Huan-Xin Jiang

Boron deficiency decreases growth and photosynthesis, and increases starch and hexoses in leaves of citrus seedlings.

Seedlings of sweet orange (Citrus sinensis) were fertilized for 14 weeks with boron (B)-free or B-sufficient (2.5 or 10 microM H(3)BO(3)) nutrient solution every other day. Boron deficiency resulted in an overall inhibition of plant growth, with a reduction in root, stem and leaf dry weight (DW). Boron-starved leaves showed decreased CO(2) assimilation and stomatal conductance, but increased intercellular CO(2) concentrations. Activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH) and stromal fructose-1,6-bisphosphatase (FBPase) were lower in B-deficient leaves than in controls. Contents of glucose, fructose and starch were increased in B-deficient leaves while sucrose was decreased. Boron-deficient leaves displayed higher or similar superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR) and glutathione reductase (GR) activities, while dehydroascorbate reductase (DHAR) and catalase (CAT) activities were lower. Expressed on a leaf area or protein basis, B-deficient leaves showed a higher ascorbate (AsA) concentration, but a similar AsA concentration on a DW basis. For reduced glutathione (GSH), we found a similar GSH concentration on a leaf area or protein basis and an even lower content on a DW basis. Superoxide anion (O(2)(-)) generation, malondialdehyde (MDA) concentration and electrolyte leakage were higher in B-deficient than in control leaves. In conclusion, CO(2) assimilation may be feedback-regulated by the excessive accumulation of starch and hexoses in B-deficient leaves via direct interference with chloroplast function and/or indirect repression of photosynthetic enzymes. Although B-deficient leaves remain high in activity of antioxidant enzymes, their antioxidant system as a whole does not provide sufficient protection from oxidative damage.

Effects of manganese-excess on CO2 assimilation, ribulose-1,5-bisphosphate carboxylase/oxygenase, carbohydrates and photosynthetic electron transport of leaves, and antioxidant systems of leaves and roots in Citrus grandis seedlings.

BackgroundVery little is known about the effects of manganese (Mn)-excess on citrus photosynthesis and antioxidant systems. Seedlings of sour pummelo (Citrus grandis) were irrigated for 17 weeks with nutrient solution containing 2 μM (control) or 500 μM (excess) MnSO4. The objective of this study were to understand the mechanisms by which Mn-excess leads to a decrease in CO2 assimilation and to test the hypothesis that Mn-induced changes in antioxidant systems differ between roots and leaves.ResultsMn-excess decreased CO2 assimilation and stomatal conductance, increased intercellular CO2 concentration, but did not affect chlorophyll (Chl) level. Both initial and total ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity in Mn-excess leaves decreased to a lesser extent than CO2 assimilation. Contents of glucose, fructose, starch and total nonstructural carbohydrates did not differ between Mn-excess leaves and controls, while sucrose content was higher in the former. Chl a fluorescence (OJIP) transients from Mn-excess leaves showed increased O-step and decreased P-step, accompanied by positive L- and K-bands. Mn-excess decreased maximum quantum yield of primary photochemistry (Fv/Fm) and total performance index (PItot,abs), but increased relative variable fluorescence at I-steps (VI) and energy dissipation. On a protein basis, Mn-excess leaves displayed higher activities of monodehydroascorbate reductase (MDAR), glutathione reductase (GR), superoxide dismutase (SOD), catalase (CAT) and guaiacol peroxidase (GPX) and contents of antioxidants, similar ascorbate peroxidase (APX) activities and lower dehydroascorbate reductase (DHAR) activities; while Mn-excess roots had similar or lower activities of antioxidant enzymes and contents of antioxidants. Mn-excess did not affect malondialdehyde (MDA) content of roots and leaves.ConclusionsMn-excess impaired the whole photosynthetic electron transport chain from the donor side of photosystem II (PSII) up to the reduction of end acceptors of photosystem I (PSI), thus limiting the production of reducing equivalents, and hence the rate of CO2 assimilation. Both the energy dissipation and the antioxidant systems were enhanced in Mn-excess leaves, while the antioxidant systems in Mn-excess roots were not up-regulated, but still remained high activity. The antioxidant systems in Mn-excess roots and leaves provided sufficient protection to them against oxidative damage.

