2017 ASHS Annual Conference

Two lettuce (Lactuca sativa L.) cultivars Romaine del Sol and Green Leaf Lettuce were grown in a hydroponic system or in containers with soil in a greenhouse to determine their tolerance to increasing salinity. Plants were exposed to irrigation water with increasing salinity (2.1, 3.6, 5.1, and 6.6 dS/m) by supplementing the nutrient solution (2.1 dS/m) with a combination of 2:1 NaCl and CaCl. Lettuce head height, diameter, and shoot dry mass decreased at the two highest salinity levels at the final harvest. When plants were smaller, salinity had no effect on these variables. Fresh weight decreased with increasing salinity over 50% at final harvest for both cultivars. Although relative water content decreased from lowest to highest salinity at final harvest, osmotic potential of both cultivars were not affected by salinity or substrate throughout the study. Leaf number at final harvest decreased by 16% for Romaine and by 29% for Green Leaf Lettuce. Substrate had no effect at final harvest on these variables except for height on Romaine lettuce which was taller in hydroponics compared to soil grown plants, and Green Leaf Lettuce which had more leaves in hydroponics than in soil substrate. An informal taste test found that the leaves from the two highest levels of salinity from both cultivars were inedible because of a salty and bitter taste. Mineral concentration of sodium and chloride in Romaine and Green leaf increased as salinity levels increased. In both cultivars chloride concentrations were higher than sodium at each salinity level. Plants of both cultivars grown in soil had greater concentrations of both elements when compared to hydroponics. The interaction of substrate and EC was significant for chloride only in the Green Leaf cultivar where chloride uptake resulted in higher concentrations in plants growing in soil compared to hydroponics. We found a similar trend for sodium uptake in Green Leaf Lettuce. All the other nutrients were within sufficient range, with some affected by salinity, substrate, or an interaction of both variables. Romaine and Green Leaf Lettuce are more tolerant to salinity than previously reported in other lettuce cultivars, they include sodium and chloride, and show no osmotic adjustment with increasing salinity. Although lettuce grown at 5.1 dS/m and 6.6 dS/m was marginally acceptable by size standards, the lack of head formation in Romaine, and the unfavorable taste of both cultivars would render these heads unmarketable.