Aleysia Kleinert

Phosphorus deficiency affects the allocation of below-ground resources to combined cluster roots and nodules in Lupinus albus.

Lupins can rely on both cluster roots and nodules for P acquisition and biological nitrogen fixation (BNF), respectively. The resource allocation (C, N and P) between cluster roots and nodules has been largely understudied during P-deficient conditions. The aim of this investigation was therefore to determine the changes in resource allocation between these organs during fluctuations in P supply. Lupinus albus was cultivated in sand culture for 3 weeks, with either sufficient (2 mM high) or limiting (0.1 mM low) P supply. Although variation on P supply had no effect on the total biomass, there were significant differences in specialised below-ground organ allocation to cluster roots and nodule formation. Cluster root formation and the associated C-costs increased during low P supply, but at sufficient P-supply the construction and growth respiration costs of cluster roots declined along with their growth. In contrast to the cluster root decline at high P supply, there was an increase in nodule growth allocation and corresponding C-costs. However, this was not associated with an increase in BNF. Since cluster roots were able to increase P acquisition under low P conditions, this below-ground investment may also have benefited the P nutrition of nodules. These findings provide evidence that when lupins acquire N via BNF in their nodules, there may be a trade-off in resource allocation between cluster roots and nodules.

The reallocation of carbon in P deficient lupins affects biological nitrogen fixation.

It is not known how phosphate (P) deficiency affects the allocation of carbon (C) to biological nitrogen fixation (BNF) in legumes. The alteration of the respiratory and photosynthetic C costs of BNF was investigated under P deficiency. Although BNF can impose considerable sink stimulation on host respiratory and photosynthetic C, it is not known how the change in the C and energy allocation during P deficiency may affect BNF. Nodulated Lupinus luteus plants were grown in sand culture, using a modified Long Ashton nutrient solution containing no nitrogen (N) for ca. four weeks, after which one set was exposed to a P-deficient nutrient medium, while the other set continued growing on a P-sufficient nutrient medium. Phosphorus stress was measured at 20 days after onset of P-starvation. During P stress the decline in nodular P levels was associated with lower BNF and nodule growth. There was also a shift in the balance of photosynthetic and respiratory C toward a loss of C during P stress. Below-ground respiration declined under limiting P conditions. However, during this decline there was also a shift in the proportion of respiratory energy from maintenance toward growth respiration. Under P stress, there was an increased allocation of C toward root growth, thereby decreasing the amount of C available for maintenance respiration. It is therefore possible that the decline in BNF under P deficiency may be due to this change in resource allocation away from respiration associated with direct nutrient uptake, but rather toward a long term nutrient acquisition strategy of increased root growth.

Low-phosphorus conditions affect the nitrogen nutrition and associated carbon costs of two legume tree species from a Mediterranean-type ecosystem

The role of phosphorus nutrition in two-legume tree species from the Mediterranean-type ecosystem of the Cape Floristic Region (CFR) in South Africa was investigated. There is very little information about the functional adaptations of nitrogen (N) and phosphorus (P) nutrition in these legume trees growing in nutrient-poor soils. Nodulated Virgilia divaricata and V. oroboides tree saplings were grown in sterilised sand and supplied with Long Ashton nutrient solution, which was modified to contain either sufficient-phosphate (500 mM) or low-phosphate (5 mM) nutrient solution for 90 days. During low-P conditions, the growth of V. divaricata was not affected, whereas V. oroboides showed a decreaseingrowth.ThedecreaseinV.oroboidesunderlow-PconditionswasrelatedtothelowerPuptake,whichresultedin an alteration in belowground biomass allocation, which consequently affected on the N nutrition and carbon (C) cost of growth. In this regard, V. oroboides plants allocated less biomass to roots and nodules, as a proportion of whole plant growth. The impact of this was a decline in N nutrition, growth respiration and photosynthetic costs in V. oroboides. In contrast, V. divaricata maintained its P concentrations, photosynthetic costs and increased its nodule allocation under low-P conditions, to the benefit of N nutrition. The two CFR tree legumes appear to have different adaptations to low-P conditions, which may influence their N and P acquisition in their naturally low-P environment. Additional keywords: acidic soils, biological nitrogen, fynbos, nutrient deficiencies fixation, Virgilia.

Phosphorus-deficiency reduces aluminium toxicity by altering uptake and metabolism of root zone carbon dioxide.