CO2 assimilation, ribulose-1,5-bisphosphate carboxylase/oxygenase, carbohydrates and photosynthetic electron transport probed by the JIP-test, of tea leaves in response to phosphorus supply

BackgroundAlthough the effects of P deficiency on tea (Camellia sinensis (L.) O. Kuntze) growth, P uptake and utilization as well as leaf gas exchange and Chl a fluorescence have been investigated, very little is known about the effects of P deficiency on photosynthetic electron transport, photosynthetic enzymes and carbohydrates of tea leaves. In this study, own-rooted 10-month-old tea trees were supplied three times weekly for 17 weeks with 500 mL of nutrient solution at a P concentration of 0, 40, 80, 160, 400 or 1000 μM. This objective of this study was to determine how P deficiency affects CO2 assimilation, Rubisco, carbohydrates and photosynthetic electron transport in tea leaves to understand the mechanism by which P deficiency leads to a decrease in CO2 assimilation.ResultsBoth root and shoot dry weight increased as P supply increased from 0 to 160 μM, then remained unchanged. P-deficient leaves from 0 to 80 μM P-treated trees showed decreased CO2 assimilation and stomatal conductance, but increased intercellular CO2 concentration. Both initial and total Rubisco activity, contents of Chl and total soluble protein in P-deficient leaves decreased to a lesser extent than CO2 assimilation. Contents of sucrose and starch were decreased in P-deficient leaves, whereas contents of glucose and fructose did not change significantly except for a significant increase in the lowest P leaves. OJIP transients from P-deficient leaves displayed a rise at the O-step and a depression at the P-step, accompanied by two new steps at about 150 μs (L-step) and at about 300 μs (K-step). RC/CSo, TRo/ABS (or Fv/Fm), ETo/ABS, REo/ABS, maximum amplitude of IP phase, PIabs and PItot, abs were decreased in P-deficient leaves, while VJ, VI and dissipated energy were increased.ConclusionP deficiency decreased photosynthetic electron transport capacity by impairing the whole electron transport chain from the PSII donor side up to the PSI, thus decreasing ATP content which limits RuBP regeneration, and hence, the rate of CO2 assimilation. Energy dissipation is enhanced to protect P-deficient leaves from photo-oxidative damage in high light.

Phosphorus alleviates aluminum-induced inhibition of growth and photosynthesis in Citrus grandis seedlings.

Limited data are available on the effects of phosphorus (P) and aluminum (Al) interactions on Citrus spp. growth and photosynthesis. Sour pummelo (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 50, 100, 250 and 500 microM KH(2)PO(4)x 0 and 1.2 mM AlCl(3). 6H(2)O. Thereafter, P and Al in roots, stems and leaves, and leaf chlorophyll (Chl), CO(2) assimilation, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Chl a fluorescence (OJIP) transients were measured. Under Al stress, P increased root Al, but decreased stem and leaf Al. Shoot growth is more sensitive to Al than root growth, CO(2) assimilation and OJIP transients. Al decreased CO(2) assimilation, Rubisco activity and Chl content, whereas it increased or did not affect intercellular CO(2) concentration. Al affected CO(2) assimilation more than Rubisco and Chl under 250 and 500 microM P. Al decreased root, stem and leaf P, leaf maximum quantum yield of primary photochemistry (F(v)/F(m)) and total performance index (PI(tot,abs)), but increased leaf minimum fluorescence (F(o)), relative variable fluorescence at K- and I-steps. P could alleviate Al-induced increase or decrease for all these parameters. We conclude that P alleviated Al-induced inhibition of growth and impairment of the whole photosynthetic electron transport chain from photosystem II (PSII) donor side up to the reduction of end acceptors of photosystem I (PSI), thus preventing photosynthesis inhibition through increasing Al immobilization in roots and P level in roots and shoots. Al-induced impairment of the whole photosynthetic electron transport chain may be associated with growth inhibition.

Roles of Organic Acid Anion Secretion in Aluminium Tolerance of Higher Plants

Approximately 30% of the worlds total land area and over 50% of the worlds potential arable lands are acidic. Furthermore, the acidity of the soils is gradually increasing as a result of the environmental problems including some farming practices and acid rain. At mildly acidic or neutral soils, aluminium(Al) occurs primarily as insoluble deposits and is essentially biologically inactive. However, in many acidic soils throughout the tropics and subtropics, Al toxicity is a major factor limiting crop productivity. The Al-induced secretion of organic acid (OA) anions, mainly citrate, oxalate, and malate, from roots is the best documented mechanism of Al tolerance in higher plants. Increasing evidence shows that the Al-induced secretion of OA anions may be related to the following several factors, including (a) anion channels or transporters, (b) internal concentrations of OA anions in plant tissues, (d) temperature, (e) root plasma membrane (PM) H+-ATPase, (f) magnesium (Mg), and (e) phosphorus (P). Genetically modified plants and cells with higher Al tolerance by overexpressing genes for the secretion and the biosynthesis of OA anions have been obtained. In addition, some aspects needed to be further studied are also discussed.

