MadSci Network: Botany

Re: Why and how the sunflower can follow the movement of the sun?

Date: Thu Feb 15 19:00:02 2001
Posted By: Henri Maurice, Faculty, Biological and Physical Sciences, Barat College
Area of science: Botany
ID: 978987816.Bt

This type of response is known as solar tracking and occurs in sunflower as 
well as many other types of plants.  It does not have to involve the flower; 
in many types of plants the leaves are involved.  Solar tracking is a type 
of tropism.  Tropisms are responses plants have to an environmental signal.  
The response depends upon the direction of the stimulus.  The orientation of 
the sunflower to the sun is a direct result of the direction of light.  The 
actual movement of the flower occurs as a result of motor cells in a 
structure called the pulvinus.  The pulvinus is a region that is located 
where the flower head is connected to the stem.  The motor cells get bigger 
or smaller depending on how much water they absorb or lose.  Potassium is 
involved in all this.  If the motor cell has a lot of potassium more water 
will go into the cell and the cell will inflate.  With less potassium, the 
cell loses water and deflates.  Which cells inflate and deflate causes the 
flower head to move as the sun moves.  At night when the sun sets the flower 
head will move and face the direction the sun should rise in the next 
morning.  This happens because of the amount of water contained in the motor 
cells of the pulvinus.

Salisbury and Ross.  1992.  Plant Physiology, 4th edition.  Wadsworth

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