P-acquisition Efficiency Research Articles

Fertilizer Addition Modifies Utilization of Different P Sources in Upland Rice on Strongly P-fixing Andosols

AimsHigh Phosphorus (P) efficiencies such as internal P utilization efficiency (PUE) and P acquisition efficiency (PAE) are crucial for upland rice production, particularly on highly P-fixing soils like Andosols. While the effect of root traits associated with high PAE in upland rice has been studied intensively, less attention has been given to the origin of P (native soil-P versus fertilizer-P) taken up by plants when evaluating differences in P efficiency. Here we aim to evaluate the efficiency of different upland rice genotypes to acquire native soil-P and fertilizer-P.MethodsFour upland rice genotypes with varying PAE were grown in an Andosol at low- and high-P fertilization level and harvested 9 and 34 days after emergence. Fertilizer-P was labeled with 33P to distinguish between the efficiency to acquire P originating from native soil and fertilizer by measuring plant P uptake.ResultsIncreased fertilizer supply enhanced native soil-P uptake. Under low-P conditions the genotype DJ123 showed a superior PAE and an increased acquisition of native soil-P while AB199 was identified to have a superior internal PUE under P deficient conditions. Differences between genotypes in overall PAE under high-P conditions were not significant but the distinction of P sources showed that genotype DJ123 acquired significantly more native soil-P per unit root than all other genotypes.ConclusionsOur results indicate that variations in PAE among genotypes are associated with their ability to access native soil-P. DJ123 emerged as the most adept genotype in acquiring sparingly soluble native soil-P and future studies should unravel the rhizosphere processes underlying increased PAE of native soil-P.

Enhancing phosphorus use efficiency in wheat grown on alkaline calcareous soils

Phosphorus (P) use efficiency is crucial for sustainable wheat production, particularly on alkaline calcareous soils. This study investigates the relative importance of two factors; P acquisition efficiency (PAE) and P utilization efficiency (PUtE), in determining P use efficiency (PUE) in wheat. A field trial with ten wheat genotypes was conducted under two P levels (no P application and P application at 110 kg P2O5 ha−1). Results revealed significant genetic variability in PUE, PAE, and PUtE among wheat genotypes under varying P availabilities. Genotypes MK-4 and MK-8 exhibited superior PUE, making them ideal candidates for soils with differing P levels. PAE played a more substantial role in influencing PUE, with PUtE contributing less to the variability. The findings underscore the importance of improving PAE, particularly for wheat genotypes grown in P-deficient conditions. Moreover, selecting genotypes with lower grain P concentration can enhance PUtE, contributing to improved PUE. These insights can improve breeding efforts and crop management practices to enhance P use efficiency in wheat, ultimately reducing production costs and fertilizer demand, especially in P-limited alkaline calcareous soils.

Improving phosphorus acquisition efficiency through modification of root growth responses to phosphate starvation in legumes.

Phosphorus (P) is one of the essential macronutrients for plant growth and development, and it is an integral part of the major organic components, including nucleic acids, proteins and phospholipids. Although total P is abundant in most soils, a large amount of P is not easily absorbed by plants. Inorganic phosphate (Pi) is the plant-available P, which is generally immobile and of low availability in soils. Hence, Pi starvation is a major constraint limiting plant growth and productivity. Enhancing plant P efficiency can be achieved by improving P acquisition efficiency (PAE) through modification of morpho-physiological and biochemical alteration in root traits that enable greater acquisition of external Pi from soils. Major advances have been made to dissect the mechanisms underlying plant adaptation to P deficiency, especially for legumes, which are considered important dietary sources for humans and livestock. This review aims to describe how legume root growth responds to Pi starvation, such as changes in the growth of primary root, lateral roots, root hairs and cluster roots. In particular, it summarizes the various strategies of legumes to confront P deficiency by regulating root traits that contribute towards improving PAE. Within these complex responses, a large number of Pi starvation-induced (PSI) genes and regulators involved in the developmental and biochemical alteration of root traits are highlighted. The involvement of key functional genes and regulators in remodeling root traits provides new opportunities for developing legume varieties with maximum PAE needed for regenerative agriculture.

