In an investigation to find the water potential of onion tissue, some students used epidermal tissue from the storage leaves of a red onion. The vacuoles of the cells in this tissue contain a red pigment. The students researched one method to use in this investigation. The first step in this method is to find the concentration of sucrose solution at which 50% of the cells in the epidermal tissue are plasmolysed and 50% are not plasmolysed. The students decided to use sucrose solutions of different molar concentrations in their investigation and record the effect on the number of cells plasmolysed in the onion epidermis. The students then investigated the water potential of two other tissues that have epidermal cells with coloured pigment in their vacuoles. For each of the three tissues investigated, the students determined the concentration of sucrose solution in which 50% of the cells were plasmolysed. All three investigations were carried out at 20 °C. Table 1.1 shows these results. Table 1.1 type of tissue concentration of sucrose solution in which 50% of the cells were plasmolysed/moldm⁻³ water potential of the tissue /MPa leaf epidermis of red onion 0.35 −0.85 leaf stalk epidermis of rhubarb 0.50 −1.22 petal epidermis of antirrhinum 0.44 −1.07