High P-use Efficiency Research Articles

Phosphorus deficiency tolerance in sorghum

Sorghum is a globally important commodity for food, feed, and fuel, and is known to have a high tolerance to heat, drought, and other abiotic stresses, and have a large genetic variation for grain yield under low-P conditions. Agricultural land in Indonesia is dominated by acid soils with limited P availability, of a total of 144.5 million ha, around 107.3 million ha are acid soils. Information regarding the tolerance of sorghum to P deficiency conditions is still very limited. The review aimed to discuss the adaptation strategy of sorghum to P deficiency. Studies showed that sorghum has a moderate adaptation to Al3+ stress with low P. Under various P levels, the performance of sorghum was also very diverse following the wide genetic diversity. From evaluations of several varieties and inbred lines with high tolerance to P deficiency that has been obtained, sorghum tolerance strategies to low P conditions are: (1) increase root size (length, diameter, and volume), and root proteoid on several genotypes, (2) increase secretion of oxalic acid, particularly in tolerant genotypes, the secretion of which are higher when Al3+ is present, (3) higher P-use efficiency (PUE), eventhough the specific absorption ratio (SAR) was lower than the sensitive ones. The high PUE is thought to contribute to higher grain weight compared to sensitive genotypes under P starvation condition, and (4) higher stay green percentage, which has a high correlation to grain weight/plant. Such strategies should be considered in precission breeding program of sorghum to P deficiency.
 Keywords: adaptation strategy; oxalic acid; P-use efficiency; sorghum genotypes; sorghum low P; stay green

Biochemical and Molecular Responses Underlying the Contrasting Phosphorus Use Efficiency in Ryegrass Cultivars.

Improving plant ability to acquire and efficiently utilize phosphorus (P) is a promising approach for developing sustainable pasture production. This study aimed to identify ryegrass cultivars with contrasting P use efficiency, and to assess their associated biochemical and molecular responses. Nine ryegrass cultivars were hydroponically grown under optimal (0.1 mM) or P-deficient (0.01 mM) conditions, and P uptake, dry biomass, phosphorus acquisition efficiency (PAE) and phosphorus utilization efficiency (PUE) were evaluated. Accordingly, two cultivars with high PAE but low PUE (Ansa and Stellar), and two cultivars with low PAE and high PUE (24Seven and Extreme) were selected to analyze the activity and gene expression of acid phosphatases (APases), as well as the transcript levels of P transporters. Our results showed that ryegrass cultivars with high PAE were mainly influenced by root-related responses, including the expression of genes codifying for the P transporter LpPHT1;4, purple acid phosphatase LpPAP1 and APase activity. Moreover, the traits that contributed greatly to enhanced PUE were the expression of LpPHT1;1/4 and LpPHO1;2, and the APase activity in shoots. These outcomes could be useful to evaluate and develop cultivars with high P-use efficiency, thus contributing to improve the management of P in grassland systems.

Comparative transcriptome analysis reveals a rapid response to phosphorus deficiency in a phosphorus-efficient rice genotype.

Phosphorus (P) is an essential plant nutrient. Most rice growing lands lack adequate P, requiring multiple P fertiliser applications to obtain expected yields. However, P fertiliser is environmentally damaging, and already unaffordable to the marginal farmers. This warrants developing P-efficient rice varieties that require less P to produce the expected yield. However, genetic factors underlying P-use efficiency (PUE) in rice remain elusive. Here, we conducted comparative transcriptome analysis using two rice varieties with contrasting PUE; a P-efficient landrace DJ123 and a P-inefficient modern cultivar IR64. We aimed to understand the transcriptomic responses in DJ123 that allow it to achieve a high PUE under low P conditions. Our results showed that both DJ123 and IR64 had replete tissue P concentrations after 48 h of P deprivation. Yet, DJ123 strongly responded to the external low P availability by inducing P starvation-inducible genes that included SPX2, PHO1, PAPs and SQDs, while these genes were not significantly induced in IR64. We envisage that the ability of DJ123 to rapidly respond to low P conditions might be the key to its high PUE. Our findings lay a valuable foundation in elucidating PUE mechanism in rice, thus will potentially contribute to developing P-efficient modern rice variety.