Higher diversity and contribution of soil arbuscular mycorrhizal fungi at an optimal P-input level

As not only an indicator but also key determinants of soil health, the performance of arbuscular mycorrhizal (AM) fungi under intensive cropping systems remains incompletely understood, particularly with regard to quantitative contributions to crop phosphorus (P) acquisition and scientific managements of P-input levels. Here, we firstly investigated the diversity and vitality of AM fungi along with decreasing P-input levels from P100 (local regular rate) to P50 (50 % off) in a greenhouse experiment to explore the optimal P-input level, and further determined the apparent contribution of AM fungi to maize (Zea mays L.) P acquisition at optimal level in a field microplot experiment with root mycorrhization inhibition by benomyl application and in another pot experiment upon AM fungal inoculation in a sterilized soil. Decreasing P input decreased the amount but increased the efficiency of plant P acquisition, and increased the diversity and colonization of AM fungi, in addition to shaping the community composition. Notably, P80 (20 % off) appeared to be an optimal level that balanced mycorrhizal vitality, maize growth and P-acquisition efficiency. The apparent contribution of AM fungi to maize P acquisition determined in both field and pot experiments were approximately 36 % and 21 % at P80 and P100, respectively, while grain yields at P80 with mycorrhizae reached the level equivalent to that of P100 without mycorrhizae, indicating a replacement of 20 % of P fertilizers by adequate mycorrhizal management. It highlights that stronger AM associations by optimizing P input allow for relatively high crop P-acquisition efficiency upon high mycorrhizal benefits, favoring agricultural production in a sustainable manner with reduced dependence on inputs.

Organic anions facilitate the mobilization of soil organic phosphorus and its subsequent lability to phosphatases

PurposeOrganic anions commonly released from plant roots and microorganisms are widely reported to mobilize soil phosphorus (P). We characterized soil organic P that was mobilized by organic anions and assessed its amenability to hydrolysis by phosphatase enzymes.MethodsSix soils differing in organic P concentration were extracted with citrate, malate or oxalate solutions and incubated with preparations of phosphomonoesterase, phosphodiesterase, or phytase. Organic P compounds present in these extracts were putatively identified and quantified with solution 31P-NMR spectroscopy and the enzyme-labile P fractions were assessed by changes in molybdate-reactive P (MRP) concentration.ResultsOrganic P mobilization varied markedly among the organic anions. Extraction with 10 mM citrate was most effective and extracted 7.8-fold more total P than the water controls across all soils. Approximately 95% of the extracted P was non-MRP. The organic anions increased both the amount of P extracted and the proportion of the total extracted P that was phosphatase-labile. Phytase was generally the most effective enzyme with up to 60% of the total non-MRP being amenable to hydrolysis by phytase across all extracts. The presence of inositol hexakisphosphates in the extracts, as well as other forms of organic P including nucleic acids and phospholipids, was verified by 31P-NMR with concentrations dependent on both organic anions and soil type.ConclusionThe combination of organic anions and phosphatases represents a key mechanism by which plants and microorganisms can enhance the bioavailability of soil P. This has important implications for understanding P dynamics in natural and managed ecosystems and for ongoing efforts to improve the P-acquisition efficiency of agricultural plants.

Understanding the Adaptive Mechanisms of Plants to Enhance Phosphorus Use Efficiency on Podzolic Soils in Boreal Agroecosystems.

Being a macronutrient, phosphorus (P) is the backbone to complete the growth cycle of plants. However, because of low mobility and high fixation, P becomes the least available nutrient in podzolic soils; hence, enhancing phosphorus use efficiency (PUE) can play an important role in different cropping systems/crop production practices to meet ever-increasing demands in food, fiber, and fuel. Additionally, the rapidly decreasing mineral phosphate rocks/stocks forced to explore alternative resources and methods to enhance PUE either through improved seed P reserves and their remobilization, P acquisition efficiency (PAE), or plant's internal P utilization efficiency (IPUE) or both for sustainable P management strategies. The objective of this review article is to explore and document important domains to enhance PUE in crop plants grown on Podzol in a boreal agroecosystem. We have discussed P availabilities in podzolic soils, root architecture and morphology, root exudates, phosphate transporters and their role in P uptake, different contributors to enhance PAE and IPUE, and strategies to improve plant PUE in crops grown on podzolic soils deficient in P and acidic in nature.

