Hydroponic lettuce yields are improved under salt stress by utilizing white plastic film and exogenous applications of proline

Greenhouse crops are often affected by salinity due to water quality decay associated with crop fertilization and irrigation managements as well as the fact that greenhouse environment may become even hotter during summer time. Thus, combined effects of salt, heat and light stresses can affect plants grown in greenhouse. This research work addressed the combined effect of white greenhouse covering film and foliar proline spray application to reduce the detrimental effects of salinity on two cultivars of lettuce (Lactuca sativa cv. Teide and cv. Impulsion) grown on a floating system. Five different experiments were conducted in two twin greenhouses covered with plastic films characterized by different light permeability. The experiments aimed at identifying the most suitable nutrient solution (exp. #1), and assessing how the effects of mild salinity (0–15 mM NaCl) would be alleviated by the greenhouse covering film (exp. #2, #3, #4 and #5) and foliar proline spray application (0–15 μM) (exp. #4 and #5). Results showed that the white covering film changed the spectral light intensity and decreased the Photosynthetically Active Radiation (PAR) of the light transmitted causing a delay in the plant growth and leaf chlorophyll content. Although salinity negatively affected plant growth and leaf photosynthesis of both cultivars, using the white film partially mitigated the influence of salt stress. The beneficial effects of the white film on salt stress mitigation were more evident during summer and in the heat sensitive genotype (cv. Teide) in terms of greater total and marketable yield as compared to control conditions. Exogenous application of foliar proline (up to 5 μM) increased the yield under control condition and enhanced the plant response to salinity. Overall, for summer cultivation of cv. Teide, in presence of saline water (15 mM NaCl), the combination of both white covering film and proline application enabled to preserve efficiently the plant growth and final yield.