Effects of different isosmotic salt solutions on leaf gas exchange of hydroponically-grown Valerianella locusta

Abstract

Nowadays there is an increasing interest in hydroponic floating system cultivation of several leafy greens. Floating could also be used to apply moderate salinity stress in order to increase the nutritional quality of fresh-cut leafy greens. In this study, we aimed to assess the effect of different types and rates of salinity on the leaf gas exchange of corn salad (Valerianella locusta). Five nutrient solutions were used: a basic nutrient solution used as control (Std), and four saline nutrient solutions that were prepared by adding to the basic nutrient solution, NaCl or CaCl2 at two concentrations (low and high). At each salinity level, the salt concentrations were iso-osmotic. Hence, the two concentrations for NaCl were 20 mM and 40 mM, LNa and HNa and for CaCl2 were 13.3 mM and 26.5 mM, LCa and HCa (low and high, respectively). For the light response curves of the photosynthetic rate (A), transpiration rate (E), stomatal conductance (gs) and intercellular CO2 concentration (Ci), youngest fully expanded leaves were placed inside the chamber of LCpro T (ADC) and irradiated with 0, 43, 87, 174, 261, 435, 696, 870, and 1044 μmol m-2 s-1. From 435 μmol m-2 s-1 and on, Std maintained the highest A. From 0 to 696 μmol m-2 s-1 HCa had the lowest E, while the LNa treatment had the highest. For gs, differences appeared after the 87 μmol m-2 s-1 point. Std had the highest gs while HCa had the lowest values. Moreover, Ci values were higher in the HNa and lower in the LCa. Finally, the Water Use Efficiency (WUE) was calculated as the A/E ratio and appeared greater in the HCa and lowest in the LNa. The obtained results indicate that even though the dominant factor for the impact of salinity on corn salad is the osmotic potential level, the source salinity should also be considered accordingly.