Tomato plant responses induced by sparingly available inorganic and organic phosphorus forms are modulated by strigolactones

Background and aimsPhosphorus (P) is an essential nutrient for plant growth, but is also one of the least accessible in soil. Plants have evolved several strategies to cope with P deficiency and recently the role of the phytohormones strigolactones (SLs) in modulating tomato plants acclimation to P shortage has been described. How SLs regulate the use of P from sparingly accessible P sources, such as organic P or precipitated metal-P systems, is however still unknown in tomato.MethodsIn this study, we compared P acquisition strategies of wild-type (WT) and SL-depleted tomato plants grown hydroponically in the presence of dissolved inorganic phosphate (Pi), dissolved myo-inositol hexaphosphate (myoInsP6), or their coprecipitated form following Fe(II) oxidative precipitation.ResultsIrrespective of the P treatment, SL-depleted plants accumulated more P in their tissues than the WT, possibly due to the constitutively higher expression of high-affinity P transporters and activity of P-hydrolyzing enzymes. Wild-type plants were conversely more effective at acidifying their growth medium and exuding more organic compounds in the presence of dissolved myoInsP6 or coprecipitated forms of P, but this behaviour did not translate into a higher P acquisition.ConclusionsThe two genotypes activated different subsets of responses to bypass low P bioavailability, although the P acquisition efficiency (PAE) was not effectively increased. Strigolactone-depleted plants achieved higher PAE values than WT plants regardless of the applied P form, highlighting a central role of SLs in controlling P uptake and optimizing the cost/benefit ratio of P acquisition.

Cover crop species and mycorrhizal colonization on soil phosphorus dynamics

Phosphorus (P) plays an important role in the physiological plant processes; however, in tropical soils, P is one of the most limiting nutrients for crop yield due to its low mobility, determined by the adsorption of this element to the soil. To enhance P-use and P-acquisition efficiency, this research aimed to evaluate the effect of cover crops and mycorrhizal colonization on changing soil P fractions, both organic and inorganic. A complete randomized block design was adopted in a field experiment, with four replications using four cover crops: i) pigeon pea (Cajanus cajan); ii) crotalaria (Crotalaria juncea); iii) millet (Pennisetum glaucum); iv) brachiaria (Brachiaria ruziziensis) and v) fallow, as control. The arbuscular mycorrhizal fungus colonization, the fungal spore density, soil P fractionation, and N, P, and K uptake were assessed. Our results show that cover crops affected the soil P cycling in deeper soil layers. Cover crops may increase arbuscular mycorrhizal inoculum potential for the succeeding crop in rotation or intercropping, when used as a green manure.

Decline of seedling phosphorus use efficiency in the heterotic pool of flint maize breeding lines since the onset of hybrid breeding

AbstractImproved management and breeding increased maize (Zea mays L.) yields over the last century, but nutritional efficiency was usually not the focus. This study investigates whether old and recently released flint and dent maize seedlings vary in the phosphorus (P) acquisition and utilization. P use efficiency (PUE) and related traits were measured and compared at two P levels in a calcareous soil. PUE and P acquisition efficiency (PAE) from founder flints to elite flints declined over the last decades. This was associated with smaller root systems, reduced ability to exploit external P, decreased rhizosphere pH and shorter root hairs in low P. Comparing flints with doubled haploid landraces (DH_LR), old and elite dents and hybrids revealed that dents started to acquire exogenous P earlier and had improved PUE. Most DH_LRs had similar PUE as elite flints. When evaluating root traits associated with P efficiency, seed P was also critical, and it is important to stack different root traits to optimize PUE, P utilization efficiency (PUtE) and PAE in breeding programmes. The root hair length, the ability to acidify the rhizosphere and the root diameter in flint and dent pools may be utilized to improve P use in maize germplasm.

