Effect of salinity and oxygen level on lettuce grown in a floating system

The effect of increasing levels of salinity on lettuce grown in a floating system with or without aeration of the nutrient solution was investigated. The basic nutrient solution was added with increasing NaCl concentrations (0, 10, 20 and 30 mmol L-1) and each salinity level was combined with two oxygen levels as obtained by insufflating or not air in the nutrient solution. Butterhead lettuce 'Justine' was transplanted on the polystyrene panels of the floating system on May 9(th) 2002,and harvested on June 12(th). At harvest, the following data were collected: head and root fresh and dry weight, root length, head diameter and leaf content of nitrates and chlorophyll (as assessed by using SPAD meter). The measured parameters showed statistically significant differences due both to salinity and oxygen level, with an interaction between the two factors. When the nutrient solution was not aerated, a strong effect on growth was noticed due to salinity: in comparison with control (0 NaCl), fresh weight of the heads and their diameter decreased statistically already at 10 mmol L-1 NaCl, and dry weight at 20 mmol L-1 NaCl. Besides, rising NaCl concentrations increased dry weight percentage both in the head and in the roots, increased root/shoot ratio, and reduced nitrate accumulation in the leaves, which showed increasing SPAD values. Tipburn symptoms were noticed only in the control. On the contrary, when the NaCl addition was combined with the aeration of the nutrient solution., a satisfactory growth was achieved up to 30 mmol L-1 NaCl, but some tipburn symptoms occurred due to salinity. In this case, nitrate accumulation in the leaves increased up to 20 mmol L-1 NaCl, decreasing to values comparable to control at 30 mmol L-1 NaCl.