| Al toxicity is the major factor limiting crop growth on acid soils. Evidence shows that the exclusion of Al from the root apex via root exudation of organic acids (OAs) is considered to be a major mechanism of Al tolerance in plants. Increasing experimental evidence shows that P and B supply can alleviate Al toxicity symptoms. In this paper, we investigated the effects of P-Al interactions and B-Al interactions on the organic acid metabolism in leaves and roots of Al-sensitive sour pummelo [Citrus grandis (L.) Osbeck)]. There were eight treatments for P-Al interactions and B-Al interactions, including four P levels (50, 100, 250 and 500μmol/L KH2PO4) and two Al levels (0 and 1.2 mmol/L AlCl3·6H2O) and four B levels (2.5, 10, 25 and 50μmol/L H3BO3) and two Al levels (0 and 1.2 mmol/L AlCl3·6H2O), respectively. Sour pummelo seedlings grown in pots containing sand were irrigated daily for 18 weeks with nutrient solution. At the end of the experiment, plant biomass, the contents of P, Al, B, malate and citrate, and the activities of acid-metabolising enzymes (CS, ACO, PEPC, NADP-ICDH, PEPP, NAD-MDH, NADP-ME, PK) in the roots and leaves were investigated.1 Effects of P-Al interactions on plant growth, and root and leaf organic acid metabolism of sour pmmelo seedlingsPlant biomass increased with increasing P supply in the presence of Al, suggesting that P supply can alleviate the effects of Al toxicity on plant growth. At the presence of 50 and 100μmol/L P, Al treatment increased leaf Al content. The finding that Al content in Al-treated roots increased arranging from 50 to 250μmol/L P concentration, means that increasing quantities of Al were immobilized in root tissues with increasing P supply.Both malate and citrate contents in Al-treated leaves decreased with increasing P supply. Malate content in the presence of 50μmol/L P and citrate content in the presence of 50 and 100μmol/L P were higher in leaves treated with Al than in those not treated with Al. At the presence of 50 and 100μmol/L P, both PEPC and PEPP activities were significantly higher in leaves treated with Al than in those not treated with Al, whereas reverse was the case for NADP-ICDH, NADP-ME and NAD-MDH activities. These results indicated that low-P-induced accumulation of malate and citrate in Al-treated leaves seems to be a consequence of both increased biosynthesis and reduced degradation. There was no difference in root malate and citrate concentrations among different P and Al combinations except for an increase for malate and citrate in the combination of 50μM P + 0 mM Al.2 Effects of B-Al interactions on plant growth, and root and leaf organic acid metabolism of sour pmmelo seedlingsAdequate B in nutrient solutions can prevent Al-induced inhibition of plant growth. Generally, the order of effectiveness was 25μmol/L B > 10 and 50μmol/L B > 2.5μmol/L B. None of the B treatments had a significant effect on root Al content, however, leaf Al content decreased as B supply increased from 2.5 to 25μmol/L in the presence of Al.Malate content was significantly higher in leaves treated with Al than in those not treated with Al in the lowest B supply. Citrate content was significantly higher in leaves treated with Al than in those not treated with Al in the pesence of 2.5,10 and 50μmol/L B, while there was no difference between leaves treated with Al and without Al in the presence of 25μmol/L B. Regressive analysis showed that leaf malate and citrate contents increased with increasing leaf PEPC and PEPP activities. Under Al stress, citrate content was significantly higher in leaves treated with 2.5 and 50μmol/L B than in those treated with 10 and 25μmol/L B, and the reverse was the case for root citrate content. The observed higher citrate content in Al-treated roots in the presence of 10 and 25μmol/L B could result from the organic acids or related precursors produced in the aerial parts of pummelo plants which are exported into the roots. The finding that malate content in Al-treated roots decreased with increasing B supply could be caused by decreased biosynthesis because PEPC activity in Al-treated roots showed a downward trend with increasing B supply.
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