Plant–Water Relations

Experiment 1.1

To determine the osmotic potential of any given plant material by using the incipient plasmolytic method. Also, to determine the osmotic potential of an unknown solution.

Requirements

Plant material : The leaves of Tradescantia spathacea1/Commelina/Zebrina

Family : Commelinaceae

Chemicals : Sucrose solution (1 molal), distilled water, unknown sucrose solution

Glass apparatus : Petri dishes, glass slides, cover slips, beakers, pipettes (1 ml, 10 ml)

Miscellaneous : Microscope, forceps, needle, stop watch, graph paper

Principle

An actively metabolizing cell comprises of a permeable cell wall and semipermeable plasma membrane and tonoplast. The bulk of the space in a cell is occupied by the vacuole, which is a storehouse of minerals, osmotic substances like sugars, amino acids, organic acids and secondary metabolites like anthocyanin, and so on. Under normal, fully distended conditions, cells are said to be ‘turgid’. When such cells are shifted to a hypertonic solution, the movement of water occurs from a region of high water potential to a region of low water potential resulting in the loss of turgor. This is followed by shrinking of the cytoplasm away from the cell wall, i.e., Plasmolysis. The stage when there is initial pulling away of the cell membrane from the cell wall and the turgor pressure becomes zero, is known as ‘incipient plasmolysis’. Plasmolysed cells can be deplasmolysed by placing them in hypotonic solutions. In hypotonic solutions, turgor pressure develops, which presses the membrane against the cell wall. The rigid cell wall exerts an equal and opposite pressure termed as wall pressure and the cell is said to be turgid.

Water potential is the algebraic sum of three components:

Procedure

Prepare a series of sucrose solutions ranging from 0.15 to 0.35 m, using 1 m sucrose as stock solution (Dilution Table). Take out epidermal peels from the zone near central midrib portion of lower surface of the leaf by applying differential or unequal pressure. Place 3 peels in each petri dish after a time gap of 5 minutes. Wait for 10 minutes2 or so, mount the peels on the slides in their respective solutions and observe under microscope (6×, 40×).