Novel Genes and Genetic Loci Associated With Root Morphological Traits, Phosphorus-Acquisition Efficiency and Phosphorus-Use Efficiency in Chickpea.

Chickpea—the second most important grain legume worldwide—is cultivated mainly on marginal soils. Phosphorus (P) deficiency often restricts chickpea yields. Understanding the genetics of traits encoding P-acquisition efficiency and P-use efficiency will help develop strategies to reduce P-fertilizer application. A genome-wide association mapping approach was used to determine loci and genes associated with root architecture, root traits associated with P-acquisition efficiency and P-use efficiency, and any associated proxy traits. Using three statistical models—a generalized linear model (GLM), a mixed linear model (MLM), and a fixed and random model circulating probability unification (FarmCPU) —10, 51, and 40 marker-trait associations (MTAs), respectively were identified. A single nucleotide polymorphism (SNP) locus (Ca1_12310101) on Ca1 associated with three traits, i.e., physiological P-use efficiency, shoot dry weight, and shoot P content was identified. Genes related to shoot P concentration (NAD kinase 2, dynamin-related protein 1C), physiological P-use efficiency (fasciclin-like arabinogalactan protein), specific root length (4-coumarate–CoA ligase 1) and manganese concentration in mature leaves (ABC1 family protein) were identified. The MTAs and novel genes identified in this study can be used to improve P-use efficiency in chickpea.

Evaluation of Diverse Wheat (Triticum aestivum) and Triticale (× Triticosecale) Genotypes for Low Phosphorus Stress Tolerance in Soil and Hydroponic Conditions

Identification of “donors” possessing superior traits to adapt to low phosphorus (P) soils is a primary requirement to develop efficient crop plants for sustainable agricultural production. Diverse wheat (Triticum aestivum L., 89) and triticale (×Triticosecale Wittmack, 15) genotypes were phenotyped in soil with low and sufficient P for two seasons and once in the hydroponic system. Principal component analysis of relative biomass and grain weight revealed four clusters each for wheat and triticale. In low P soil, genotypes in the efficient cluster showed lesser reduction in total biomass, grain weight, total P uptake, and harvest index (HI), and improved P acquisition efficiency (PAE) and P harvest index (PHI) as compared to the inefficient cluster. In hydroponics, reduction in total biomass was less while root biomass and PAE increased at low P in the efficient genotypes. Out of 22 (soil) and 38 (hydroponics) P-efficient wheat genotypes, nine were overlapping in both screens while in P-inefficient cluster, out of 23 (soil) and 5 (hydroponics), three were common. A few P-efficient genotypes identified were DL784-3, DBW 16, and GW 322 and triticale TREAT, whereas P inefficient were C306 and ABACUS PVR (triticale). The selections in hydroponics are likely to be relevant in the field as demonstrated by overlapping of genotypes in both the mediums. Genotypic variations in P efficiency were strongly associated with P utilization rather than P uptake.

Young navel orange rootstock improves phosphorus absorption from poorly soluble pools through rhizosphere processes

