Perennial Plant Response to Defoliation - An Overview

Overview and Objectives

• Define the terms source, sink, photosynthesis, carbohydrate, apical meristem, lateral meristem, and residual leaf area
• Compare the differences in morphology (plant structure) and physiology (plant function) between a non-defoliated and defoliated plant
• Predict how a plant will respond to defoliation based on morphology, physiology, and the environment.

Perennial Plants

It is early spring. You are driving west on Highway 2 through the native grasslands of the Nebraska Sandhills. The perennial plants that are an important part of life in this region of the world have been dormant since fall, making the countryside look desolate. However, things are about to change, not only for the plants but also for the people and cattle who depend on these plants. To truly appreciate this change, a little insight into the life of perennial plants is necessary.

Perennial plants live for three or more years, and usually produce seed or vegetative forms of reproduction each year. In Nebraska and surrounding areas, perennial herbaceous plants die back to ground level at the end of the growing season, remain dormant during the winter, and resume growth in the spring. Perennial grasses and broadleaves (also known as forbs) are herbaceous plants that are common in a diversity of landscapes including wildlands and intensively cultivated crop fields and gardens. Grasses have a hollow or pithy stem and slender leaves with parallel venation. Grass stems are also jointed, meaning there are nodes located along the stem. An example of a perennial grass is little bluestem (Schizachrium scoparium), Nebraska’s state grass. Forbs are herbaceous, broad-leaf plants that have solid or pithy stems and relatively broad leaves. An example of a perennial forb is Missouri goldenrod (Solidago missouriensis), Nebraska’s state flower.

The position and composition of leaves, stems, roots, and other parts of perennial plants are specific to an individual plant’s needs in order to survive. The following list details the plant parts of both grasses and forbs.

Figure 1: Examples of grass (left) and broadleaf (right).

• Leaf: Leaves are the energy production centers of the plant. They are responsible for performing photosynthesis (discussed later) which in turn produces the carbohydrates needed for plant growth. Leaves can also act as temporary storage organs for carbohydrates during maximum production phases of the growing season.
• Inflorescence: An inflorescence is a clustered flower system that produces the seed head of a grass plant.
• Node: A node is the point along the stem where the leaf is attached
• Internode: An internode is the space along the stem in between nodes, present in both grasses and broadleaves.
• Tiller: A tiller is specific to grass plants. Also known as the shoot, it is composed of a growing point, stem, leaves, nodes, and buds.
• Branch: A branch, specific to forbs, is a lateral shoot composed of stem, leaves, nodes, buds, and lateral meristem.
• Rhizomes and Stolons: Rhizomes and stolons are both stem structures that grow parallel to the soil surface and are present in many different grasses and forbs. Rhizomes grow below ground while stolons grow above the soil surface. Both contain nodes that have the ability to develop into new plants.
• Apical meristem (growing point): Apical meristem is involved in the primary growth of plants including root and shoot development. The apical meristem is composed of undifferentiated cells capable of developing into many different tissues. Apical meristem tissues are located at the tips of roots and the base of shoots in grasses during vegetative growth. As grass shoots begin to elongate the growing point moves from the base of the plant to a higher position along the main shoot. Apical meristem is found at the root tips and tip of the main shoot in forbs. These areas of plants are where new growth is initiated.
• Intercalary Meristem: The intercalary meristem is directly involved in secondary growth such as internode and leaf growth in grasses and branch development in forbs. Intercalary meristem is located at the base of leaf blades, leaf sheaths, and internodes in grasses.
• Roots: The root structure of plants can be taproot or fibrous. Both types of systems contain apical meristem tissue. The main functions of roots include absorbing nutrients and water from the soil, storing carbohydrates, and anchoring the plant in the soil. The root system of perennial plants stays intact from year to year with the potential to initiate new growth each season.
• Axillary buds: Buds are described as undeveloped shoots, leaves, or flowers. The axillary buds, also known as lateral buds, are located in the leaf axil (the area created between the stem and leaf) of forbs.

How Perennial Plants Grow

Perennial plants must survive year after year and for each of the four seasons. Each season (spring, summer, fall, and winter) plays a role in plant survival and productivity. This section describes the growth cycle of a warm-season perennial plant through a year.

