by C. Wayne Cook & Joe Trlica

Colorado State University

INTRODUCTION

Plants are very important organisms in ecosystems because they capture sunrays, carbon dioxide and water and convert them to carbohydrates through carboxylation in their chloroplasts in the process of photosynthesis. For photosynthesis to occur in plants, CO2 from the air must diffuse into the leaves of plants. Through this process and additional chemical reactions carbohydrates are formed, which are energy substances used in cells. In ecosystems this is known as primary production and when the plant production is consumed by animals it is converted to animal tissue and this is commonly known as secondary production.

When carnivore animals consume other animals, energy is transferred to the third trophic level. We also have feeding animals that consume dead organic matter either plant or animal or both. These animals or organisms are known as detritus feeders and are very essential in keeping ecosystems clean and healthy.

The products produced in plants consist largely of carbohydrates and are often referred to as plant food reserves. Concentration of carbohydrates in storage organs within the plants change with the input of photosynthetic products and translocation of materials from aerial portions of the plant. The rate of carbohydrate use by plants, and export from storage to other areas of the plants, changes this concentration. Thus, any factor affecting photosynthesis or utilization of carbohydrates for respiration or growth will affect the level of plant reserves.

RESERVE CONSTITUENTS

Concentration of carbohydrates in plant storage organs changes with the input of photosynthetic products and translocation of materials from various portions of the plants. The rate of carbohydrate use and export from storage to other areas of the plants also changes this concentration. Thus, any factor affecting the photosynthesis or utilization of carbohydrates for respiration or growth may affect the level and makeup of plant reserves.

The carbohydrates of greatest importance as food reserves (available carbohydrates) are sugars, starch, dextrins (glucose residues or glucosans), inulin (fructose residues), and fructosans. The more complex polysaccharides such as cellulose, hemicellulose, and pentosans are primarily structural materials and are not used significantly as reserves.

Glucose, fructose, and sucrose are the three most important reserve constituents of plants. Glucose and fructose are reducing sugars but sucrose, or common table sugar which is a compound of glucose and fructose (disaccharide), is a nonreducing sugar. Glucose and fructose occur frequently in plant tissue as free sugars, but other monosaccharides are present only in small quantities and generally in the phosphorylated form. These sugars may be converted to other forms when excesses accumulate. Sucrose is the only disaccharide that is common in plants, and it is one of the main sugars that is transported within the plant.

Starch is a dominant carbohydrate reserve in many plant species and is formed by the conjunction of glucose units. In some grasses, starch may be almost absent in the storage organs. When present, starches are converted to soluble sugars as they are needed. Conversion of starch to sugars occurs when reserves are used for active growth. Starch appears to be the most readily available and osmotically inert reserve carbohydrate in perennial plants.

Inulin is a polysaccharide of which fructose is the repeating unit and can be compared to starch which possesses glucose as the repeating unit. Inulin replaces starch as a reserve carbohydrate in some plants, but many plants have both starch and inulin.

The carbohydrate reserve substances vary among plant species and even within a single plant species throughout the seasons and under various climatic conditions. In many grasses, fructosans are the main reserve substance. In other plants, sucrose is the most important reserve carbohydrate. In still other grassspecies, starch and dextrins are the dominant reserve sources.

Reserves are also synthesized into cellulose, lignin, organic acids, nitrogenous compounds, and a multitude of other constituents constituting cell walls and protoplasm in the plant. Thus, reserves may act as building blocks for construction of various plant tissues and organs. The reserves also serve as a source of energy for plant functions at all times.

USE OF CARBOHYDRATE STORAGE

Carbohydrate reserves are used by the plant for respiration and maybe for slight growth during the winter, and for initial growth and subsequent rapid growth during the spring, and for secondary growth in the fall. If, however, the plant is defoliated at any time during growth, reserves may be utilized for the production of new photosynthetic tissue. When plants are forced into dormancy by drought, their sustenance is dependent on carbohydrate reserves.

Plants endowed with a large capacity to store carbohydrates may be better adapted to survive heavy grazing and unfavorable environmental conditions. Also plants with the ability to rapidly build leaf tissue with current photosynthetic products may be better able to survive those stresses.

