Seedlings of Citrus grandis (L.) Osbeck cv. Tuyou were irrigated daily for 5 months with nutrient solution containing 0 (control), 0.2, 0.6 or 1.6 mM aluminum (Al) from AlCl₃·6H₂O. Shoot growth was more sensitive to Al toxicity than root growth, gas exchange, chlorophyll (Chl) concentration, polyphasic Chl a fluorescence (OJIP) induction and related parameters. Leaves of Al-treated plants showed decreased CO₂ assimilation and Chl concentration, yet intercellular CO₂ concentration increased and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was unchanged. Chlorophyll a fluorescence induction analysis of Al-stressed leaves showed a large rise at the O-step and a large depression at the P-step, accompanied by two new bands at 300 μs (K-band) and at about 150 μs (L-band). Maximum fluorescence, maximum quantum yield of primary photochemistry, oxygen-evolving complex (OEC), quantum yield of electron transport, quantum yield of electron transport from Q_A⁻ to the Photosystem I end electron acceptors, IP phase and total performance index were decreased in leaves of Al-treated plants, whereas minimum fluorescence, relative variable fluorescence at the J-step and I-step, and dissipated energy were increased. We propose that impaired electron transport capacity accompanied by lack of reducing equivalents were the main factors contributing to decreased CO₂ assimilation in Al-treated plants. Aluminum-induced photoinhibition occurring at both the donor (i.e., the OEC) and the acceptor sides of Photosystem II may be associated with growth inhibition. Besides decreased light absorption due to reduced Chl concentration, enhanced energy dissipation protected the leaves of Al-treated plants from photo-oxidative damage in high light.

Phosphorus (P) is an essential macronutrient for plant growth, development and production. However, little is known about the effects of P deficiency on nutrient absorption, photosynthetic apparatus performance and antioxidant metabolism in citrus. Seedlings of ‘sour pummelo’ (Citrus grandis) were irrigated with a nutrient solution containing 0.2 mM (Control) or 0 mM (P deficiency) KH₂PO₄ until saturated every other day for 16 weeks. P deficiency significantly decreased the dry weight (DW) of leaves and stems, and increased the root/shoot ratio in C. grandis but did not affect the DW of roots. The decreased DW of leaves and stems might be induced by the decreased chlorophyll (Chl) contents and CO₂ assimilation in P deficient seedlings. P deficiency heterogeneously affected the nutrient contents of leaves, stems and roots. The analysis of Chl a fluorescence transients showed that P deficiency impaired electron transport from the donor side of photosystem II (PSII) to the end acceptor side of PSI, which showed a greater impact on the performance of the donor side of PSII than that of the acceptor side of PSII and photosystem I (PSI). P deficiency increased the contents of ascorbate (ASC), H₂O₂ and malondialdehyde (MDA) as well as the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) in leaves. In contrast, P deficiency increased the ASC content, reduced the glutathione (GSH) content and the activities of SOD, CAT, APX and monodehydroascorbate reductase (MDHAR), but did not increase H₂O₂ production, anthocyanins and MDA content in roots. Taking these results together, we conclude that P deficiency affects nutrient absorption and lowers photosynthetic performance, leading to ROS production, which might be a crucial cause of the inhibited growth of C. grandis.

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.