Alkalinity of water is considered critical for plant growth. Interactions between water alkalinity and solution pH on lettuce were investigated. Seeds of romaine type lettuce were sown in pots containing perlite and irrigated with a high alkaline nutrient solution at four pH values (5, 6, 7, and 8). Considering the high amount of Fv/Fm at pH 5; higher shoot growth (at pH 5) could be due to the improvement of the efficiency of the photosynthesis apparatus of plants and consequently higher photochemical quantum yield. Although, magnesium (Mg) concentration in leaves increased with elevating pH up to 7 and then decreased at 8, but iron (Fe), manganese (Mn), and zinc (Zn) concentrations decreased at higher solution pH levels, constantly. In commercial lettuce production in which water with high pH and carbonate species is normally used, it is better to use safe acids to reduce the pH in nutrient solutions to 5.
