Abstract
In recent decades, increasing human pressure has caused the gradual deterioration of the physical and chemical properties of water and soil. Salinity is an important factor influencing the quality of water. The aim of this comprehensive research was to determine the effect of increasing concentrations of sodium chloride, which is a salinity inducer, on the yield, photosynthesis efficiency (expressed with chlorophyll fluorescence measurement) and content of selected nutrients in the leaves of hydroponically grown lettuce (Lactuca sativa L.). Experiments were conducted at the following concentrations of NaCl: 0 (control treatment), 10, 20, 40, and 60 mmol L−1. Studies were conducted in two independent seasons: spring and autumn. The plants exposed to NaCl stress modified their chemical composition by lowering the uptake of (for 60 mmol L−1 NaCl in relation to control): N (−11%), K (−35.7%), and Mg (−24.5%), while increasing the sodium content (+2400%). The Na:K ratio was significantly narrowed (from 76:1 to 2.6:1). The increase in the Cl level in the lettuce leaves may also have caused a decrease in the content of nitrates. As a result of disturbed ionic balance, the RWC was significantly reduced (−6.2%). As a result of these changes, the yield of the biomass of the aerial parts decreased (more than two-fold for the highest NaCl concentration in relation to control) whereas the dry matter content increased (+32%). The measurement of fluorescence showed significant changes at the PSII level. Salinity modified the energy flow rate (F0, FM, FV, FV/FM) as well as the specific energy flows through the reaction centre (ABS/RC, TR0/RC, ET0/RC, DI0/RC). The PSII functioning index, calculated on the basis of energy absorption (PIAbs), also changed. The salinity induced with NaCl significantly worsened the physiological reactions of the plants in the PSII, changed the ionic balance, which resulted in a significantly lower yield of the plants. Due to increasing water quality problems, it will be necessary to use, in agriculture on a much larger scale than before, saline water treatment systems (e.g., highly effective nanofiltration and/or reverse osmosis).
Highlights
During ontogenesis plants modify the course of physiological processes, and their growth and development is partially influenced by environmental factors [1]
The day before harvesting the plants, the OJIP test was applied to measure the following chlorophyll fluorescence parameters in each cycle: F0—initial fluorescence, FM— maximum fluorescence intensity, FV—maximum variable fluorescence, FV/FM—maximum photochemical quantum PSII after dark adaptation, ABS/RC—the light energy absorbed by the PSII antenna photon flux per active reaction centre, TR0/RC—total energy used to reduce QA by the unit reaction centre of PSII per energy captured by a single active RC, ET0/RC—rate of electron transport through a single RC, DI0/RC—non-photochemical quenching per reaction centre of PSII; total dissipation of energy not captured by the RC in the form of heat, fluorescence and transfer to other systems, PIAbs—performance index for energy conservation from excitation to the reduction of intersystem electron acceptors
The dry matter content is a determinant of post-harvest durability and it is an important parameter of the commercial value of vegetables for direct consumption
Summary
During ontogenesis plants modify the course of physiological processes, and their growth and development is partially influenced by environmental factors [1]. Soil salinity, which may be caused by both natural and anthropogenic factors, is an example of negative changes in the chemical and physical properties of soils This effect may be caused by the irrigation of plants with water with a high EC level due to a high concentration of sodium chloride. Some of the publications concern lettuce seedlings (short term experiments) [26] or salt stress effect is modified by other factors, e.g., application of silicon [27] For this reason, and in view of the fact that the quality of vegetables is strongly correlated with the conditions of their cultivation, research was undertaken to comprehensively investigate the response of lettuce (Lactuca sativa L.) to the increasing concentration of NaCl in a nutrient solution
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