morphology and physiology than the wild grasses. Another finding of this year’s water relation study is that the maximum leaf osmotic potentials (obtained when leaves are fully saturated with water) of both Kentucky bluegrass and western wheatgrass tend to be lower on the heavily grazed pasture than on the moderately grazed pasture and the exclosure. This indicates the existence of osmotic adjustment, suggesting that plants in the heavily grazed pasture may have experienced more drought stress. This supports our observation in 2002 that under heavy grazing, soil-plant water status can be worse than with moderate grazing or no grazing.
Leaf photosynthesis-light relationship (CRP site)
The second focus of field measurements this summer was to characterize the photosynthetic parameters of range plants. The measurements were made in both the grazing intensity site at the CGREC and the Conservation Reserve Program (CRP) site south of the Center.
Everyday, we eat vegetables, cereals and meat, etc., all of which ultimately comes from photosynthesis of green plants. Photosynthesis is an energy conversion system: it converts solar energy into chemical energy, and uses this energy to fix carbon dioxide from the free air to form carbohydrates. In our measurements we are not concerned about the physical-chemical mechanisms involved, but what single leaves are doing as a whole in terms of photosynthesis. In particular, we want to know how photosynthesis of a single grass leaf responds to light. Like the relationship between leaf water potential and water content, the photosynthesis-light relationship is not a new research topic either. However, for agricultural applications, a site and species-specific determination of this basic relationship is important. Assuming all other aspects of the environment (temperature, humidity, etc.) remain unchanged, in darkness, photosynthesis itself is zero. With a slight increase of light, photosynthesis will increase almost instantly and behaves somewhat linearly within a narrow range of lower light levels. This linear relationship is called photosynthetic efficiency, which is one of the parameters for describing single leaf photosynthesis. As light intensity further increases, the photosynthetic rate will still increase but at a slower rate. Eventually, the further increase of light will not increase photosynthetic rate and photosynthesis reaches the light-saturated level. This is the maximum rate of photosynthesis, and is another parameter for describing leaf photosynthesis. In plant physiology, photosynthesis is closely related with stomatal conductance. Plant leaves are porous structures with numerous tiny openings called stomas located at the upper and /or lower surfaces. Stomatal conductance is a macroscopic measure of how fast water or carbon dioxide transfers through the leaf epidermal layer. Conductance for carbon dioxide is lower than for water because each of the molecules of the former is heavier. It’s apparent that an appropriate degree of stomatal opening is essential for photosynthesis to happen.
The purpose of photosynthetic measurements at the CRP site is to characterize the photosynthetic capacity of three species (alfalfa, tall wheatgrass and intermediate wheatgrass) and estimate photosynthesis parameters useful for modeling of the grass-legume dynamics in this area. The measurements were made on six clear or mostly clear days in mid-June, mid-July and mid-August. The growing season of 2003 was characterized with a wet spring and an early summer with progressive drought stress. In June, all the three species had high rates of photosynthesis and stomatal conductance. With the progression of the season and the gradual increase of drought, species responded differently. In July, high photosynthesis was measured as in June in three species, but stomatal conductance decreased in intermediate wheatgrass. Also leaf water potential in intermediate wheatgrass was lower. In August, when it was very dry, alfalfa regrew and maintained a high photosynthetic rate though stomatal conductance and leaf water potentials decreased somewhat; but in the two grasses, especially intermediate wheatgrass, photosynthetic rate and stomatal conductance dropped. In August, for all three species, leaf vapor pressure deficit increased appreciably. Alfalfa may have avoided drought stress by possessing a deeper root system and tall wheatgrass avoided drought stress by having rigid leaves. We see from these physiological measurements that intermediate wheatgrass is more sensitive to drought. A high drought resistance and high photosynthetic rate may be translated into high production, however, production depends on several other factors also, such as phenology, allocation of photosynthate, etc.
Leaf photosynthesis-light relationship (grazing intensity site)
In the grazing intensity site we measured photosynthesis-light response curves for two species: Kentucky bluegrass and western wheatgrass. In the Hurley Pasture model, the same parameters are used to describe leaf photosynthesis, and it is necessary to check the variability of the parameters in our grazing system and interpret the results in an ecological setting. This is the main purpose of this year’s photosynthesis measurements at the grazing intensity site. We observed that with the progress of season and also drought, photosynthesis and stomatal conductance decreased in both Kentucky bluegrass and western wheatgrass from June to August. In the very dry August, measurable Kentucky bluegrass plants were only available in the exclosure and for the most part both photosynthetic rate and stomatal conductance were very low. This provides estimates for minimum stomatal conductance obtainable in the field in our grazing trial (in the measurement of 2002, this estimate was not obtained because of the failure of the leaf temperature sensor). On the other hand, photosynthetic rate and stomatal conductance of western wheatgrass, while decreasing in the dry August conditions, remained higher than that of Kentucky bluegrass.
From the first inspection of the data, the effects of grazing treatments on photosynthesis response curves seem unclear. However, in several cases, plants (of both species) from the moderately grazed pasture showed lower photosynthetic rates and stomatal conductance compared to those on the heavily grazed pasture or the exclosure. This is a little surprising, because previous studies have shown that forage production in the moderate grazing treatment is better than either the non-grazing exclosure or heavy grazing treatment. So, intuitively, we suppose leaf photosynthesis may be higher on the moderate grazing treatment. However, as the measured results show unexpected tendencies, there must be a reason that needs careful investigation. Based on measurements this year, we will need to consider other factors, including leaf nitrogen content, leaf soluble carbohydrate concentration, and biomass data to understand these results.
A drought study in the grazing intensity site began this spring (Don Kirby, Paul Nyren and Ryan Limb). Measurements were taken in the drought plots (receiving 70% of average rainfall) and the control plots (receiving natural rainfall). From the photosynthesis data, however, the effects of drought are not clear for this first year. We will continue to monitor the field plots next year, but we hopefully will have a bigger gas exchange chamber so that an area of grassland rather than several individual leaves of a grass plant will be measured for carbon dioxide fluxes.
Ecological changes on rangelands are to a large extent the results of eco-physiological processes that happen at a shorter time scale (seconds, minutes, hours, etc.). Considering the many challenges for the uncertain future, for example, the changing environment and genetics of the biological organisms, the marriage of the science of range management and eco-physiology will become more and more important, and mathematical modeling is an effective medium for this integration. The development of computer technology facilitates the transfer of the modeling tools from the hands of mathematicians and physicists to rangeland researchers. The Hurley Pasture Model is perhaps a good example of this.
In 2002 and 2003, our field measurements have been focused on the aboveground processes, especially leaf physiology. In the next year, we will consider both the aboveground and the belowground processes with a focus on carbon and nitrogen dynamics in a grazing system. The work will be in co-operation with scientists from the Department of Animal and Range Science, and the Department of Soil Science, NDSU, and USDA-ARS, Mandan, ND.
Acknowledgments
The collaborators of this year’s fieldwork are Paul Nyren, Director/Range Scientist; Bob Patton, Assistant Range Scientist; Brian Kreft, Livestock Specialist; and Anne Nyren, Administrative Officer at the CGREC, NDSU; and Don Kirby, Professor; and Ryan Limb, graduate student at Department of Animal and Range Science, NDSU. We are preparing three journal papers based on this year’s measurements.
| NDSU Central Grasslands Research Extension Center |
| Home | 2003 Annual Report |