AP Environmental Science Edinburg North High School

 Increased salinity of a soil can occur in arid regions where crops are irrigated (Miller and Spoolman 2009).  The salts decrease the osmotic potential of the soil making it difficult for plants to take up water (Molnar 2005).  The Lower Rio Grande Valley (LRGV) of south Texas has been characterized as having a subtropical and semi-arid climate (Lonard et al. 1991).

 Objective: Describe the salt tolerance of the Grand Rapids Lettuce.  We tested the null hypothesis that salt concentration will not have an effect on germination rates of this lettuce cultivar.

The following salt (NaCl) concentrations, by weight, were tested: 0.01%, 0.05%, 0.10%, 0.50%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0% In addition, a control group used water only.

Nine replicates for each test concentration as well as for the control group were established For each replicate, 10 lettuce seeds were placed on a paper towel, in a Petri dish.

The paper towel in each dish was completely saturated with the respective test salt solution. Seeds were allowed to germinate for 7-days. After 7-days, the number of seeds germinated was recorded for each replicate.

 Mean germination frequencies were calculated for each test group, then plotted against concentration.  The median lethal dose (LD 50 ) for the lettuce cultivar was interpolated from the graph.

No response was observed between 0 and 0.01%. Ten out of 10 (or 100%) of the seeds germinated. The threshold of toxicity of NaCl was observed at between 0.01% and 0.50%. Viability dropped precipitously above 0.50%. One- hundred percent mortality was observed at 1.50%. The median lethal dose was estimated to be 0.80%.

 The null hypothesis that salt concentration would have no effect on germination rate of Grand Rapids Lettuce was rejected.  Salt tolerance of Grand Rapids Lettuce was moderate.

 Feasibility of growing this lettuce cultivar in the LRGV requires further investigation. Water requirements Soil salinity Projected increases in soil salinity from irrigation requirements