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How the sunflower moves as per the direction of the sun? How the sunflower moves as per the direction of the sun? Sunflowers have the ability to follow the sun's path because of motor cells in the pulvinus, a flexible, joint-like segment in their stems just below each flower head, a process called heliotropism. When lack of sunlight is on the back of the sunflower head, motor cells in the pulvinus pump potassium ions. The potassium changes the pressure, elongating the cells that are away from the light, forcing the sunflower head to bend perpendicular to the sunlight. As the flower head senses that it is no longer receiving enough sun, it makes adjustments accordingly, causing the flower head to follow the sun across the sky. When the flower head matures, the pulvinus loses its flexibility and the flower head remains facing the east. This chapter focuses on how plants respond to external and internal cues.

Learning Objectives

After completing the topic, the student will be able to:

  • Define, discuss and distinguish the terms etiolation and de–etiolation in plants and appreciate the importance of phytochrome molecule.
  • Analyze how control and coordination in plants is driven by plant hormones.
  • Explore the number of applications of plant hormones in agriculture, horticulture, and biotechnology.
  • Examine the different types of tropisms and nasties that are shown by plants and investigate the difference between them.
  • Investigate how plant’s biological clocks help them to prepare for the day.
  • Explore how plants respond to different types of stress conditions and discuss the area of plant stress research.
  • Define and discuss the term “apoptosis/senescence” and examine the role of hormones in it.
Plants grow towards light Plants grow towards light One of the most commonly observed tropic responses in plants is phototropism, in which plant stems grow towards light.
Plant Responses

Plants have the means to respond to their environment in order to successfully live. Being rooted to the ground, plants must respond to whatever environmental change comes their way.

For example, bending of a grass seedling toward light begins with the plant sensing the direction, quantity, and color of the light. After integrating this information, cells in the tip of the seedling undergo a complex series of biochemical changes that influence distribution of chemical signals in the plant.

Ultimately, changes in distribution of these growth-regulating chemicals lead to the bending of the seedling toward the light. This is one example of how a plant's morphology and physiology are constantly tuned to its surroundings by complex interactions between environmental stimuli and internal signals.

Plant response towards stimuli Plant response towards stimuli The closed leaves of the sensitive mimosa (Mimosa pudica) after being touched.
Plant response to stimuli

This chapter focuses on how plants respond to external and internal cues. At the organismal level, plants and animals respond to environmental stimuli by very different means.

Animals, being mobile, respond mainly by behavioral mechanisms, moving toward positive stimuli and away from negative stimuli. Being stationary, a plant generally responds to environmental cues by adjusting its pattern of growth and development. All organisms receive specific environmental signals and respond to them in ways that enhance survival and reproductive success.

Plants, too, have cellular receptors that they use to detect important changes in their environment, whether the change is an increase in the concentration of a growth hormone, an injury from a caterpillar munching on leaves, or a decrease in day length a winter approaches.

For a stimulus to elicit a response, certain cells must have an appropriate receptor, a molecule affected by the stimulus. Upon receiving a stimulus, a receptor initiates a series of biochemical steps, a signal transduction pathway, that couples reception to response. This phenomenon is called as signal transduction of plants. Plants are sensitive to a wide range of stimuli, each initiating a specific signal transduction pathway.