Metabolic Pathway of α-Ketoglutarate in Citrus Leaves as Affected by Phosphorus Nutrition

Abstract

The uptake ad metabolism of a-15-14Clketoglutarate by phospborusdeficient and full nutrient (control) iemo (Citus lmn) leaves were studied over various thme intervals. After 45 minutes In P-deficient leaves, the bulk of Incorporated 14C appeared In organk acids and much less In amino acids, while In the control leaves, the 14C contests of organic and amino acids were equal. In P-efcient leaves, after loger Incubation times the 14C content of organic acids and anino acids Increased, whRe that of CO and residue fractions remaied iow. In full nutrient leaves the "C content of amino acids and organic acids decreased after ionger iocubaton time and Increased in the insolube residue and CO2. In full utient leaves the organic and amino acid metaboUsm were ciosely related and accompanied by protein synthesis and CO2 release, whie in P-deficient leaves an accelerating accumulation of arginine and citric acid was iHnked together With hibitio of protein syntheis and CO liberation.Accumulation of arginine is a well recognized metabolic disturbance induced by phosphorus deficiency in various plants (6,10,11,21). In P-deficient lemon seedlings we found an inverse relationship between their growth and arginine accumulation: as the rate of growth declined the concentration of arginine rose, while the other amino acids and protein remained low in comparison with seedlings grown in full nutrient solution (1). This deviation from the normal pattern of amino acid synthesis may be attributed to a decrease in activity of transaminases, enzymes which require pyridoxal phosphate as a coenzyme (2). In a study of the correlation among phosphate ratio, GOT2 activity, and PLP content, we found that the reduced activity of enzyme could be restored by infiltration of pyridoxal phosphate or mineral phosphate into the P-deficient leaves (2). On the other hand, determinations oforganic acids disclosed extensive accumulation ofcitrate in P-deficient lemon leaves (unpublished data).In the present study we followed the incorporation of a-ketoglutarate into various cell metabolities in P-deficient and normal citrus leaves. In each jar, a small vial containing 1.5 ml of methanol-ethanolamine (4:1, v/v), was placed to trap the "CO2.At the end of the incubation period the radioactivity of CO2 was counted in 10 ml of toluene fluor. During the incubation period, the jars were shaken gently in a water bath at 35 C. The incubation periods were 45, 90, and 180 min. Each combination of treatments (mineral nutrition and incubation period) was tested in three replicates.At the end of the incubation period the buffer solution was filtered through Miracloth. The discs were rinsed with deionized H20, blotted on filter paper, and boiled in 15 ml of 100%o ethanol for 10 min.Extraction. The discs were homogenized for 2 min in a VirTis blender set at speed 60, and centrifuged at 10,000g. The pellet was reextracted four times by shaking for 10 min with 10-ml portions of 80% (v/v) ethanol and then centrifuged (22).The insoluble residue was dried over silica ge...

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

“…Unlike in the present work, P deficiency increased the concentration of citrate in rape and hedge mustard shoots, whereas the concentration of malate was affected little in hedge mustard shoots and even declined in rape shoots (Hoffland et al, 1992). In lemon leaves, the P-deficiency-induced increase in the concentration of malate was much less than that of citrate (Achituv and Bar-Akiva, 1978). In P-deficient rice leaves, the marked decrease in oxalate concentration was accompanied by increased malate and citrate concentration (Nanamori et al, 2004).…”

“…Achituv and Bar-Akiva (1978) showed that citrate concentration in P-deficient lemon (Citrus limon Burm.) leaves was almost 20 times higher than in controls, while P-deficiency-induced increase in malate concentration was much less.…”

Magnesium deficiency-induced changes in organic acid metabolism of Citrus sinensis roots and leaves

“…In China, Mg-deficiency is frequently observed in Citrus plantations and is responsible for loss of productivity and fruit quality (Ling et al 2009). Previous studies showed that malate and citrate were the major OAs in both leaves (Achituv and Bar-Akiva 1978) and fruit juice (Yamaki 1989) of Citrus. In plant cells, malate synthesis mainly occurs in the cytosol and is catalyzed by phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) and NAD-malate dehydrogenase (NAD-MDH, EC 1.1.1.37) (Moing et al 2000).…”