Rice Genotype Responses to Acetate to Improve No‐Tillage and Minimal‐Tillage Systems

The goal of this work was to evaluate the response of 25 rice genotypes to the toxic effect of acetate, which is commonly produced in low‐drainage and organic‐matter‐rich soils. The work was performed in hydroponics with four acetate doses. The experimental design was random blocks with three replications on a factorial scheme. The variables root (RL) and shoot (SL) length, number of roots (NR), and root (RDM) and shoot (SDM) dry matter were measured. Analyses of variance, relative performance, and regression adjustments were used. Only the dose × genotype interaction effect for SL, NR, and SDM were not significant. Among the variables analyzed, RL was influenced the most by the acetate treatments. The regressions established for this variable revealed 6 tolerant and 19 sensitive genotypes. Most of the tolerant genotypes were irrigated japonica.     

Potassium and sodium fertilization affects leaf nutrient content and growth of ‘Shawnee’ blackberry

Aluminum (Al) has many detrimental effects on plant growth, and shoots and roots are normally affected differently. A study was conducted to determine differences among sorghum [Sorghum bicolor (L.) Moench] genotypes with broad genetic backgrounds for growth traits of plants grown at 0,200,400,600, and 800 μM Al in nutrient solutions (pH 4.0). Genotypes were categorized into “Al‐sensitive”, “intermediate Al‐tolerant”, “Al‐tolerant”, and SC 283 (an Al‐tolerant standard). As Al increased, shoot and root dry matter (DM), net main axis root length (NMARL), and total root length (TRL) became lower than controls (0 Al). Aluminum toxicity and/or nutrient deficiency symptoms become more severe, and shoot to root DM ratios and specific RL (TRL/root DM) values also changed as Al in solution increased. Root DM had greater changes among genotypes than shoot DM, and NMARL at 400 μM Al, and TRL at 200 μM Al had greater differences among genotypes than root DM, ratings for toxicity and/or deficiency symptoms, and other DM and RL traits. The wide differences among genotypes for NMARL and TRL could be used more effectively to evaluate sorghum genotypes for tolerance to Al toxicity than the other growth traits.     

Growth traits and mineral concentrations of maize hybrids grown on unlimed and limed acid soil

Growing crop plants tolerant to acid soils is an alternative for successful production on acid soils with limited inputs, especially lime. Acid soil‐ or aluminum (Al)‐tolerant plants offer considerable protection against soil acidity problems. Thirteen maize (Zea mays L.) hybrids developed for production under various environmental conditions were grown (greenhouse) on two acid soils (unlimed and limed) to determine differences among hybrids for growth traits, mineral acquisition, and relative tolerance to acid soil. Porters soil induced greater acid soil stress on maize than did Lily soil, although shoot/root dry matter (DM) ratios were affected more in plants grown on Lily than on Porters soil. Shoot and root DM and total root length (RL) over all hybrids followed sequences of Limed Lily ≥ Limed Porters > Unlimed Lily > Unlimed Porters, and the trait with the greatest variation among hybrids was total RL. Specific RL (total RL/root DM) over all hybrids followed a sequence of Limed Lily=Limed Porters=Unlimed Lily>Unlimed Porters, with relatively small variations among hybrids. Shoot DM/RL among hybrids followed a sequence of Unlimed Porters ≥ Unlimed Lily > Limed Lily = Limed Porters, and had the least variation among hybrids. Two Brazilian hybrids (HD 91102 and HD 9176) had highest DM and total RL to indicate relatively high tolerance to acid soil stresses, while other hybrids (ten from the United States and one from Brazil) had relatively small differences for growth traits to indicate moderate to low tolerance to acid soils. Although genotypes differed widely for mineral element concentrations, no significant differences in mineral elements between more and less tolerant genotypes were noted.     

Variation and Heritability of Sodium Chloride Tolerance in Spring Wheat at Early Stage of Plant Development

The response of 5‐week‐old wheat seedlings of 68 genotypes was assessed in three salinity levels (i.e., control, 12, and 24 dS/m^?1) developed in solution cultures on the basis of fresh root and shoot weight. The 68 genotypes responded differently to increasing salinity levels in the growing medium. The genotypes S‐24 and DN‐33, which produced greater root weights than those of DN‐30, appeared as the most tolerant, whereas WC‐78, DN‐27, and DN‐32 were the most sensitive genotypes. The estimates of broad sense heritability (h² _B) for absolute fresh root weight were 0.35, 0.038, and 0.018 in the control, 12, and 24 dSm^?1, respectively, whereas those for relative fresh root weight were 0.08 and 0.05 in low and high salinities, respectively.     

Genetic Variation in Root Traits and Nutrient Acquisition of Lentil Genotypes

ABSTRACT

Lentil (Lens culinaris L.), a pulse crop, is grown in nutrient-poor soils in many developing countries, often with little or no fertilization. Knowledge on root traits of lentil and the assessment of their role in nutrient capture would help to sustain its production in these nutrient-poor soils. Root traits (root length, root hairs, root-induced acidification, and phosphatase enzymes) of 10 lentil genotypes (Barimasur-3, Barimasur-4, PLX-79542, GP-8407-5, GP-8403, BLX-79542, L-5 × 8704(2), L-107 × 87012, L-5 × 87272 and 8406-122) were investigated and then related to the plant uptake of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), and cobalt (Co) in laboratory and pot experiments. There were significant (p < 0.05) differences in root length (RL) and root-hair density (number mm^?1 root) among the genotypes. The genotypes did not differ to induce rhizosphere acidification and acid phosphatase activity (aptase). Uptake of most nutrients differed significantly (p < 0.05) among the genotypes, but root length (RL) was, in general, weakly correlated to the uptake of the most nutrients in the shoot dry matter (DM). The genotypes with prolific root-hair formation (Barimasur-4 and Barimasur-3) were particularly superior in uptake of those nutrients (K, P, Fe, Mn, Cu, Zn, Mo) whose availability in soils is usually low and whose transport to the roots is diffusion limited. The results of this investigation, though based on a small sample of lentil accessions/cultivars, suggest that genetic variation in lentil root traits and nutrient uptake can be pronounced. Screening of a large number of local and exotic cultivars or lines of lentil should be conducted by including more root traits (N₂ fixation, organic acids, mycorrhizae) to find nutrient-efficient germplasm to promote lentil production.     

