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Explain the cells and the ionic mechanisms that enable transpiration from plants. Transpiration occurs at pores in leaves called stomata, which are surrounded by two guard
cells. K+ ions and anions (Cl') are transported into guard cells. The increase in solute (K+
and Cl‘) in the guard cells creates a low water potential in the cell which draws in water,
making the guard cells swell, thereby opening the stomatal pores. When stomatal pores
open, water can transpire from the plant to the atmosphere. ,Whatjredhennajormechanisms ~Whi<th4httbk water and—nutrients- to be .
transported from roots to leaves through the xylem? Brieﬂy describe each
mechanism or theory. Root pressure: accumulation of solutes in xylem creates a negative water potential in the
xylem which drives water into the xylem. The casparian strip prevents solute and water leakage.
Transpiration-Adhesion-Cohesion theogi: T ranspiration: water transpiration to the atmosphere creates a water potential gradient
which draws water up the xylem. Adhesion: hydrogen bonding of water leads to adhesion of one water molecule to the next
creating a continuous “thread” of water which is pulled up the xylem. Cohesion: water adheres to the sides of xylem walls [which pulls water up the xylem (capillaﬁtyﬂ-
Names plant hormones and brieﬂy list their main functions. Auxin: stem elongation; photon-opisrn and gravitropism
Cﬂolcinins: cell division; delay aging Gibberellins: stem elongation; fruit growth; germination
Abscisic acid: inhibits growth; closes stomata Ethylene: enhance fruit ripening Which is the controlling factor of the growth and development of a plant: hormonal
balance or hormones acting in isolation? Give an example of hormonal action that
supports your choice. Hormonal balance controls the growth and development of a plant. An example of this is
the control of apical dominance. Auxin and cytokinins act in an antagonistic fashion.
Auxin transported down the shoot from the terminal bud prevents the growth of auxiliary
buds. If the terminal bud (and therefore the auxin) is removed, the auxiliary buds do grow.
Cytokinins enter the shoot system from roots and they signal the auxillary buds to grow.
Auxin cannot stop a bud from growing once it has started. Lower buds break dormancy
earlier than buds close to the apex (due to the relative distance to auxin'and cytoltinin
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- Spring '08