Changes in organic acid metabolism differ between roots and leaves of Citrus grandis in response to phosphorus and aluminum interactions

Seedlings of sour pummelo (Citrus grandis) were irrigated daily for 18 weeks with nutrient solution containing four phosphorus (P) levels (50, 100, 250 and 500 microM KH2PO4) and two aluminum (Al) levels [0 (-Al) and 1.2 mM AlCl3 x 6H2O (+Al)]. Both malate and citrate concentrations in +Al leaves decreased with increasing P supply, but their concentrations in -Al leaves did not change in response to P supply. The concentrations of malate under 50 microM P and of citrate under 50 and 100 microM P were higher in +Al leaves than in -Al ones, but malate concentration was lower in +Al leaves than in -Al ones under 500 microM P. There was no difference in root malate and citrate concentrations among different P and Al combinations except for an increase in malate and citrate under 50 microM P+0 mM Al and a slight decrease in malate under 50 microM P+1.2 mM Al. The activities of acid-metabolizing enzymes (citrate synthase, aconitase, phosphoenolpyruvate carboxylase, NADP-isocitrate dehydrogenase, phosphoenolpyruvate phosphatase, NAD-malate dehydrogenase, NADP-malic enzyme and pyruvate kinase) in most cases were less affected by P and Al interactions in roots compared to the leaves. Our results support the hypothesis that changes in organic acid metabolism differ between roots and leaves of C. grandis in response to P and Al interactions.

Root release and metabolism of organic acids in tea plants in response to phosphorus supply

Self-rooted, 10-month-old, uniform tea [Camellia sinensis (L.) O. Kuntze cv. Huangguanyin] plants were supplied for 17 weeks with 0, 40, 80, 160, 400, or 1000μM phosphorus (P) to investigate the effects of P supply on root citrate and malate release, the concentrations of malate and citrate and the activities of acid-metabolizing enzymes in leaves and roots. Root malate release and accumulation was induced by both 0 and 40μM P, while root citrate release and accumulation was induced only by 0μM P. Phosphorus-deficiency-induced malate and citrate release coincided with higher concentrations of root malate and citrate. The higher concentrations of malate and citrate were accompanied by increased activities of phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate phosphatase (PEPP), citrate synthase (CS) and NAD-malic enzyme (NAD-ME) and decreased activities of pyruvate kinase (PK), NADP-ME and NADP-isocitrate dehydrogenase (NADP-IDH) in roots. In contrast to roots, malate accumulated in the leaves only in response to 0μM P, and no change was observed in citrate levels. The P-deficiency-induced leaf malate accumulation coincided with increased activities of NADP-ME, NAD-ME and PK. Overall, the P-deficiency-induced changes in organic acid (OA) metabolism differed between roots and leaves. The high tolerance of tea plants to P-deficiency might be involved in two major processes: (a) increasing the availability of P by inducing root release of OA anions; and (b) improving the ability to use P efficiently by inducing bypass enzymes involved in tissue P economy.

Boron-aluminum interactions affect organic acid metabolism more in leaves than in roots of Citrus grandis seedlings

Sour pummelo (Citrus grandis) seedlings were irrigated with nutrient solution containing four boron concentrations (i.e., 2.5, 10, 25 and 50 μM H3BO3) and two aluminum concentrations [i.e., 0 (-Al) and 1.2 mM AlCl3 · 6 H2O (+Al)]. It was found that B did not affect, but Al increased, the Al content in the roots. The Al and citrate contents in the -Al leaves either did not change or slightly increased with increasing B concentration. On the other hand, the Al and citrate contents in the +Al leaves rapidly decreased as B concentration increased from 2.5 to 50 μM, then decreased at the highest B concentration. The Al and citrate contents were higher in the +Al than in the -Al leaves, except for at 25 μM B when they were similar. The leaf malate content did not change in response to B or Al, except for an increase in the +Al leaves and a decrease in the -Al leaves at 2.5 μM B. Similarly, the root malate and citrate contents did not change in response to B with or without Al, except for a decrease in the malate and citrate contents in the +Al roots at 50 μM B and an increase in the citrate content in the -Al roots at 50 μM B. The activities of acid-metabolizing enzymes were less affected by B-Al interactions in the roots than in the leaves.