Nitrate-uptake restraint in Banksia spp. (Proteaceae) and Melaleuca spp. (Myrtaceae) from a severely phosphorus-impoverished environment

AimsSouth-western Australia has some of the most phosphorus (P)-impoverished soils in the world. Hakea prostrata (Proteaceae) has nitrate (NO3−)-uptake restraint, associated with its high P-use efficiency. This research explores how common this trait is in other Proteaceae and in co-occurring plant species in south-western Australia.MethodsBanksia attenuata (Proteaceae), B. telmatiaea (Proteaceae), Melaleuca seriata (Myrtaceae) and M. rhaphiophylla (Myrtaceae) were grown with no NO3−, Medium (500 µM) NO3− or High (2500 µM) NO3− treatments with no other source of nitrogen (N). Plants were harvested after treatments. Leaf nitrate and phosphate concentrations were determined, as well as biomass, total N and total P concentrations of leaves, stems and roots separately.ResultsBanksia attenuata, B. telmatiaea and M. seriata had similar total N content when supplied with High NO3− as they did when supplied with Medium NO3−. All four species had the same low leaf NO3−-N concentration in High and Medium NO3− treatments, no more than 6% of leaf total N. All species had similar leaf NO3− concentrations as those of plants growing in their natural habitat where the available soil nitrate concentration was much lower. These results are in sharp contrast with plants that are typically studied, for example Arabidopsis thaliana.ConclusionsAll four species exhibited NO3−-uptake restraint to different degrees. Nitrate-uptake restraint appears a convergent trait common to many species that evolved in severely P-impoverished landscapes, allowing them to be more P-efficient.

Synergies in sustainable phosphorus use and greenhouse gas emissions mitigation in China: Perspectives from the entire supply chain from fertilizer production to agricultural use

Synergies to achieve high phosphorus (P) use efficiency (PUE) and mitigate greenhouse gas (GHG) emissions are critical for developing strategies aimed toward agricultural green development. However, the potential effects of such synergies in the entire P supply chain through optimizing P management in crop production are poorly understood. In this study, a partial life cycle of a GHG emissions model was developed to quantify the P-related GHG emissions in the entire P supply chain in China. Our results showed that 16.3 kg CO2-equivalent (CO2-eq) was produced from the entire P supply chain per unit of P used for grain agriculture (maize, rice, and wheat). P-related GHG emissions in China increased more than five-fold from 1980 (7.2 Tg CO2-eq) to 2018 (44.9 Tg CO2-eq). GHG emissions were found to be strongly associated with the intensity of grain production in China, and they varied considerably across production regions owing to the differences in the P fertilizer production efficiency. Mineral P fertilizer use in crop production was the primary source of P-related GHG emissions. The results suggest that sustainable P management by matching mineral P fertilizer rates and fertilizer types with crop needs can mitigate GHG emissions by 10.8–27.7 Tg (24.0–65.1%). Moreover, this can improve PUE and reduce mineral P input by 0.7–1.4 Tg (24.0–46.0%). These findings highlight that potential synergies between high PUE and low P-related GHG emissions can be achieved via sustainable P management, thereby enhancing green agricultural development in China and other regions worldwide.

