Abstract
Seedlings of “Xuegan” (Citrus sinensis) and “Sour pummelo” (Citrus grandis) were irrigated daily with a nutrient solution at a pH of 2.5, 3, 4, 5, or 6 for 9 months. Thereafter, the following responses were investigated: seedling growth; root, stem, and leaf concentrations of nutrient elements; leaf gas exchange, pigment concentration, ribulose-1,5-bisphosphate carboxylase/oxygenase activity and chlorophyll a fluorescence; relative water content, total soluble protein level, H2O2 production and electrolyte leakage in roots and leaves. This was done (a) to determine how low pH affects photosynthesis, related physiological parameters, and mineral nutrient profiles; and (b) to understand the mechanisms by which low pH may cause a decrease in leaf CO2 assimilation. The pH 2.5 greatly inhibited seedling growth, and many physiological parameters were altered only at pH 2.5; pH 3 slightly inhibited seedling growth; pH 4 had almost no influence on seedling growth; and seedling growth and many physiological parameters reached their maximum at pH 5. No seedlings died at any given pH. These results demonstrate that citrus survival is insensitive to low pH. H+-toxicity may directly damage citrus roots, thus affecting the uptake of mineral nutrients and water. H+-toxicity and a decreased uptake of nutrients (i.e., nitrogen, phosphorus, potassium, calcium, and magnesium) and water were likely responsible for the low pH-induced inhibition of growth. Leaf CO2 assimilation was inhibited only at pH 2.5. The combinations of an impaired photosynthetic electron transport chain, increased production of reactive oxygen species, and decreased uptake of nutrients and water might account for the pH 2.5-induced decrease in CO2 assimilation. Mottled bleached leaves only occurred in the pH 2.5-treated C. grandis seedlings. Furthermore, the pH 2.5-induced alterations of leaf CO2 assimilation, water-use efficiency, chlorophylls, polyphasic chlorophyll a fluorescence (OJIP) transients and many fluorescence parameters, root and leaf total soluble proteins, H2O2 production, and electrolyte leakage were all slightly greater in C. grandis than in C. sinensis seedlings. Hence, C. sinensis was slightly more tolerant to low pH than C. grandis. In conclusion, our findings provide novel insight into the causes of low pH-induced inhibition of seedling growth and leaf CO2 assimilation.
Highlights
Acidic soils that limit crop growth and productivity are often observed all over the world, especially in the tropics and subtropics
Our results showed that pH 2.5 caused an increased O-step and P-step in C. sinensis and C. grandis leaves compared with pH 5, and that the pH 2.5-treated C. sinensis and C. grandis leaves had positive I, J, K, and L-bands around 30 ms, 2 ms, 300 μs, and 130 μs as compared with the pH 5-treated leaves, respectively
We found that pH 2.5 greatly inhibited the CO2 assimilation in C. grandis and C. sinensis leaves, and that this inhibition was slightly greater in C. grandis than in C. sinensis leaves (Figure 3A)
Summary
Acidic soils that limit crop growth and productivity are often observed all over the world, especially in the tropics and subtropics. Poor crop growth and yield on acidic soils is usually due to the combination of toxicities of H+, Al, and manganese (Mn) and a lack of nutrients—namely phosphorus (P), calcium (Ca), magnesium (Mg), potassium (K), and molybdenum (Mo)— and a reduced uptake of water (von Uexküll and Mutert, 1995; Bian et al, 2013). Anugoolprasert et al (2012) reported that the uptake of N, P, K, Ca, and Mg, and their concentration in roots, leaflets, petioles and whole plant, were not altered over the range of pH 3.6 to 5.7 for 4.5 months; this possibly explains the normal growth of sago palm seedlings at pH 3.6. Anugoolprasert et al (2012) reported that the uptake of N, P, K, Ca, and Mg, and their concentration in roots, leaflets, petioles and whole plant, were not altered over the range of pH 3.6 to 5.7 for 4.5 months; this possibly explains the normal growth of sago palm seedlings at pH 3.6. Kidd and Proctor (2001) have suggested that the direct toxicity of H+ was the primary cause of the poor growth in H+-intolerant plants growing in very acidic soils
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