Problem :
Define a hormone and list the five different classes of plant hormones.
A hormone is a chemical that affects the ways in which an organism functions.
It is produced in one part of the plant body but, by traveling to target
cells throughout the body, affects many other parts as well. The five classes
of plant hormones are auxins, cytokinins, gibberellins,
inhibitors, and ethylene.
Problem :
What is phototropism, and which hormone is responsible for it?
Phototropism is the growth of a plant toward a light source, resulting from the
rapid elongation of cells on the dark side of the plant. The hormone that
causes phototropism is auxin.
Problem :
One of the plant hormones delays senescence (aging), and another stimulates it.
Which hormone is responsible for each task?
Cytokinins delay senescence, especially in leaves. Ethylene, on the other hand,
encourages senescence, especially in terms of fruit development and leaf
abscission.
Problem :
Why is the balance between auxins and cytokinins important to the plant?
The balance between these two hormones regulates the growth of roots and shoots
so that the two portions of the plant remain balanced themselves.
Problem :
Explain the acid growth hypothesis. What phenomenon in plants does this
hypothesis account for?
The acid growth hypothesis explains the occurrence of phototropism. According
to the hypothesis, auxins trigger proton pumps in cell membranes, lowering the
pH in the cell wall to such an extent that the hydrogen bonds holding its
cellulose fibers together break apart. These broken bonds give the cell wall
greater flexibility and expandability, so that more water can enter the cell by
diffusion, causing the cell itself to elongate.
