Plant Growth And Development

Plant Growth And Development

Growth is the process by which a plant increases in the number and size of leaves and stems. The result of plant growth is forage production and the amount harvested by animal or machine is forage yield. The growth of both plants and animals requires energy. Animals get their energy by digesting the plants they eat. Plants get their energy from the sun through photosynthesis. Photosynthesis is the process where the green pigment in the plant's leaf (chlorophyll) absorbs energy from sunlight and, using this energy, water, and carbon dioxide, produces oxygen and simple sugars. The plant then uses these sugars to make more complex sugars and starches for storage as energy reserves, to make cellulose and hemicellulose for cell walls or with nitrogen, to make proteins. How the plant uses its energy depends on the developmental stage of the plant and on environmental conditions.
When leaves are removed from a grass or clover plant, new leaves develop and grow from buds on the crown or stems of the plant. This growth requires energy which comes from reserve carbohydrates (sugars and starches) or from actively photosynthesizing leaves remaining on the plant. Most closely harvested plants have a predictable plant growth-energy reserve cycle (Figure 1). Manipulating this cycle is a useful tool in managing forages.
Let us look at orchardgrass as an example. When the leaves of an orchardgrass plant are harvested, new leaves will start to grow. If all the leaves are removed, the new growth will have to come from energy reserves stored in the leaf bases of the plant. Over a period of days the usable carbohydrate reserves will decrease. If more leaf area is left on the plant, the use of reserve carbohydrates will be less and regrowth will be faster since it can also be powered by photosynthesis.
As new growth increases the leaf area, more sunlight is intercepted and photosynthesis increases, providing more energy for additional leaf growth. At some point photosynthesis is great enough to produce more sugar than is needed for leaf growth. This results in an increase in the reserve carbohydrates in the plant. As the leaf area increases further, leaves start shading one another, and net growth slows as older leaves in deep shade do not get enough sunlight and begin to die.
Plant development is the process of a plant changing from one growth stage to another. This could be the development of tillers on a grass plant or flower buds on a legume plant. Plant development is the major factor affecting forage quality (Table 2). As plants change from vegetative to reproductive stages, forage quality decreases. When in the reproductive stage, both grasses and legumes produce stemmy growth. As a plant matures, it increases in fiber and decreases in digestibility, crude protein, and intake by livestock. The rate and timing of reproductive development is determined by species, day length (photoperiod), and temperature.

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