Spring

Warm-season grasses and forbs initiate new growth in mid-to late- spring with warming temperatures that cause buds to break dormancy. The buds are located at ground level or just below the soil surface as is the case with grasses and forbs. During this phase of plant growth, the growing point in grasses is located at the base of the plant. In forbs, growing points are located at the tip of the main shoot and tip of the developing branches.

Carbohydrates, both sugars and starches, play a key role in the initiation of new growth. Carbohydrate reserves (stored as starch), which have been stored in the stems, roots, rhizomes, and stolons during the winter, are used by the plant to provide energy and carbon to begin spring growth. Once green tissue has emerged, photosynthesis can begin. Photosynthesis is an important process that occurs in plants, but why is it so important? The purpose of photosynthesis is to provide energy and the raw material for plant maintenance and growth. Photosynthesis takes CO2 (carbon dioxide) from the air to build carbohydrates. With photosynthesis actively occurring, the plant can grow without depending on carbohydrate reserves. During this spring growth period, the areas of carbohydrate source and sink in the plant changes. Initially the newly developing tissue is a sink requiring carbohydrates from a source (the carbohydrate reserves). After green tissue has emerged and is able to perform photosynthesis it then becomes a source of carbohydrate for other plant parts and functions.

Spring prairie landscape. (Walter Schacht, 2005)	Spring prairie closeup. (Walter Schacht, 2005)

This is the period of the growing season when most of the vegetative and reproductive growth occurs. Growth places a high demand on carbohydrates. The plant uses the carbon in carbohydrates to build more cells and as a chemical energy source. In the event that the plant requires more energy than is provided by photosynthesis, carbohydrates from the reserves will be used. As grass plants continue to grow, the growing point is elevated with the elongation of the shoots. The main growing point in forbs is also elevated as the plant grows, but its position relative to the other parts of the plant does not change. (see 'Perennial Life Cycle' animation below

Summer prairie. (Walter Schacht, 2005)

As late summer and fall approaches, the rate of plant growth begins to decrease. Perennial plants are now active in replenishing carbohydrate reserves to store enough energy for next spring’s growth. Since the amount of active plant growth has decreased, the overall carbohydrate demand is reduced. Therefore, the majority of the carbohydrates produced will go directly to the storage portions of the plant. The areas of the plant such as stolons, rhizomes, and roots now become the main sink while the leaves remain the source of carbohydrate production.

Fall prairie. (Walter Schacht, 2005)

During the winter no growth is occurring. However, some carbohydrates are used from the reserves to maintain the plant and prepare the dormant buds for spring.

Winter prairie. (Walter Schacht, 2005)

Carbohydrates and Defoliation

What happens to all the carbohydrates?

First of all, why do we even care about carbohydrates? Carbohydrates are compounds produced during photosynthesis. In plants, they have two main purposes. First, they provide building blocks for plant structural components, such as cellulose (important in building cell walls). Secondly, carbohydrates are molecules that deliver energy for plant growth.

Throughout the growing season, carbohydrates are constantly flowing through the plant going from locations that have an excess supply, called ‘sources’, to locations that have a need, called ‘sinks’. This is called the ‘source-sink relationship’.

The source-sink relationship can be compared to the relationship between a factory, a warehouse, and a retail store. Let’s use the hybrid car market as an example. A factory (source) manufactures hybrid cars and then ships them to the dealerships (sink) to sell. The dealership is selling as many cars as they receive from the factory. Then, the factory expands and produces twice as many cars as before. Meanwhile, the demand at the dealership is the same resulting in an excess of cars being produced at the factory. So, the factory starts sending the excess cars to the warehouse to store until they are needed. Later that year, the gas prices skyrocket and suddenly, there is a great demand for fuel-efficient hybrid cars. To keep up with the high demand, both the factory and the warehouse send cars to the dealership.

Now apply this to a perennial plant. The factory is the leaf material, where the carbohydrates are produced. The warehouse is the roots and crown where excess carbohydrates are stored until they are needed. And, the dealership is any part of the plant using the carbohydrates for building structure or for energy.

In early spring, there is no green leaf material for photosynthesis, so all carbohydrates used to start growth of the first leaves must come from storage in the roots and crown. In this case, the leaves are the ‘sink’, and the roots/crown are the ‘source’. Once the amount of leaf area becomes great enough, it is able to produce enough carbohydrates to supply the plants needs and carbohydrates no longer need to be taken from storage in the roots/crown. Eventually, later in the season, the leaf area will be great enough that it is producing more carbohydrates than the plant needs. The excess carbohydrates are then sent for storage in the roots/crown.