Characterization on the P-associated and agronomic traits as well as associated molecular processes in wheat under Pi deprivation condition

Phosphorus (P) acts as one of the essential macronutrients and plays critical roles in regulating growth, development, and yield formation capacity of the crop plants. Elucidating the physiological and molecular processes underlying plant P deprivation responses benefit the crop cultivation with high P use efficiency (PUE). In this study, we investigated the P-associated and agronomic traits as well as the transcriptome profile under contrasting inorganic phosphorus (Pi) levels combined with deficit irrigation, using two wheat cultivars contrasted by PUE (i.e., high PUE Shixin 828 and P deprivation sensitive Jimai 518). The deficient-P (DP) treatment decreased P accumulative amounts, photosynthetic function, and biomass in plants at various growth stages and reduced yields with respect to sufficient-P (SP) condition. Although the two cultivars were comparable on growth and P-associated traits as well as yields under SP, Shixin 828 displayed better growth traits, more P accumulation, and higher yields than Jimai 518 under DP, suggesting that the high PUE cultivars with enhanced P uptake positively affects photosynthetic function, biomass production, and productivity of plants under P deprivation conditions. A large quantity of genes with differential expression patterns, including 2948 upregulated and 1844 downregulated, were identified based on RNA-seq analysis in the Shixin 828 plants treated with P deprivation. These DE genes were shown to be associated with biological process (i.e., metabolic process and cellular process etc.), cellular components (cell body and organelle etc.), and molecular function (binding and catalytic activity), and phytohormone signaling pathways. Transgene analysis on TaZFP1, a gene in ZFP transcription factor family that exhibited upregulated expression in P-deprived plants, validated its role in enhancing P accumulation and P starvation adaptation of plants. Our results suggested that plant P deprivation response is underlying modulation of various physiological processes via modifying gene transcription at global level.

Investigating the effect of Aspergillus niger inoculated press mud (biofertilizer) on the potential of enhancing maize (Zea mays L.) yield, phosphorous use efficiency, and phosphorous agronomic efficiency

The world is facing tremendous challenges in tackling waste diversion from landfill to resource recovery. Conversion of organic waste into biofertilizer would reduce its environmental impacts by improving soil nutrient contents level and decreasing the requirements of mineral fertilizer (MF). Phosphorus (P) is the primary least available mineral in many cropping environments. The current study was aimed to isolate an efficient P-solubilizing fungi and to investigate its influence on total organic carbon (TOC), C/N ratio, total nitrogen (TN), total phosphorous (TP), total potassium (TK), and germination index (GI) of press mud to convert into biofertilizer. P from biofertilizer was then evaluated in different treatments in terms of maize biological yield, grain yield, P use efficiency (PUE), and P agronomic efficiency (PAE) in comparison to MF. Among seven P-solubilizing fungal isolates, A. niger was selected for its highest P-solubilizing activity in solid (1.5 cm) and liquid media (389 ug/ml) by solubilizing insoluble tricalcium phosphate (TCP). It was also found to mobilize zinc (Zn) from insoluble zinc oxide (ZnO). Bio-augmentation of press mud with A. niger shortens maturity period, improved nutrient contents and GI. Analysis of maize in different treatments revealed that P from mineral and biofertilizer enhanced crop yield. The competency of treatments was measured on higher PUE and PAE. Higher grain yield and harvest index (HI) were achieved in T5(100%Org+50%MF) containing a higher amount of P from mineral and biofertilizer. However, higher PUE and PAE were found in the order T6>T5>T2>T3>T4>T1, demonstrating the importance of integrated and balanced use of fertilizers.

Enhancing phosphorus availability, soil organic carbon, maize productivity and farm profitability through biochar and organic–inorganic fertilizers in an irrigated maize agroecosystem under semi‐arid climate

