Changes in Photosynthesis, Yield, and Quality of Baby Lettuce under Salinity Stress

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The objective of this work was to study the responses of two baby lettuce (Lactuca sativa L.) cultivars, namely, green Paris Island and red Sanguine, to different NaCl concentrations (0, 5, 10, and 20 mM). The study was conducted in a floating system and plants were harvested at the stage of 5-6 leaves, outside leaf 6-10 cm in length (25 and 31 days after sowing for green and red lettuce, respectively). Photosynthetic parameters (photosynthesis rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration) and yield (leaf fresh weight) as well as color [L* (lightness), C* (saturation), Hº (object’s color)], pigments (Chla, Chlb, total chlorophyll, carotenoids and anthocyanins) and quality indices (appearance, freshness, taste and texture) were determined. The results indicated that, in both cultivars, leaf fresh weight was reduced at 20 mM NaCl; the reduction depended on the limited water accessibility and photosynthetic rate was suppressed in high salt concentration through stomatal limitations aiming at the retention of water but resulting in restricted availability of CO2. Nevertheless, no limitations were observed in photosynthetic pigments and, thus, stomatal closure was the dominant factor limiting photosynthesis. On the other hand, salinity improved anthocyanin content and coloration in red lettuce and enhanced freshness in green lettuce.