Chlorophyll a Fluorescence and Chloroplastidic Pigments as Indicators of Aluminum Toxicity in Hydroponic Lettuce

Abstract

Aluminum (Al) toxicity is a major limiting factor for plants, but its effects in hydroponic systems, particularly on chlorophyll and the photosystem, remain poorly understood. Among horticultural crops, lettuce is notable for its production versatility in diverse systems, which may be susceptible to exposure to this metal. Therefore, this study aimed to evaluate the effect of Al on the growth, tissue accumulation, chloroplastidic pigments, and photosystem of lettuce grown in a hydroponic system. For this purpose, lettuce plants (cv. Regina) were exposed to 0, 50, 100, 250, and 500 µM of aluminum nitrate in Hoagland solution. Tissue aluminum content, growth parameters, chloroplastidic pigments, and chlorophyll a fluorescence were evaluated from the 42nd to the 70th day after seedling emergence (DAE). Al accumulated preferentially in the roots. Translocation to the leaves was highest at 100 µM and decreased at higher concentrations. Growth was initially inhibited (day 42) at the highest concentrations, followed by a significant increase in biomass and leaf number (from day 56 onward) compared to the control. However, Al stress caused a progressive and dose-dependent reduction in chlorophyll a , b , and carotenoid contents. Fluorescence analysis on the 70th day revealed a drastic drop in photochemical efficiency under light (Fv’/Fm’) and the vitality index (Rfd), with an increase in non-photochemical quenching (qN). Although the lettuce showed increased biomass, possibly due to the fertilizing effect of nitrate (NO 3 ), this growth occurred at the expense of photosynthetic health, compromising photochemical efficiency and pigment content.

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