AbstractBiochar amendments offer promising potential to improve soil fertility, soil organic carbon (SOC) and crop yields; however, a limited research has explored these benefits of biochar in the arid and semi‐arid regions. This two‐year field study investigated the effects of Acacia tree biomass‐derived biochar, applied at 0 and 10 t ha‒1 rates with farmyard manure (FYM) or poultry manure (PM) and mineral phosphorus (P) fertilizer combinations (100 kg P ha‐1), on maize (Zea mays L.) productivity, P use efficiency (PUE) and farm profitability. The application of biochar with organic–inorganic P fertilizers significantly increased soil P and SOC contents than the sole organic or inorganic P fertilizers. Addition of biochar and PM as 100% P source resulted in the highest soil P (104% increase over control) and SOC contents (203% higher than control). However, maize productivity and PUE were significantly higher under balanced P fertilizer (50% organic + 50% mineral fertilizer) with biochar and the increase was 110%, 94% and 170% than 100%‐FYM, 100%‐PM and 100% mineral fertilizer, respectively. Maize productivity and yield correlated significantly positively with soil P and SOC contents These positive effects were possibly due to the ability of biochar to improve soil properties, P availability from organic–inorganic fertilizers and SOC which resulted in higher PUE and maize productivity. Despite the significant positive relationship of PUE with net economic returns, biochar incorporation with PM and mineral fertilizer combination was economically profitable, whereas FYM along biochar was not profitable due to short duration of the field experiments.

Determining the optimum range of soil Olsen P for high P use efficiency, crop yield, and soil fertility in three typical cropland soils

Soil Olsen P level has a major influence on crop yield, efficient P utilization, and soil fertility. In this study, the optimum Olsen P range was determined from long-term (1990–2012) field experiments in three typical soil types of China under single cropping of maize or double cropping of maize and wheat. The critical soil Olsen P value for crop yield was evaluated using three different models, and the relationships among P use efficiency (PUE), Olsen P, and total P were analyzed. The agronomic critical soil Olsen P values obtained from the three models for the neutral soil of Gongzhuling and the calcareous soil of Zhengzhou were similar; however, the values from the linear-linear and linear-plateau models for both maize and wheat were substantially lower than those from the Mitscherlich model for the acidic soil of Qiyang. The PUE response change rates (linear equation slopes) under different soil Olsen P levels were small, indicating slight or no changes in the PUE as the soil Olsen P increased in all three soils. A comparison of the Olsen P levels that achieved the maximal PUE with the agronomic critical values derived from the three models indicated that the linear-plateau model exhibited the best performance. The regression equation coefficients of Olsen P response to total P decreased as follows: Zhengzhou (73 mg g−1) > Qiyang (65 mg g−1) > Gongzhuling (55 mg g−1). The Olsen P level increased as the total P increased, which may result in a decrease in PUE. To achieve a relatively high crop yield, PUE, and soil fertility, the optimum Olsen P range should be 13–40, 10–40, and 29–40 mg kg−1 at Gongzhuling, Zhengzhou, and Qiyang, respectively.

The genome evolution and low-phosphorus adaptation in white lupin

White lupin (Lupinus albus) is a legume crop that develops cluster roots and has high phosphorus (P)-use efficiency (PUE) in low-P soils. Here, we assemble the genome of white lupin and find that it has evolved from a whole-genome triplication (WGT) event. We then decipher its diploid ancestral genome and reconstruct the three sub-genomes. Based on the results, we further reveal the sub-genome dominance and the genic expression of the different sub-genomes varying in relation to their transposable element (TE) density. The PUE genes in white lupin have been expanded through WGT as well as tandem and dispersed duplications. Furthermore, we characterize four main pathways for high PUE, which include carbon fixation, cluster root formation, soil-P remobilization, and cellular-P reuse. Among these, auxin modulation may be important for cluster root formation through involvement of potential genes LaABCG36s and LaABCG37s. These findings provide insights into the genome evolution and low-P adaptation of white lupin.

Linkages between Phosphorus and Plant Diversity in Central European Forest Ecosystems—Complementarity or Competition?

