How Does Cattle Grazing Affect the Growth of Grass Leaves?
Preliminary Report

Xuejun Dong, Paul Nyren, Bob Patton, Brian Kreft and Anne Nyren, NDSU Central Grasslands Research Extension Center



Table of Contents

Introduction

Specific leaf area and its two components: leaf thickness and density

Leaf osmotic potential and soluble carbohydrate content

Maximum leaf photosynthesis and leaf nitrogen content

Summary


Tables

Table 1

Table 2


Figures

Figure 1

Figure 2

 

 

Related journal publication: Applied Ecology and Environmental Research. 2011. 9(4): 311-331.




Introduction


Leaf properties such as density, thickness, and chemical composition influence whole plant survival and metabolism. However, systematic measurements of these leaf attributes on rangelands with animal grazing is not well studied. In 2002 and 2003, we conducted a case study on this topic on two dominant cool-season grasses, western wheatgrass (Pascopyrum smithii (Rydb.) A. Love) and Kentucky bluegrass (Poa pratensis L.), growing in North Dakota rangelands. Western wheatgrass represents drought-adapted species, while Kentucky bluegrass represents drought-sensitive species. The study was used to answer this question: given a unit amount of dry matter, how would range plants develop their leaves under the influence of prolonged animal grazing? The study was focused mainly on the following three aspects: (1) specific leaf area and its two components of leaf thickness and density; (2) leaf osmotic potential and soluble carbohydrate content; (3) maximum leaf photosynthesis and leaf nitrogen content.



Specific leaf area and its two components: leaf thickness and density


Specific leaf area is defined as the amount of leaf area per unit of leaf dry matter. It can be used as an indirect measure of several basic leaf processes, such as leaf-water relations, photosynthetic capacity and growth potential, etc. From experience, we may expect that the two species might produce leaves with higher specific leaf area under heavy grazing, compared with non-grazing, because plants under heavy grazing usually are more tender and more nutritious. However, as shown in Tables 1 and 2, the responses of the two species are varied. Both species had similar leaf density (avg=0.315 g cm-3, p=0.186) but western wheatgrass had thicker leaves overall (0.27 vs 0.19 mm, p<0.0005). Leaf density in both species was lowest early in the growing season and increased steadily through the season. Leaf thickness was highest early in the season in both species and decreased steadily as the season progressed. In western wheatgrass, specific leaf area did not respond to grazing (Table 1). Leaf thickness and density tended to increase and decrease, respectively, with grazing only in August. Because specific leaf area can be expressed as: specific leaf area =1/(leaf density x leaf thickness), the trend found in August for western wheatgrass can be interpreted as: with the increase of grazing, the increase of leaf thickness and the decrease of leaf density cancelled the effect of each other and thus the specific leaf area did not change. In Kentucky bluegrass, specific leaf area increased significantly in both the June-July and August periods, but not in September. The significant increase in specific leaf area in the June-July period can be interpreted as the compound effect of two simultaneous non-significant decreases (density and thickness). The significant increase of specific leaf area in August was mainly caused by a major decrease in leaf density.



Leaf osmotic potential and soluble carbohydrate content


Osmotic potential measures the average solute concentration in the membrane-bounded cellular compartments of plant leaves. The decrease of leaf osmotic potential in field crops can be the result of combined effects of (a) increased photosynthesis; (b) decreased requirement for photosynthate (by other organs such as fine roots); (c) increased accumulation of osmotic-active substances (soluble carbohydrates, amino acids, etc) which help plants absorb water from the drying soils. Leaf osmotic potential at full turgor can be determined through the construction of a pressure-volume curve (Figure 1). Total leaf water potential 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 bright 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 turgor loss point is reached. If water loss continues over the turgor loss point, leaves may experience physiological difficulty and a linear relation exists between the total water potential (with only osmotic component) and water content. Extrapolate the osmotic line to the vertical axis (Figure 1), inverse the intercept value and make it negative, we have the osmotic potential at full turgor as shown in Tables 1 and 2. As expected, especially during the actively growing periods, a decreased osmotic potential was observed under heavy grazing (in both June-July and August for western wheatgrass and only in June-July for Kentucky bluegrass). In August, a significantly lower osmotic potential was observed in western wheatgrass under heavy grazing (compared with non-grazing) despite the non-significance in soluble carbon content, suggesting that substances other than soluble carbon are mainly responsible for lowering the osmotic potential. However, Kentucky bluegrass did not show a decreased osmotic potential (with grazing) in August although the measured soluble carbon was significantly higher with heavy grazing compared with the non-grazing treatment. This may suggest the soluble carbon was not osmotic-active and/or other substances (such as amino acids) might not be involved in the osmotic-regulation (increase of cellular solute concentration to decrease leaf water potential and facilitate water uptake) of Kentucky bluegrass at this time of the year.

 


Maximum leaf photosynthesis and leaf nitrogen content

 

Figure 2 shows typical light response curves in western wheatgrass and Kentucky bluegrass, in which the maximum photosynthetic rate is attained at high light. Consistent with many other studies on the relationship between leaf nitrogen and photosynthesis, western wheatgrass in our study had both a higher leaf nitrogen content and higher photosynthetic rate, compared with Kentucky bluegrass. We had expected that heavy grazing would result in an increase in both maximum photosynthetic rate and leaf nitrogen content. However, for both species the measured results in the photosynthetic rate did not show significant difference between grazing and non-grazing treatments (Tables 1 and 2). Heavy grazing tended to decrease leaf nitrogen content in western wheatgrass but the difference was significant only in August (Table 1); on the other hand, heavy grazing tended to increase leaf nitrogen in Kentucky bluegrass but only significantly in August (Table 2). We don’t have a reasonable explanation on the grazing related change in leaf nitrogen in the two species. For the case of photosynthesis, a more variable parameter, it is possible that the field measurements (made on two consecutive days, both mostly clear) might fail to capture the real differences that existed between grazing treatments.

 


Summary


(1). Producing leaves with higher specific leaf area enables the construction of more leaf area per unit of leaf mass, which is a typical strategy of fast-growing species. On the other hand, increased specific leaf area may parallel a reduction in leaf’s drought resistance. Our results predict that, compared with the drought-adapted western wheatgrass, Kentucky bluegrass in this grassland may have become more sensitive to water with the long-term animal grazing: that is, with the aid of changed leaf construction, it may exhibit a faster growth under high precipitation; however, under drought Kentucky bluegrass plants may wilt more easily, compared with plants that have not been or less subjected to long-term grazing. (2). The interpretation of the measured value of osmotic potentials involves several aspects on the carbon source-sink relations, which are not definitive based on our field measurements alone. However, our data revealed that grazing increased the osmotic-regulation for longer periods of time in western wheatgrass than in Kentucky bluegrass in the 2003 grazing season. Under drought, osmotic-regulation might have limited value to Kentucky bluegrass, because this species is intrinsically drought sensitive. Different from the drought “avoiding” western wheatgrass, Kentucky bluegrass’s strategy is to “escape” the drought: under moderate water stress, leaves of Kentucky bluegrass may partly fold and this is accompanied with reductions in leaf water potentials; with severe drought, most or all leaves dry out and the plants are in a “dormant state” until the next rain. (3). To what extent grass leaves produced under heavy grazing employ a higher photosynthetic rate is not clear from our field measurements in 2003. This problem will be considered later this year by analyzing the relevant 2004 data.

 


NDSU Central Grasslands Research Extension Center

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