Storage Organs

Carbohydrates that are manufactured in the leaves must first be moved to places of storage and perhaps later moved to other areas of the plant to be used as reserves. Food reserves are generally associated with underground or protracted organs. The most common storage organs are the roots, tubers, rhizomes, stolons, and crowns or stem bases of herbaceous species and also the twigs of woody species. However, storage may occur temporarily in most portions of the plants. The dominant reserve substance and relative importance of reserve storage areas varies among plant species.

Enzymes Play An Important Role

The types of carbohydrates formed depend upon the enzymes present. Enzyme activity may be affected by both external and internal conditions such as the pH of the cell fluids, concentration of certain sugars, hydration of cells, temperature, light intensity, and availability of soil moisture and soil nutrients. Just as enzymes are formed in the plant they may likewise be inactivated by the formation of inhibitors that are also influenced by growing conditions.

CARBOHYDRATE RESERVES AND PHENOLOGICAL DEVELOPMENT

In general, the normal trend of seasonal carbohydrate balance in the storage organs is similar for most perennial plants (Figure 1). Carbohydrates are influenced by growth behavior as well as environmental conditions.

Variation By Species

The duration of the decline and the extent of depletion during early growth differs among individual species. The periods of low reserve concentration may vary rather widely among plant species. In the majority of plant species, minimum reserves in storage organs occur during early vegetative growth. After initial growth has produced considerable leaf area, carbohydrates are synthesized in excess of growth requirements. At this time, some plant species replenish the reserve (Figure 1) while in other species reserves remain at a relatively low level until later stages of maturation.

Storage Of Reserves Are Variable

In general it has been found that carbohydrates are elaborated in the leaves of plants in excess after flowering and are subsequently translocated to the roots or other storage organs for storage. If regrowth takes place during the autumn, reserves in the storage organs may be utilized and reserve levels decline. There is a slight decline in stored reserves during the winter because of respiration and possibly slight growth.

Kinsinger, in the Great Plains area, found that total carbohydrates (reducing sugars, sucrose, and starch) were low when growth of plants was rapid. When plants approached dormancy, carbohydrates reached their peak. Warm season plants utilized reserve carbohydrates until August when storage occurred, but the peak in carbohydrate storage was not reached until the fall or early winter.

It has generally been observed that increases in starch and sugar concentrations in stems and roots of plants occur under slight drought conditions. Plant samples taken before and after drought have revealed that protected and moderately grazed plants accumulated excess food reserves when entering drought.

Grazing during drought reduces the carbohydrate content in roots of range plants, and thus appeared to affect their resistance to injury from low soil moisture. Cook stated that roots of crested wheatgrass which received additional water had significantly higher average percentages of cellulose, sucrose, and other carbohydrates while the roots from drier areas had significantly more protein.

Variation By Season And Degree Of Maturity

From 10 to as much as 45 percent of the annual growth of plants may be produced in early phenological stages before root reserves cease to decline. Translocation and storage of reserves generally are most active in the autumn when plants are completing their annual growth cycle. If regrowth takes place during the autumn, reserves in the storage organs may again decline (Figure 2). There is a general decline in stored reserves during the winter because of respiration. Respiration during the winter and the production of spring growth together may consume from 47 to as much as 75 percent of the carbohydrate reserves accumulated during the previous autumn.

When plants approach maturation, carbohydrate storage reaches its peak. Warm season grasses utilized reserve carbohydrates late into the summer before storage occurs, so the peak in carbohydrate storage may not be reached until later. In cool-weather species, peaks in carbohydrate storage are often reached in early summer as the plants mature.

GRAZING AND CARBOHYDRATE RESERVES
Animals select portions of the plant as a function of stage of growth and plant association. Even plants of the same species in a pure stand are utilized differently depending on the amount of old growth intermingled with new growth and the general state of vigor. Heavy use usually results in lowered vigor and increased palatability which makes the plant doubly susceptible to additional grazing which may cause permanent damage and eventual death.

Defoliation in Early Growth

If the plant is defoliated in early growth, food manufacture is very low until new aboveground growth restores the photosynthetic tissue (Figure 2). In this case, the plant must draw again upon reserves or grow at a slower rate within the ability of the remaining photosynthetic tissue to furnish food. If the carbohydrate reserves are exhausted by previous defoliation, food to supply both the tops and roots must be manufactured by new leaves. Growth of the limited photosynthetic tissue is thus severely reduced.

When the reserve is severely lowered, regrowth from only a few tiller or latent buds will be stimulated. Reduced aerial growth eventually results in reduced root growth. When this occurs the vigor of the plant is seriously impaired.