The phosphorus nutrition status of European forests has decreased significantly in recent decades. For a deeper understanding of complementarity and competition in terms of P acquisition in temperate forests, we have analyzed α-diversity, organic layer and mineral soil P, P nutrition status, and different concepts of P use efficiency (PUE) in Fagus sylvatica L. (European beech) and Picea abies (L.) H. Karst. (Norway spruce). Using a subset of the Second National Soil Survey in Germany, we correlated available data on P in the organic layer and soil with α-diversity indices for beech and spruce forests overall and for individual vegetation layers (tree, shrub, herb, and moss layers). Moreover, we investigated α-diversity feedbacks on P nutrition status and PUE of both tree species. The overall diversity of both forest ecosystems was largely positively related to P content in the organic layer and soil, but there were differences among the vegetation layers. Diversity in the tree layer of both forest ecosystems was negatively related to the organic layer and soil P. By contrast, shrub diversity showed no correlation to P, while herb layer diversity was negatively related to P in the organic layer but positively to P in soil. A higher tree layer diversity was slightly related to increased P recycling efficiency (PPlant/Porganic layer) in European beech and P uptake efficiency (PPlant/Psoil) in Norway spruce. The diversity in the herb layer was negatively related to P recycling and uptake efficiency in European beech and slightly related to P uptake efficiency in Norway spruce. In spruce forests, overall and herb species richness led to significantly improved tree nutrition status. Our results confirm significant, non-universal relationships between P and diversity in temperate forests with variations among forest ecosystems, vegetation layers, and P in the organic layer or soil. In particular, tree species diversity may enhance complementarity and hence also P nutrition of dominant forest trees through higher PUE, whereas moss and herb layers seemed to show competitive relationships among each other in nutrient cycling.

Relationships of phosphorus concentration in reproductive organs with soil phosphorus availability for tropical rain-forest trees on Mount Kinabalu, Borneo

Abstract:Bornean rain forests on phosphorus (P)-poor soils exhibit a high P-use efficiency in the production of reproductive organs (i.e. the inverse of P concentration in reproductive-organ litter). The mechanism underpinning this high P-use efficiency is not known, but is hypothesized to result from dilution of P in a given type of reproductive organ and/or a shift of the community composition of flower/fruit types with decreasing P availability. These hypotheses were tested using eight forests with different soil P availabilities on Mount Kinabalu, Borneo. Mean P concentration per forest by genus in inflorescences was significantly positively correlated with P availability, while that in seeds or pericarps was not significantly correlated. This trend was consistent across 21 genera that we analysed, suggesting that P concentration in seeds is maintained in exchange with the dilution of P in inflorescences. The composition of fruit types in tree community was estimated based on the relative abundances of genera in each forest. The relative abundance of capsulate species, which required less P in pericarps, tended to increase in tree community with decreasing P availability. Therefore, both mechanisms were involved in P-use efficiency. This work provides an insight into the reproductive adaptation of trees to P deficiency.

Strategies of organic acid production and exudation in response to low-phosphorus stress in Chinese fir genotypes differing in phosphorus-use efficiencies

Plants with high P-use efficiency adapted more effectively via increased organic acid content in roots and root exudates than those with low P-use efficiency. Plants with high P-use efficiency also redistributed P in their tissues as an adaptive response, when organic acid exudation did not improve P-use efficiency. Increased organic acid exudation enables plants to cope with low-phosphorus (P) stress conditions. We aimed to clarify the variation in organic acid exudation strategies among different breeding genotypes presenting high-phosphorous (P) use efficiency. In this study, Chinese fir [Cunninghamia lanceolata (Lamb.) Hook.] seedlings differing in P-use efficiency were compared regarding low-molecular weight organic acid content in roots and root exudates under low-P stress conditions, and dry weight and P-use efficiency also were examined. Compared to the genotype characterized by low P-use efficiency (M24), passive low-P tolerance (M1) or active capture of soil P (M4) genotypes effectively increased organic acid content in roots and root exudates. Although M1 was more sensitive to low P than M4, the former exhibited a specific tolerance to low P. Accordingly, the production of organic acids was slower, and the resultant stress response was less effective in M1 than in M4. Organic acid exudation did not improve P-use efficiency, given the absence of insoluble P in the rhizosphere, but M1 and M4 redistributed P in the tissues as an adaptive response. M4 also increased the root/shoot ratio and allocated more assimilation to root system to enhance P acquisition under low-P condition. In conclusion, the studied genotypes revealed different organic acid exudation strategies. Plants with high P-use efficiency adapted more effectively than those with low P-use efficiency. Metabolic and energy-use adjustments by plant organs, which facilitate plants’ passive adaptation under P-stress, are important survival strategies that allow adaptation to low-P environments.

