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Perennial Plant Response to Defoliation - An Overview

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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)

Summer

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)

Fall

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)

Winter

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.

The following spring the carbohydrate reserves stored from the previous growing season are used to initiate new plant growth. This cycle will continue much the same year after year depending on the growing environment.

Winter prairie.
(Walter Schacht, 2005)

Review the life cycle of a perennial grass by ’clicking’ on Perennial Life Cycle of the animation in the figure below.

 

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