The role of phosphorus (P) status in root-zone CO(2) utilisation for organic acid synthesis during Al(3+) toxicity was assessed. Root-zone CO(2) can be incorporated into organic acids via Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31). P-deficiency and Al(3+) toxicity can induce organic acid synthesis, but it is unknown how P status affects the utilisation of PEPC-derived organic acids during Al(3+) toxicity. Two-week-old Solanum lycopersicum seedlings were transferred to hydroponic culture for 3 weeks. The hydroponic culture consisted of a standard Long Ashton nutrient solution containing either 0.1μM or 1mM P. Short-term Al(3+) toxicity was induced by a 60-min exposure to a pH-buffered solution (pH 4.5) containing 2mM CaSO(4) and 50μM AlCl(3). Al(3+) toxicity induced a decline in root respiration, adenylate concentrations and an increase in root-zone CO(2) utilisation for both P sufficient and P-deficient plants. However during Al(3+) toxicity, P deficiency enhanced the incorporation and metabolism of root-zone CO(2) via PEPC. Moreover, P deficiency led to a greater proportion of the PEPC-derived organic acids to be exuded during Al(3+) toxicity. These results indicate that P-status can influence the response to Al(3+) by inducing a greater utilisation of PEPC-derived organic acids for Al(3+) detoxification.

Adaptive strategies for nitrogen metabolism in phosphate deficient legume nodules

Legumes play a significant role in natural and agricultural ecosystems. They can fix atmospheric N2 and contribute the fixed N to soils and plant N budgets. In legumes, the availability of P does not only affect nodule development, but also N acquisition and metabolism. For legumes as an important source of plant proteins, their capacity to metabolise N during P deficiency is critical for their benefits to agriculture and the natural environment. In particular for farming, rock P is a non-renewable source of which the world has about 60-80 years of sustainable extraction of this P left. The global production of legume crops would be devastated during a scarcity of P fertiliser. Legume nodules have a high requirement for mineral P, which makes them vulnerable to soil P deficiencies. In order to maintain N metabolism, the nodules have evolved several strategies to resist the immediate effects of P limitation and to respond to prolonged P deficiency. In legumes nodules, N metabolism is determined by several processes involving the acquisition, assimilation, export, and recycling of N in various forms. Although these processes are integrated, the current literature lacks a clear synthesis of how legumes respond to P stress regarding its impact on N metabolism. In this review, we synthesise the current state of knowledge on how legumes maintain N metabolism during P deficiency. Moreover, we discuss the potential importance of two additional alterations to N metabolism during P deficiency. Our goals are to place these newly proposed mechanisms in perspective with other known adaptations of N metabolism to P deficiency and to discuss their practical benefits during P deficiency in legumes.

Carbon Metabolism and Costs of Arbuscular Mycorrhizal Associations to Host Roots

Arbuscular mycorrhizal (AM) fungi form beneficial associations with host root systems, during which the soil nutrient acquisition by the fungal symbiont induces an alteration in AM root carbon metabolism and belowground carbon costs of the host plant. The reciprocal exchange of host-derived carbohydrates for the symbiont-acquired nutrients may be controlled by both partners in the symbiosis. The carbohydrates not only serve as the fuel for fungal growth and maintenance but also provide energy for nutrient uptake and assimilation of inorganic minerals such as nitrogen and phosphorus. The belowground carbon cost may be further influenced by biotic and abiotic factors that affect the basic carbon metabolism of the AM roots. In natural and agricultural soils, major factors affecting carbon economy include the concentration and source of nitrogen or phosphorus nutrition, the developmental stages of the host, and the presence of additional symbionts, such as nitrogen-fixing bacteria in legume nodules. Since each of these factors may complicate the measurement of the carbon economy, several methods are proposed to evaluate the carbon costs of nutrient uptake and assimilation by AM roots.

The role of phosphorus deficiency in nodule microbial composition, and carbon and nitrogen nutrition of a native legume tree in the Cape fynbos ecosystem

In phosphorus (P)-poor ecosystems, microbial communities can play a major role in the nitrogen (N) mineral nutrition during N2 fixation in legumes. This study investigated the role of P nutrition on the composition of N2-fixing bacterial community in Virgilia divaricata root nodules, grown under glasshouse conditions. V. divaricata seeds were germinated in Fynbos soil as a natural inoculum, and, thereafter, transferred into sterile quartz-sand cultures and supplied with 500 µM P and 5 µM P, respectively. The N2-fixing bacterial communities in the rhizosphere and root nodules were examined on the basis of the polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE) banding patterns of 16S rDNA and sequencing methods. The GenBank blast results showed that V. divaricata was nodulated by a wide range of root-nodule bacterial strains also found in the rhizosphere. These included Burkholderia phytofirmans, Burkholderia sp. and Bradyrhizobium sp., during both high and low P supply. The 15N natural-abundance data also confirmed that 40–50% of the N nutrition was from symbiotic N2 fixation. This is not only evidence of nodulation, but an indication of the adaptation of a range of N2-fixing bacterial strain species to the nutrient-poor, sandy, acidic soil of the Mediterranean-type ecosystems of the fynbos vegetation in the Cape Floristic Region (CFR). Legume species V. divaricata is highly adapted to the low-nutrient soils of its native range by its association with the symbiotic N2-fixing bacteria.