Tropical maize selection indexes genotypes for efficiency in use of nutrients: phosphorus

ABSTRACT Brazil generates an annual demand for more than 2.83 million tons of phosphate fertilizers. Part of this is due to low P use efficiency (PUE) by plants, particularly in current maize cultivars. Thus, the aim of this study was to create indexes that allow accurate selection of maize genotypes with high PUE under conditions of either low or high P availability. The experiment was conducted in a greenhouse (20º45'14"S; 42º52'53"W) at the Universidade Federal de Viçosa in October 2010. We evaluated 39 experimental hybrid combinations and 14 maize inbred lines with divergent PUE under two conditions of P availability. The relative importance of the traits studied was analyzed and estimated by principal component analysis, factor analysis, and establishment of selection indexes. To obtain genotypes responsive to high P availability, the index SIHP (selection index for high phosphorus) = 0.3985 RDM + 0.3099 SDM + 0.5567 RLLAT + 0.2340 PUEb - 0.1139 SRS is recommended. To obtain genotypes tolerant to low P availability, the index SILP (selection index for low phosphorus) = 0.3548 RDM + 0.3996 RLLAT + 0.3344 SDM + 0.0041 SH/RS - 0.1019 SRS is suggested.

Evaluation of early maturing cowpea (Vigna unguiculata) germplasm for variation in phosphorus use efficiency and biological nitrogen fixation potential with indigenous rhizobial populations

SUMMARYCowpea genotypes that efficiently utilize phosphorus (P) with high potential for biological nitrogen (N) fixation (BNF) are vital to sustainable cropping systems in West Africa. A total of 175 early maturing cowpea genotypes were evaluated in 2010 and 2011 for P use efficiency (PUE) and BNF with an indigenous rhizobial population at Shika in the Northern Guinea savanna of Nigeria. There were significant genotypic variations for all 11 variables measured. The P utilization index, percentage N derived from the atmosphere and total N fixed ranged between 2·10–4·67, 31·3–61·86% and 11·86–50 kg/ha, respectively. The 175 early maturing cowpea genotypes were divided into five categories using principal component analysis (PCA), whereby total N fixed was associated with N and P uptake and plant biomass yield. Complete linkage cluster analysis revealed a total of three distinctive clusters having remarkable correspondence with the PCA. Some genotypes were identified as potential candidates for further breeding programmes using high PUE genotypes with relatively high capacity for BNF and indigenous rhizobial populations.

Effect of rice (Oryza sativa) establishment methods and levels of irrigation and phosphorus on P use efficiency of zero-tillage maize (Zea mays) under rice based cropping system

