Transpiration

Transpiration

Abstract

This lab demonstrates how different environments effect the rate of transpiration in plants. A potometer is set up and placed in four different environments: one simply at room temperature (the control), one with a fan pointing toward it creating a gentle breeze, on with a floodlight on it and a beaker felled with water as a heat sink, and one where the leaves are misted then covered with a transparent plastic bag. It is imperative that the tubing doesn't have any bubbles, otherwise the experiment will not work. After it equilibrates for ten minutes, readings are taken every three minutes for thirty minutes to determine how much water was lost. By dividing the amount of water lost by the leafs surface area, the results of the four conditions can be compared accurately.

The plant placed in the light transpired the most, followed by room temperature, then the fan and mist.

It is evident that water potential greatly impacts transpiration. Since water moves from an area of higher water potential to lower water potential, the drier environments in the lab produced higher rates of transpiration.

Introduction

Plants lose most of their water through transpiration, which is the evaporation of water from the plant surface. They also lose much of their water through guttation which is the loss of liquids from the ends of vascular tissues at the margins of the leaves. It is imperative that this water lost is replaced; otherwise the plant will wilt and most likely die.

Water from the roots in the xylem travels up from cell to cell and over long distances in the plant to replace lost water. This process is governed by the differences in water potential. More water potential means that there are more free water molecules available, and thus more potential energy. The factors contributing to water potential are gravity, pressure and solute concentration. Water always moves from an area of higher water potential to an area of lower water potential. The movement of water is able to occur due to the properties of water, osmosis, adhesion and cohesion, and root pressure. Cohesion is when water molecules are attracted to each other due to hydrogen bond formation. Adhesion is when water molecules stick to something, in this case he walls of the xylem cells, which offsets the forces of gravity.

Within the root, minerals are actively transported in accumulating in the xylem. As a result, water potential is lowered because the solute (minerals) leave less water available. Water moves into the root by osmosis (the movement of water from a higher concentration to a lower concentration).

The stomata are small pores in the leaves where evaporation of water occurs. They surround the mesophyll cells of the leaf. These spaces have moist air, so their water potential is greater than the air outside, so water will evaporate from the leaf. Adjacent mesophyll cells replace the water lost in the air spaces. The adjacent mesophyll cells now have a lowered water potential because the cytoplasm becomes more concentrated. The cascade continues with the water, and it moves into the mesophyll cells by osmosis from surrounding cells that have a greater water potential (xylem). The water moving into the mesophyll cells exerts a pull on the column of water molecules existing in the existing in the xylem from the leaves down to the roots.

This upward transpirational pull on the fluid in the xylem causes a tension (negative pressure) to form within the xylem. As a result, the walls of the xylem are propelled inward. Further, tension causes the water potential in the xylem to decrease. The phenomenon of water traveling up from the soil, through the cortex of the roots to the leaves is a result of the decreasing water potential that began with the evaporation of water.

Hypothesis: If the lab is set up correctly, then the plants in drier environments (less water potential in the air than in the plant) will transpire the most. The flood light should transpire the most followed by the either the control or the fan, and the plant that has been misted will transpire the least.

Materials and Methods

Shallow tray filled with water, timer, a beaker containing water, scissors, .1-mL pipette, a plant cutting, ring stand, clamps on a ring,