Biol 1440, Principles of Biology I

Writing Lab Reports
Example for Seed Germination

The Effect of Sodium Chloride on Root Growth in Garden Peas

 By: Grady A. Student

Lab Section: Tuesday 10:50-1:30 p.m.

Instructor: Dr. C. Darwin

 Hypothesis/Rationale

 As the concentration of sodium chloride increases in water used to imbibe pea seeds, the root growth of pea seedlings will decrease. The rationale for this hypothesis is that increasing sodium chloride concentrations will lower the chemical potential of water and make the water necessary for germination and growth harder for the seed to acquire.  

 Materials and Methods

 Pea seeds (Pisum sativum L.) (obtained from the lab instructor) were sorted as to size and surface characteristics. Only smooth coated seeds of similar size were used in subsequent experiments. Five seeds were placed in plastic germination dishes on top of a 30-cm length of paper toweling folded to fit in the dish. Seeds were treated with 20 ml of either distilled water (control) or with one of seven sodium chloride concentrations ranging from 20-500 mM. Each germination dish was then placed in a plastic “zip-lock” bag to retard the evaporation of water and then incubated at 25C for 1 week. Any root growth was measured using a metric ruler. All treatments were replicated in triplicate.

 Results

 The response of pea seeds to increasing sodium chloride concentrations in the imbibition medium was complex. For example, if the added sodium chloride concentration was equal to or less than 50 mM, subsequent root lengths were similar to or slightly greater than growth seen in seedlings imbibed in distilled water (Figure 1). In contrast, as sodium chloride concentrations were increased beyond 50 mM, roots exhibited steadily decreasing lengths with the shortest roots seen in peas germinated in 500 mM sodium chloride (Figure 1).

 Conclusion

 The hypothesis was not supported by the data. Although imbibition solutions containing sodium chloride concentrations above 50 mM did reduce root growth (suggesting reduced water uptake) smaller amounts of salt either had no effect or produced a slightly longer root. A possible explanation for the lack of root growth inhibition in the control and low sodium chloride experimental groups was that the potential of water inside the cells of the seedlings was lower than the external water. Under these conditions, water uptake would continue to be favored by the water potential gradient. The slight stimulation of root growth seen in the 20 mM treatment is more difficult to explain. One possibility is that pea seeds never experience distilled water environments in nature and it may be that water uptake is too fast under these conditions and may be slightly inhibitory to root growth.

 

 Figure 1.  The effect of sodium chloride on root growth in peas.  Each data point is the average of three replicates.