Variable P supply affects N metabolism in a legume tree, Virgilia divaricata, from nutrient-poor Mediterranean-type ecosystems

Virgilia divaricata Adamson is a forest margin legume that is known to invade the N- and P-poor soils of the mature fynbos, implying that it tolerates variable soil N and P levels. It is not known how the legume uses inorganic N from soil and atmospheric sources under variable P supply. Little is known about how P deficiency affects the root nodule metabolic functioning of V. divaricata and the associated energy costs of N assimilation. This study aimed to determine whether P deficiency affects the metabolic status of roots and nodules, and the impact on the routes of N assimilation in V. divaricata.V. divaricata had reduced biomass, plant P concentration and biological nitrogen fixation during P deficiency. Based on adenylate data, P-stressed nodules maintained their P status better than P-stressed roots. V. divaricata was able to alter C and N metabolism differently in roots and nodules under P stress. This was achieved via internal P cycling by possible replacement of membrane phospholipids with sulfolipids and galactolipids, and increased reliance on the pyrophosphate (PPi)-dependent metabolism of sucrose via UDP-glucose (UDPG) and to fructose-6-phosphate (Fru-6-P). P-stressed roots mostly exported ureides as organic N and recycled amino acids via deaminating glutamate dehydrogenase. In contrast, P-stressed nodules largely exported amino acids. Compared with roots, nodules showed more P conservation during low P supply. The roots and nodules of V. divaricata metabolised N differently during P stress, meaning that these organs may contribute differently to the success of this plant in soils from forest to fynbos.

Dynamic responses of photosynthesis and the antioxidant system during a drought and rehydration cycle in peanut plants

Drought stress is one of the most important environmental factors that adversely affect the productivity and quality of crops. Most studies focus on elucidating plant responses to this stress but the reversibility of these effects is less known. The aim of this work was to evaluate whether drought-stressed peanut (Arachis hypogaea L.) plants were capable of recovering their metabolism upon rehydration, with a focus on their antioxidant system. Peanut plants in the flowering phase (30 days after sowing) were exposed to drought stress by withholding irrigation during 14 days and subsequent rehydration during 3 days. Under these conditions, physiological status indicators, reactive oxygen species production and antioxidant system activity were evaluated. Under drought stress, the stomatal conductance, photosynthetic quantum yield and 13C:12C ratio of the peanut plants were negatively affected, and also they accumulated reactive oxygen species. The antioxidant system of peanut plants showed increases in superoxide dismutase-, ascorbate peroxidase- and glutathione reductase-specific activities, as well as the total ascorbate content. All of these responses were reversed upon rehydration at 3 days. The efficient and dynamic regulation of variables related to photosynthesis and the antioxidant system during a drought and rehydration cycle in peanut plants was demonstrated. It is suggested that the activation of the antioxidant system could mediate the signalling of drought stress responses that enable the plant to survive and recover completely within 3 days of rehydration.

Short-term supply of elevated phosphate alters the belowground carbon allocation costs and functions of lupin cluster roots and nodules.

The legume Lupinus albus is able to survive under low nutrient conditions due to the presence of two specialized below ground organs for the acquisition of nitrogen and phosphate, respectively.In this regard, cluster roots increase phosphate uptake and root nodules acquire atmospheric N₂via biological nitrogen fixation(BNF). Although these organs normally tolerate low phosphate conditions, very little is known about their physiological and metabolic flexibility during short-term changes in phosphate supply. The aim of this investigation was therefore to determine the physiological and metabolic flexibility of these organs during short-term supply of elevated phosphate nutrition. L. albus was cultivated in sand culture for 4 weeks at 0.1 mM phosphate supply, and then supplied with 2 mM phosphate for 2 weeks. Short-term elevated phosphate supply caused increased allocation of carbon and respiratory costs to nodules, at the expense of cluster root function. This alteration was also reflected in the increase in nodule enzyme activities related to organic acid synthesis, such as Phosphoenol-pyruvate Carboxylase (PEPC), Pyruvate Kinase (PK), Malate Dehydrogenase(NADH-MDH) and Malic Enzyme (ME). In cluster roots, elevated phosphate conditions caused a decline in these organic acid synthesizing enzymes. Phosphate recycling via Acid Phosphatase (APase),declined in nodules with elevated phosphate supply, but increased in cluster roots. Our findings suggest that during short-term elevated phosphate supply, there is a great degree of physiological and metabolic flexibility in lupin nutrient acquiring structures, and that these changes are related to the altered physiology of these organs [corrected].