Effect On Range Condition

Exhaustion of carbohydrate reserves and reduced plant growth and vigor can be the primary cause of changes in range condition. The more palatable species are grazed more intensively and frequently than less palatable plants. the carbohydrate reserves in the heavily grazed plants are gradually reduced while the less palatable species have optimum reserves. Thus further grazing stress and drought will cause a change in species composition favoring those with high reserves and low palatability.

Frequency and Intensity of Defoliation and Effect on Carbohydrate Levels

Most defoliation during the growing period results in reduced carbohydrate reserves (Figure 3). Even so, they are not generally reduced below the critical level. Frequent defoliation, however, will result in a decrease in the concentration of total available carbohydrates and a reduction in the quantity or size of the storage organs. In general, any situation that interferes with normal growth of the photsynthetic tissue of range plants will restrict development of roots and normal reserve storage and as a result curtail subsequent growth.

The effects of grazing on carbohydrate levels to be attained at the end of the growing season depends on the number of times the plant is grazed and the proportion of photosynthetic tissue that remains after each grazing event. The more frequent and more intense the grazing treatments are, the greater will be the reduction in the number of roots and rhizomes and the amount of food reserves present. When abusive treatment is continued, it eventually kills the plant.

It is believed that a plant that prolongs replenishment of reserves following initial spring growth is more susceptible to grazing stress during the growing period than one that replenishes its reserve rapidly following initial growth (Figure 4). Likewise a plant that develops rapidly during early growth and draws heavily on the reserves before replenishment may be more susceptible to damage from early grazing than a plant that grows more slowly and does not draw extensively on the reserves.

Desert range plants will not tolerate heavy and continuous spring use because they do not have an opportunity for regrowth and carbohydrate replenishment during the dry summer season that follows. Most range plants are most adversely affected if grazed during late spring or summer as they approach maturity.

Grazing Systems As Tool For Replenishment Of Carbohydrate Reserves

Various grazing systems are designed to allow replenishment of the carbohydrate reserves. For example, deferred-rotation grazing allows the plant to set seed one year out of every 3 or 4 and rest-rotation grazing allows the plant to go ungrazed for one year out of every 3 or 4 years, depending on the grazing prescription arrived at for the particular area. The adverse feature of these grazing systems is the assumption that, regardless of conditions, the deferment or rest will always compensate for any over-use or critical seasonal use during the previous years. The grazing system must be flexible so that during a drought year or immediately following a drought year, plants are allowed additional time or less pressure from grazing to compensate for the previous low storage of food reserves.

Since species of plants vary greatly in the extent of depletion of carbohydrate reserves during spring growth and in the rate and period of replenishment, it is impossible to perfect a method of grazing that will benefit all plant species in the same manner. Therefore, every grazing system should somehow be designed to meet the physiological requirements of the key species for the area involved.

When plants are grazed twice during the growing season, they may be required to draw on their reserves for regrowth at least during 3 separate intervals. If growing conditions are not favorable following the period of use, the plants will not have an opportunity to store optimum reserves. In the following year the period of use may be rotated to a later date before the plants have set seed. In this case the reserves may not have been translocated from the aerial portion of the plant. However, during the second year there would be no opportunity to store optimum reserves and the plant, as a result, has lost more vigor than it can regain in a complete year of rest or a period of deferment. This could be disastrous if the year of rest or deferment is an unusually dry year. It is necessary then to design a grazing system that will consider these variable circumstances.

Grazing management systems, if wisely used, constitute good range management planning, but a paper plan without regard to variations in plant growth and carbohydrate balances may cause severe damage to the range.

MANAGEMENT OF RANGELANDS AND CARBOHYDRATE RESERVES

Proper management of range plants does not necessarily imply that a maximum level of carbohydrate reserves must be maintained, but care must be taken to provide that these reserves do not fall below a critical level. Plants intensively used should have a period of rest for restoration of the reserve level during the same year that heavy use occurred. Plants on unfavorable sites should be used conservatively or during seasons when the effects of grazing will be least harmful.

Intensive management of rangelands may someday be based primarily on carbohydrate reserves and plant vigor. Therefore, a thorough knowledge of carbohydrate synthesis, translocation, storage and use within a plant will aid in identifying a range manager who can care for range plants with all the solace and attention that a cowboy gives his horse.