A field experiment was conducted during two consecutive seasons of kharif and rabi of 2007-08 and 2008-09 at College farm, College of Agriculture, Rajendranagar, Hyderabad on sandy clay loam soil. The experiment was laid out in split-plot design replicated four times. During kharif, the experimental field was divided into two blocks in each replication wherein two methods of rice crop establishment (Transplanted and Aerobic method) were evaluated. During rabi, zero-tilled maize was grown in sequence to rice while considering the two previous rice (Oryza sativa L.) crop establishment methods as main-plot treatments and two levels of irrigation (1.0 and 0.8 IW:CPE) and four levels of phosphorus (0,30,60 and 90 kg P2O5/ha) as sub-plot treatments. The objective of the study was to evaluate the effect of rice crop establishment methods, levels of irrigation and phosphorus on P nutrition of zero-tilled maize (Zea mays L.) under rice-maize system. The grain yield of maize improved significantly from 0 to 60 kg P2O5/ha application. Higher P-use efficiency was recorded in maize crop grown after transplanted rice than that of aerobic rice and the irrigations scheduled at IW: CPE ratio of 1.0 than with IW: CPE ratio of 0.8 during both the years. There was significant decrease in P- use efficiency with increase in P2O5 levels from 30 to 60 and 90 kg/ha (53.91 to 24.95 and 57.17 to 25.61 kg grain/kg P2O5/ha during 2007-08 and 2008-09 respectively). The balance sheet of available soil P at the end of two years in rice-maize crop sequence indicated that, the uptake of P by maize crop was much higher than the rice crop and the depletion of available soil P was maximum when phosphorus was applied to rice during kharif and without phosphorus application to maize during subsequent rabi. However, maximum accumulation of phosphorus was noticed when two crops were fertilized with 60 kg P2O5/ha for both rice and maize. Based on the yield-input relationship, the optimum level of phosphorus was 85.4 kg P2O5/ha for realizing higher yield of 7 309 kg/ha.

Chinese fir root response to spatial and temporal heterogeneity of phosphorus availability in the soil

Deficiency in availability of phosphorus (P) in forest soils of southern China, where Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) stands have been established, greatly affects Chinese fir productivity. Deploying clones with high P-use efficiency could be a more attractive option than P fertilization to enhance the commercial productivity of Chinese fir. Thus, an understanding of the adaptive responses of roots to spatiotemporal variability of P, which is often ignored in previous studies, is essential. In this study, we report the root responses of a Chinese fir clone with high P-use efficiency to the spatially heterogeneous P availability in soil during various periods of time when P availability was controlled. The results showed significantly (p < 0.05) more root proliferation in the form of root length, root surface area, root volume, and root dry matter accumulation in P-replete than in P-poor patches when the patch persisted for a short period of time (50 days). As the patches persisted longer (100–200 days), root proliferation in P-replete patches appeared to decline. P-replete patches that persisted for short (50 days) and long (200 days) periods resulted in high root P accumulation, whereas P-replete patches that persisted for 100–150 days resulted in lower root P accumulation than other patches. It seems that the observed responses in the roots of the Chinese fir clone to heterogeneous P availability might include a comprehensive adjustment of root growth and intraroot P translocation in patches so as to maintain internal P homeostasis, as well as a trade-off between energy cost and production of new roots when the patch persists longer.

N and P colimitation of N2-fixing and N-supplied fynbos legumes from the Cape Floristic Region

Legume species in the fynbos vegetation of the Cape Floristic Region, that fix N2 in soils with low P, may have evolved for enhanced acquisition and efficient use of P. It was hypothesized that N2-fixing and combined-N supplied (N-supplied) A. linearis, P. calyptrata and C. genistoides are adapted to low P and would be relatively unresponsive to increased P of 100 μM. 18 legume species were evaluated for their nodulation response to low P availability. The N X P interaction was then examined in A. linearis, P. calyptrata and C. genistoides reliant on either N2-fixation or 300 μM N (NH4NO3), and receiving 0.1, 1.0, 10 and 100 μM P (NaH2PO4). In the species selection experiment, A. linearis, P. calyptrata and C. genistoides, with the greatest nodule fresh weight (FW) and nodule FW to root FW ratio, were the most prolific nodulating species. In the N X P experiment, with low P supply, the biomass of N2-fixing P. calyptrata and C. genistoides was consistently greater than that of N-supplied plants. In contrast, with high P supply of 100 μM P, all N-supplied plants accumulated more biomass than the corresponding N2-fixing plants. High P-use efficiency, poor down-regulation of P uptake and P storage was evident in A. linearis and P. calyptrata. The growth response to P and the significant N X P interactions indicate that N2-fixing and N-supplied plants were not adapted to low P, but rather colimited by both N and P.

