Increased pH-mediated alleviation of copper-toxicity and growth response function in Citrus sinensis seedlings

Abstract Little is known about pH-mediated alleviation of copper (Cu)-toxicity in plants. ‘Xuegan’ [Citrus sinensis (L.) Osbeck] seedlings were irrigated with nutrient solution at a pH 3.0, 4.0 or 4.8 and a concentration of 0.5 (control) and 300 (Cu-toxicity) μM CuCl2 for 17 weeks. Thereafter, a total of 125 physiological parameters including biomass, nutrient elements in roots, stems and leaves, relative water content (RWC) in roots and leaves, and pigments, gas exchange, chlorophyll (Chl) a fluorescence (OJIP) transients and related fluorescence parameters in leaves were investigated in order to understand the physiological mechanisms for increased pH-mediated alleviation of Cu-toxicity. Our results clearly demonstrated that both pH 3.0 (at a concentration of 0.5 or 300 μM Cu) and 300 μM Cu (at a pH of 3.0, 4.0 or 4.8) were toxic to C. sinensis seedlings, as indicated by decreased biomass and/or CO2 assimilation. Raised pH decreased Cu uptake per plant (root dry weight) and Cu concentration in roots, stems and leaves and mitigated Cu-toxic damage to root growth and function, thus improving the uptake of water (root and leaf RWC) and some nutrients (nitrogen, potassium, phosphorus, magnesium, calcium, sulfur and iron), as well as the distribution of some nutrient elements (potassium, sulfur and zinc) in leaves, hence mitigating Cu-toxic impairment on the whole electron transport chain and increasing plant growth, leaf pigment level and CO2 assimilation. Principal component analysis (PCA) indicated that Cu-toxic-effects on all these physiological parameters as a whole decreased with increase of pH. Low pH (pH 3.0) effects on OJIP transients and most of physiological parameters were less pronounced in control than in Cu-toxic seedlings. Interactive effects of pH Cu were significant on 84 physiological parameters. To conclude, low pH intensified Cu-toxic effects on C. sinensis seedlings; Cu-toxicity aggravated low-pH effects on C. sinensis seedlings.