An increased plant capacity to efficiently use the poorly available pools will be very helpful in citrus production. Information on the various mechanisms evolved by citrus rootstocks to optimize soil phosphorus (P) access is limited. Thus, the objectives of this study were: To evaluate P acquisition efficiency (PAE) in Thomson Navel Orange (Citrus sinensis L. Osbeck.) plants grafted on three rootstocks and adaptive mechanisms developed by each to these rootstocks to increase P uptake in one calcareous soil. For this purpose, a pot experiment was conducted in a completely randomized design with three replications. Treatments consisted of three rootstocks of citrus, including Sour Orange (SO), Swingle Citrumelo (SC) and Troyer Citrange (TC). 18 months after planting seedlings in pots, plants were harvested and the rhizosphere soils were separated. Dissolved organic carbon (DOC), the population of fungi and bacteria, microbial biomass P (MBP), alkaline (AlP) and acid (AcP) phosphatases, available P (P-Olsen), and P fractionation according to the modified Hedley’ sequential fractionation procedure were determined in the rhizosphere soils. After the harvest, P acquisition efficiency of plants (PAE) was determined. The results of this study showed that SO promoted higher scion P uptake and increased PAE. The fungal and bacterial population, MBP, AcP in the SO rhizosphere soils was greater than the SC and TC rhizosphere soils (P < 0.05). P availability closes the root surface of SO rootstock was higher in comparison to the other rootstocks. Moreover, the effect of the rhizosphere on various fractions of P was under rootstock control. The highest amount of exchangeable P and Fe and Al soil P in the rhizosphere soils was found in SO grafting, while the lowest P associated with Ca compounds and residual P in the rhizosphere soils was found in SO-grafting. This necessarily implies the P mobilization of poorly available pools are vital mechanisms for P acquisition by plants on SO in this P-poor soil. In conclusion, the results of the present study showed that different PAE of rootstocks may be associated with the various mechanisms of action of each rootstock involved in P absorption.

Arbuscular mycorrhizal enhancement of phosphorus uptake and yields of maize under high planting density in the black soil region of China

Arbuscular mycorrhizal (AM) symbioses are an attractive means of improving the efficiency of soil phosphorus (P) that difficult to be used by plants and may provide a sustainable way of maintaining high yields while reducing P applications. However, quantifying the contribution of indigenous AM fungi on phosphorus uptake and yields of maize (Zea mays L.) under field conditions is not particularly clear. Mesh-barrier compartments were applied to monitor the distribution of hyphal P uptake throughout the experimental period under different planting densities and soil depths, over two consecutive years. AM symbioses enhanced plant P-acquisition efficiency, especially during the silking stage, and hyphae of AM fungi was assessed to contribution 19.4% at most to total available P content of soil. Moreover, the pattern of AM depletion of soil P generally matched shoot nutrient demand under the high planting density, which resulted in significantly increased yield in 2014. Although the hyphal length density was significantly decreased with soil depth, AM fungi still had high potential for P supply in deeper soil. It demonstrates the great potential of indigenous AM fungi to maize productivity in the high-yield area of China, and it would further provide the possibility of elimination P fertilizer applications to maintain high yields.

Targeting Low-Phytate Soybean Genotypes Without Compromising Desirable Phosphorus-Acquisition Traits.

Phytate-phosphorus (P) in food and feed is not efficiently utilized by humans and non-ruminant livestock, potentially contributing to high losses of P to the environment. Crops with high P-acquisition efficiency can access soil P effectively. It remains elusive whether crop genotypes with high P-acquisition efficiency can also have low seed phytate concentrations. A core collection of 256 soybean [Glycine max (L.) Merr.] genotypes from China with diverse genetic background were grown in the same environment and seeds were sampled to screen for seed phytate-P concentration. Some of these genotypes were also grown in a low-P soil in the glasshouse to measure root morphological and physiological traits related to P acquisition. Large genotypic variation was found in seed phytate-P concentration (0.69–5.49 mg P g–1 dry weight), total root length, root surface area, rhizosheath carboxylates, and acid phosphatase activity in rhizosheath soil. Geographically, seed phytate-P concentration was the highest for the genotypes from Hainan Province, whereas it was the lowest for the genotypes from Inner Mongolia. Seed phytate-P concentration showed no correlation with any desirable root traits associated with enhanced P acquisition. Two genotypes (Siliyuan and Diliuhuangdou-2) with both low phytate concentrations and highly desirable P-acquisition traits were identified. This is the first study to show that some soybean genotypes have extremely low seed phytate concentrations, combined with important root traits for efficient P acquisition, offering material for breeding genotypes with low seed phytate-P concentrations.