Question : What would happen to the dealership if a tornado suddenly destroyed the hybrid car factory? It would go out of business because it didn’t have enough cars to sell. It would send what cars it had to the warehouse for storage until the factory was rebuilt. It would get cars from the warehouse to sell. It would have a “going out of business” sale.

Question : What would happen if all of the green leaf material was suddenly cut off? The plant would immediately starve to death because it no longer has leaves to supply the needed carbohydrates. The plant would ship all of its carbohydrates to the roots/crown to store. The plant would use carbohydrates stored in the roots/crown to re-grow the leaves. The plant would get up and move so it wouldn’t be cut down again.

Defoliation of Grasses and Broadleaves

Defoliation is the removal of above ground plant material. A few ways that defoliation can be accomplished is by mowing, burning, grazing, hail, frost, or applying herbicide. In many cases defoliation is done intentionally. What happens to a plant once it has been defoliated?

There are three main questions to address in order to determine how the plant will respond after defoliation.

1. Where is the growing point located?

The location of the growing point throughout the season is critical in predicting the response of a plant after defoliation. During the first part of the growing season, the growing point in grasses is located at the base of the plant, but as the season progresses the growing point moves to a higher positioned along the tiller. This is important in determining how quickly regrowth occurs. If the grass plant is defoliated above the growing point, regrowth will occur rather quickly. This is because the growing point is still intact and growth can continue from the same tiller. However, if defoliation happens below the growing point, regrowth will be slower. Once the growing point is removed, growth no longer occurs from that tiller. Therefore, new growth must begin from dormant buds rather than continuing from the original tiller.


Figure 2: The blue dot in the picture above shows the location of the growing point in grass.

Forbs have many growing points all along the shoot. The main growing point is at the tip of the shoot. When this growing point is removed, secondary growing points along the shoot are activated and lateral growth occurs. Even when individual, secondary growing points are removed growth will still occur. It is only when all plant material is removed below all growing points that growth ceases and a dormant bud must initiate new growth.

Figure 3: This figure shows the areas a broadleaf can be defoliated. Growing points are located at the tips of the plant and at each branch.

2. What part of the growing season was the plant defoliated?

The time during the growing season when plant tissue is removed determines the response of the plant. Removing tissue during the spring and summer, with growing points still intact, will most likely not affect the ability of the plant to grow actively for most of the season. This is when growing conditions are optimum (i.e., good soil moisture, high temperatures, and good light conditions) and the photosynthetic rate is high; therefore the plant can easily replace the removed tissue. There is also still enough time remaining in the growing season for adequate regrowth and storage of carbohydrates. However, if the plant is defoliated in late summer-fall, when the focus is on storing carbohydrates, this will have a negative effect on the initial growth next spring. Without sufficient storage of carbohydrates in late summer and early fall, dormant buds in the spring will not have the energy required to initiate growth.

3. How much green tissue is left after defoliation?

The amount of residual leaf area remaining after defoliation must be adequate for continuing photosynthesis in order to get significant regrowth. Photosynthesis can be greatly inhibited with the removal of leaf tissue because green tissue is required for photosynthesis. This directly affects the source-sink relationship in the plant. Without the green tissue to perform photosynthesis there is no production of carbohydrates to supply the sinks with energy.

Review the response of a perennial grass plant and broadleaf to defoliation by ’clicking’ on the icon.

Summary

Summary Many grasses and forbs are perennial plants that have morphological and physiological characteristics necessary for survival and production year after year. Growth is initiated each year from buds at the base of perennial plants. Apical and intercalary meristems develop and are sites of plant growth through the growing season. Photosynthesis in green plants is the process of converting solar energy to chemical energy and bringing carbon into the plant. The immediate products of photosynthesis are carbohydrates which are used for maintenance and production in plants. Carbohydrates can be stored in the plant for later use when amounts produced are in excess of current needs.

Perennial plants are commonly defoliated during some part of the year. Perennial plant response to defoliation is largely determined by (1) location of the growing point, (2) timing of the defoliation, (3) amount of green plant tissue remaining following defoliation, and (4) amount of energy reserves. Defoliation is not necessarily detrimental to a perennial grass plant when the growing point is not removed, much of the growing season remains, and much green leaf tissue and/or energy reserves remain following defoliation.