Ecophysiological response in leaves of Caragana microphylla to different soil phosphorus levels

Phosphorus (P) is one of the limiting mineral nutrient elements in the typical steppe of Inner Mongolia, China. In order to find out the adaptive strategy of Caragana microphylla to low soil P status, we grew plants in P-deficient soil in April 2009 and gave a gradient of P addition ranging from 0 to 60 mg(P) kg-1(soil) from May 2010. Leaf traits were measured in September 2010. Both leaf growth and light-saturated photosynthetic rate (P max) were similar among different groups. Leaf nitrogen (N):P ratio indicated that the growth of C. microphylla was not P-limited in most of the Inner Mongolia typical steppe, which had an average soil available P content equal to 3.61 mg kg-1. The optimal P addition was 20 mg(P) kg-1(soil) for two-year-old plants of C. microphylla. Leaf mass area (LMA) and leaf dry matter content (LDMC) were enhanced with low P, and significantly negatively correlated with photosynthetic N-use efficiency (PNUE). Photosynthetic P-use efficiency (PPUE) increased with decreasing soil P and increasing leaf inorganic P (Pi): organic P (Po) ratio, and showed no significant negative correlation with LMA or LDMC. P max of C. microphylla did not decline so sharply as it was anticipated. The reason for this phenomenon might be due to the increased PPUE through regulating the leaf total P allocation. C. microphylla had high P-use efficiency via both high PPUE and long P-retention time at low-P supply. The adaptation of C. microphylla to low-P supply provided a new explanation for the increased distribution of the species in the degraded natural grassland in Inner Mongolia, China.

Phosphorus supply level affects the regulation of phosphorus uptake by different arbuscular mycorrhizal fungal species in a highly P-efficient backcross maize line

Arbuscular mycorrhizal (AM) fungi are known to facilitate effective acquisition of phosphorus (P) by host plants in low P soils. However, the contribution of mycorrhizal traits to high P-use efficiency in modern-bred maize genotypes is still not clear. In the present study one backcross maize inbred line 224 (bred for high P-use efficiency) was used as the host plant associated with AM fungal species (Rhizophagus irregularis or Glomus mosseae) grown at a range of soil P treatments (10, 20, 30, 40, 50, 100 mg P kg–1, Experiment 1) or foliar P applications (0, 0.025%, 0.5% m/v, Experiment 2). The experiments were to test the hypothesis whether the change point of the mycorrhizal growth and P responsiveness of 224, as well as the expression of ZEAma;Pht1;6 was at or near the optimal P supply level. In addition, different AM inoculants might differ in regulating P uptake of the host. Our results indicated that inbred line 224 was highly responsive to mycorrhizal inoculation. In Experiment 1, root colonisation rate, hyphal length density and alkaline phosphatase increased with the increase of soil P supply level. However, the mycorrhizal growth response (MGR) and P accumulation in shoot (MPR) were greatly affected by soil P supply level and varied between the two fungal species. Maize plants exhibited higher MGR and MPR at lower P supply when inoculated with R. irregularis, and at intermediate P supply when inoculated with G. mosseae. In Experiment 2, shoot P uptake was significantly increased by foliar P supply and inoculation, whereas shoot growth was significantly affected by P supply and the interaction. The expression of the AM-inducible Pi transporter gene ZEAma;Pht1;6 was neither significantly affected by soil (except at 100 mg P kg–1, Experiment 1) or foliar P supply level, nor by fungal species. Root P uptake efficiency (RPUE) was generally greatly increased by mycorrhizal colonisation at all P supply levels in both experiments, and significant correlations were observed between mycorrhizal variables and RPUE in Experiment 1. Our results indicate that the formation of mycorrhizal association could increase RPUE and thus may be partly attributed to high P-use efficiency of inbred line 224. The different responsiveness of mycorrhizal fungi to soil-available P implies the importance for the development of precision strategies to optimise the potential function of AM fungi under different P fertilisation management regime in agricultural soils.