Phosphorus Uptake and Growth of Wild-Type Barley and Its Root-Hairless Mutant Cultured in Buffered-and Non-Buffered-P Solutions

Root hairs play an important role in phosphorus (P) nutrition of plants. To better understand the relationship between root hairs and P acquisition efficiency (PAE) in barley, experiments were conducted with the wild-type barley (cv. ’Pallas’) and its root-hairless mutant (brb). A hydroponic split-root system was used to supply P as Ca3(PO4)2 (tri-calcium phosphate, TCP) to one-half and other nutrients to the other half of the root system. Using TCP as a sole P source can simulate a soil solution with buffered low P concentration in hydroponics to induce prolific root hair growth. Root morphology, plant growth, and P uptake efficiency were measured with 50 mg L−1 TCP supplied to the roots in the split-root system and 0, 35, or 1000 μM NaH2PO4 in a non-split-root hydroponic system. The wild-type plants developed root hairs, but they did not contribute to the significant genotypic differences in the P uptake rate when a soluble P source was supplied in the non-split root system, indicating that root hair formation does not contribute to P uptake in a non-split root solution. On the other hand, when grown in a split-root system with one-half of the roots supplied with TCP, the wild-type showed 1.25-fold greater P uptake than the root hairless mutant. This study provides evidence that root hairs play an essential role in plant P uptake when P bioavailability is limited in the root zone.

Intrinsic root morphology determines the phosphorus acquisition efficiency of five annual pasture legumes irrespective of mycorrhizal colonisation.

Mycorrhizal fungi are ubiquitous in agroecosystems and form symbiotic associations that contribute to the phosphorus (P) acquisition of many plants. The impact of mycorrhizas is most pronounced in P-deficient soil and commonly involves modifications to the root morphology of colonised plants. However, the consequences of mycorrhizal colonisation on root acclimation responses to P stress are not well described. Five annual pasture legumes, with differing root morphologies, were grown to determine the effect of mycorrhizal colonisation on shoot yield, root morphology and P uptake. Micro-swards of each legume were established in pots filled with a topsoil layer that had been amended with five rates of P fertiliser. The topsoil overlaid a low-P subsoil that mimicked the stratification of P that occurs under pasture. Mycorrhizal colonisation improved P acquisition and shoot yield in the low-P soil treatments, but did not reduce the critical external P requirement of the legumes for near-maximum yield. The yield responses of the mycorrhizal plants were associated with reduced dry matter allocation to topsoil roots, which meant that the P acquisition benefit associated with mycorrhizal colonisation was not additive in the P-deficient soil. The contribution of the mycorrhizal association to P acquisition was consistent among the legumes when they were compared at an equivalent level of plant P stress, and was most pronounced below a P stress index of ~0.5. The intrinsic root morphology of the legumes determined their differences in P-acquisition efficiency irrespective of mycorrhizal colonisation.

Novel QTL Conferring Phosphorus Acquisition and Utilization Efficiencies in Barley.

Phosphorus (P) deficiency in agricultural soil is a major constraint for crop production and increasing P acquisition efficiency (PAE) of plants is considered as one of the most cost-effective solutions for yield increase. The objective of this study was to detect quantitative trait loci (QTL) controlling (PAE) and P utilization efficiency (PUE) in barley under applied (+P) and non-applied P (−P) conditions. Based on the analysis of a recombinant inbred lines (RILs) population derived from a cross between a malting barley variety and a wild barley accession, 17 QTL controlling PAE, PUE and yield traits were detected. The phenotypic variation explained by each of these QTL ranges from 11.0 to 24.7%. Significant correlation was detected between most of P-related traits and yield traits. Five QTL clusters were identified on four different chromosomes (1H, 3H, 5H, and 7H). Two of the QTL clusters, located on chromosome 1H (for GPUP/PUP) and 7H (for SPUE/SPC), respectively, are novel. Fourteen genes located in the interval harboring the major QTL were identified as candidates associated with P efficiency. The stable QTL for PAE, PUE and yield-related traits could be important for breeding P-efficient barley varieties.

Screening for Phosphorus (P) Acquisition Efficient Genotypes and Identification of Sequence Variations among P-Efficient and Inefficient Genotypes in Chickpea (Cicer arietinum L.)

