4: Plant Physiology and Regulation
Plant physiology focuses on the chemistry and physics of how plants function. Plants capture light energy and produce sugars through photosynthesis and break down these sugars through aerobic cellular respiration. They respond to a variety of environmental conditions through growth changes, life events like germination or flowering, and even, in special cases, through movement. Plants rely on soils for mineral nutrients and water, and biogeochemical cycles replenish soils with these nutrients. Once water and minerals are absorbed, they must be transported through the xylem, and this movement is driven by the loss of water vapor from leaves ( transpiration ) and the cohesive and adhesive properties of water. Likewise, sugar-rich assimilate must be moved, or translocated , through the phloem. Five main types of hormones in plants are responsible for relaying messages throughout the plant body. Throughout this unit are examples of how plants regulate their internal conditions whether it the concentration of carbon dioxide in the leaves; the positioning of stems, roots, and leaves; or the movement and retention of water (Figure \(\PageIndex{1}\)).
Attribution
Melissa Ha ( CC-BY-SA )
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- 4.1: Photosynthesis and Respiration
- Photosynthesis is the process by which plants, algae, and photosynthetic bacteria capture carbon dioxide and synthesize sugars using light energy. Oxygen is released as a byproduct in this process. There are two steps of photosynthesis: the light-dependent reactions and the light-independent reactions. Sugars are commonly broken down to release usable energy through the process of aerobic cellular respiration.
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- 4.2: Environmental Responses
- Animals can respond to environmental factors by moving to a new location. Plants, however, are rooted in place and must respond to the surrounding environmental factors. Plants have sophisticated systems to detect and respond to light, gravity, temperature, and physical touch. Receptors sense environmental factors and relay the information to effector systems—often through intermediate chemical messengers—to bring about plant responses.
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- 4.5: Transport
- The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.
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- 4.6: Development
- Development refers to the process by which a plant changes over its life. Growth occurs at apical, primary, and lateral meristems. Embryogenesis is the development of the embryo inside the seed. A mature seed consists of the seed coat surrounding the embryo, which typically contains a epicotyl, hypocotyl, radicle, and cotelydon(s), but the precise structure differs between eudicots and monocots. Upon germination, the plant resumes growth. A mature plant flowers according to the ABCDE model.
Thumbnail image: A normal Arabidopsis plant (left) and a mutant that does not respond properly to the hormone auxin. Image by William M. Gray ( CC-BY ).