Response of Hydroponic Bibb Lettuce (Lactuca sativa) to Chloride Additives in Integrated Aquaculture Systems
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Agricultural managers are continually looking for ways to increase production while at the same time reduce costly inputs. One such way is an integration of two or more crops or production systems. Sustainability is a key word in our world today and one component of sustainability is the reduction of waste and the reuse of all available resources and refuse. Aquaponics is the combination of plant production and fish production whereby the byproducts of one system are used as an input for the other, thereby creating a symbiotic effect. One such system that has been used and shows promise is the combination of intensive production of tilapia (Oreochromis spp) and greenhouse production of lettuce (Lactuca sativa L.). Monoculture systems have traditionally relied on proven methods and inputs. However, in cross-cultural systems, what has proven to be “standard practice” for one production system may actually be harmful to the other production system. In intensive tank culture of fish, it is a common practice to add salt in the form of NaCl or CaCl2 to the tank in order to ease stress on fish. Fish tend to have a high tolerance for salt and benefit in many ways from it. However, plants typically do not have such a tolerance. This study was conducted at facilities at Auburn University, Alabama, to determine the tolerance of hydroponic Bibb lettuce to various chloride concentrations resulting from the addition of NaCl or CaCl2 to the hydroponic solution. In a 29.3 x 9.1 m (96 x 30 ft) double insulated greenhouse located at the E.W. Shell Fisheries Center, North Auburn Unit lettuce seeds (Lactuca sativa L. ‘Charles’) were sown in one inch (1”) oasis blocks in July, 2010. In August, three weeks after sowing, the blocks were placed in a hole cut in 20 cm x 20 cm (8” x 8”) ½” thick styrofoam squares. The squares were placed on top of 6 liter (1 gal.) buckets containing a standard hydroponic fertilizer and concentrations of NaCl or CaCl2 measured in ppm chloride ranging from 0 to 500 ppm. A one (1) inch airstone was put in each pot powered by an 80 watt compressor. Salinity, measured in parts-per-thousand (ppt), electro-conductivity (EC) and pH of the treatment solutions were monitored using a YSI Model 63 meter (YSI Inc., Yellow Springs OH). Chlorophyll level in the foliage was measured with a SPAD-502 meter (Minolta, Spectrum Industries Inc.). Water loss from plant uptake and evaporation was replaced with fresh water throughout the experiment. Approximately 30 days after transplanting, plants were harvested and fresh shoot weight (FSW), fresh root weight (FRW), and a Growth Index were taken. The Growth Index was calculated by taking the average of the height and two perpendicular widths of each plant. Plant tissue was then dried in a forced-air oven for nine (9) days at 45o C (113o F) and dry shoot weight (DSW) and dry root weight (DRW) measurements were taken. Results from these experiment showed that Bibb lettuce (Lactuca sativa L. ‘Charles’) is not adversely affected by either NaCl or CaCl2 at levels ranging from 0 to 500 ppm (0.5 ppt) chloride. A second set of experiments was begun in December 2010 with increased concentrations of NaCl or CaCl2 in order to determine salt concentrations needed to observe adverse effects on Bibb lettuce. The second experiment was conducted at the Paterson Greenhouse Complex located on the main campus of Auburn University in a 14.6 x 6.1 m (48 x 20 ft) gas-heated greenhouse where the air temperature was maintained between 21o and 35oC (70 to 95oF). Concentrations of NaCl or CaCl2 were from 0 ppm to 20,000 ppm (20 ppt) chloride. Experimental setup was the same. Water quality parameters were again monitored and growth parameters taken 35 days later at harvest. Results showed that SPAD readings and pH ranges were poor indicators for measuring the effects of chloride concentrations on Bibb lettuce. Treatments of NaCl or CaCl2 over 5000 ppm (5 ppt) chloride were lethal to Bibb lettuce. Significant adverse affects were seen at concentrations above 2000 ppm (2 ppt) chloride for both salts. Regression lines of the Growth Index, FSW, FRW, DSW, and DRW suggests that the decline begins prior to where differences are significant. Our research suggests that managers not view the 2000 ppm (2 ppt) chloride level as a tipping point for lettuce but as the point at which significant adverse physiological root-mediated plant responses occur. More research is needed to determine a more precise point at which physiological health and growth is significantly affected. Perhaps more importantly, additional research is needed to determine a more specific salt level at which plant health and growth begins to be adversely affected in hydroponic Bibb lettuce.