Low phosphorus (P) availability is the major abiotic constraint in chickpea (Cicer arietinum L.) cultivation across the globe as well as in India. Present study contemplated to identify high P-acquisition efficient chickpea genotypes suitable for P-deficient regions and sequence variation was detected among the tested P-acquisition efficient and inefficient genotypes. Initial screening was conducted with 104 diverse chickpea genotypes at two locations characterized by P-sufficient and deficient conditions. A panel of 14 chickpea genotypes having contrasting P-acquisition efficiency was extracted for further assessment under P-sufficient and deficient conditions. Along with the significant genetic variations, the P-efficient chickpea genotypes had high P-acquisition efficiency than P-inefficient genotypes in both the conditions. P-acquisition efficient genotypes recorded high biomass and good yield potential in P-deficient condition in comparison to P-inefficient genotypes. Sequence analysis of the CaSPX3 domain containing protein gene from P-acquisition efficient and inefficient genotypes identified two single nucleotide polymorphisms in the 5¢ - untranslated region. Present findings might be of great importance in future marker-assisted breeding in chickpea especially for P-deficient soil.

Root proliferation and phosphorus acquisition in response to stratification of soil phosphorus by two contrasting Trifolium subterraneum cultivars

Phosphorus (P) is usually stratified in the topsoil layer under pasture, due to the broadcast application of fertiliser, excreta and leaf-litter deposition on the soil surface, and minimal soil disturbance. The objective of this study was to investigate root proliferation and P acquisition in response to P stratification by comparing two Trifolium subterraneum cultivars with contrasting root morphologies. Clover micro-swards were grown with deficient, constrained and sufficient P supplied in a topsoil layer overlying a P-deficient subsoil that mimicked the stratification of P that occurs under pasture. Phosphorus labelled with 33P- and 32P-radioisotope tracer was mixed throughout the topsoil and subsoil layers, respectively. The shoot yield and total plant P uptake of the cultivars increased in response to increased topsoil P supply. The length of roots produced by the cultivars was equivalent in each of the P treatments, although the specific root length achieved by the cultivars was substantially different. In the P-constrained and P-sufficient treatments, ~91% and ~ 99% of total plant P was acquired by topsoil roots, respectively. In contrast, subsoil roots acquired 60–74% of total plant P in the P-deficient treatment. Topsoil roots were most important for P acquisition when P was highly stratified, whereas subsoil roots contributed to P acquisition when P was uniformly distributed throughout the P-deficient soil profile. Selection for prolific nutrient-foraging roots, in conjunction with plasticity for subsoil exploration, may improve the P-acquisition efficiency of T. subterraneum genotypes and confer adaptability across a range of soil-P environments.

Root proliferation in response to P stress and space: implications for the study of root acclimation to low P supply and P acquisition efficiency

The experiment was conducted to understand how root morphological traits contributed to the contrasting P-acquisition efficiencies of two Trifolium subterraneum cultivars. Phosphorus acquisition, root length proliferation and root acclimation to P deficiency were investigated by varying the P supply (20–300 mg P kg−1) and planting density (1–12 plants pot−1) of clover micro-swards. The shoot yield of both cultivars increased in response to increased P supply and increased planting density. Higher planting densities increased shoot yield by reducing the time to leaf canopy closure. In P-deficient soil, the ‘less P-efficient’ cultivar acquired less P and yielded relatively poorly compared with the ‘more P-efficient’ cultivar when planting density was low (<6 plants pot−1). However, the P acquisition and yields of the cultivars were equivalent when planting density was high (~6–12 plants pot−1). Both cultivars proliferated nutrient-foraging roots in response to P limitation and space (i.e. lower planting densities). However, the proliferation responses of the cultivars differed, particularly in response to space. Differential proliferation responses, combined with differences in specific root length, explained how the cultivars differed in P-acquisition efficiency. The results indicate that inappropriate cultural conditions (e.g. planting densities) may confound attempts to define P-efficient root traits and to identify genotypes with improved P-acquisition efficiency.