Nagina 22 mutants tolerant or sensitive to low P in field show contrasting response to double P in hydroponics and pots

ABSTRACT

A simple screening method is needed to identify rice genotypes tolerant to low phosphorus (P) in soil. Nagina 22 (N22) mutants were classified as gain of function (gof) and loss of function (lof) for tolerance to very low P (soil Olsen P 2.01 mg kg^?1). Two gof and two lof mutants were grown in hydroponics and in pot experiment at three P levels (-P, +P and +2P). Comparing response at +P and +2P in hydroponics we found that shoot and root dry weight were significantly less in gof and significantly more in lof in +2P. In pots with soil, tiller number and yield/plant was 3 fold less in gof but 2.5 to 3 fold more in lof and N22 in +2P compared to +P. That 2P can be used to identify low-P tolerant genotypes was validated using 3 low-P tolerant and 3 sensitive varieties whose response to low P soil is well documented. Both shoot and root dry weight was significantly less in +2P than in +P in tolerant and significantly more in sensitive genotypes. Thus screening in hydroponics in +2P can help identify low-P- tolerant genotypes easily and rapidly avoiding field screening.     

Variation of tolerance to manganese toxicity in Australian hexaploid wheat

High concentrations of manganese (Mn), iron (Fe), and aluminium (Al) induced in waterlogged acid soils are a potential constraint for growing sensitive wheat cultivars in waterlogged‐prone areas of Western Australian wheat‐belt. Tackling induced ion toxicities by a genetic approach requires a good understanding of the existing variability in ion toxicity tolerance of the current wheat germplasm. A bioassay for tolerance to high concentration of Mn in wheat was developed using Norquay (Mn‐tolerant), Columbus (Mn‐intolerant), and Cascades (moderately tolerant) as control genotypes and a range of MnCl₂ concentrations (2, 250, 500, 750, 1000, 2000, and 3000 μM Mn) at pH 4.8 in a nutrient solution. Increasing solution Mn concentration decreased shoot and root dry weight and intensified the development of toxicity symptoms more in the Mn‐intolerant cv. Columbus than in Norquay and Cascades. The genotypic discrimination based on relative shoot (54% to 79%) and root dry weight (17% to 76%), the development of toxicity symptoms (scores 2 to 4) and the shoot Mn concentration (1428 to 2960 mg kg^–1) was most pronounced at 750 μM Mn. Using this concentration to screen 60 Australian and 6 wheat genotypes from other sources, a wide variation in relative root dry weight (11% to 95%), relative shoot dry weight (31% to 91%), toxicity symptoms (1.5 to 4.5), and shoot Mn concentration (901 to 2695 mg kg^–1) were observed. Evidence suggests that Mn tolerance has been introduced into Australian wheat through CIMMYT germplasm having “LERMO‐ROJO” within their parentage, preserved either through a co‐tolerance to Mn deficiency or a process of passive selection for Mn tolerance. Cultivars Westonia and Krichauff expressed a high level of tolerance to both Mn toxicity and deficiency, whereas Trident and Janz (reputed to be tolerant to Mn deficiency) were intolerant to Mn toxicity, suggesting that tolerance to excess and shortage of Mn are different, but not mutually exclusive traits. The co‐tolerance for Mn and Al in ET8 (an Al‐tolerant near‐isogenic line) and the absence of Mn tolerance in BH1146 (an Al‐tolerant genotype from Brazil) limits the effectiveness of these indicator genotypes to environments where only one constraint is induced. Wide variation of Mn tolerance in Australian wheat cultivars will enable breeding genotypes for the genetic solution to the Mn toxicity problem.     

Identification of an aluminum tolerant tropical cowpea cultivar by growth and biomass accumulation parameters

Ten‐day‐old seedlings of four cowpea (Vigna unguiculata Walp) genotypes were subjected to six levels of aluminum (Al) (0, 74, 148, 222, 296, and 370 μM/L) to test their tolerance to Al toxicity in a nutrient solution at pH 4.0±0.1. Seedlings were grown in the presence of Al under controlled environmental conditions in a growth chamber. The nutrient solutions were replenished once a week. After 20 days, treatments were terminated and the differences in their growth patterns were compared. Standard growth parameters, such as plant growth, dry matter production, relative growth reduction in roots (RGRS) and shoots (RGRS), and root and shoot tolerance indices (RTI and STI) have been used as markers of Al toxicity. The cowpea genotypes studied exhibited a wide range of responses in their tolerance to Al. Though the genotypes were subjected to six levels of Al, a good degree of separation in their responses was observed only at the 222 μM Al/L treatment level. Therefore, this concentration was chosen to treat and compare the performances of the genotypes. The genotype Co 3 showed an increase in growth, while Paiyur 1 and other genotypes showed severe inhibitions in the presence of Al. Furthermore, for RTI and STI, Co 3 also registered its tolerance to Al by showing increased ratios in the presence of Al. Whereas, Paiyur 1 recorded severe reductions. The RGRR and RGRS data also substantiates this finding. Based on the growth parameters, the four cowpea genotypes were ranked based on their tolerance to Al: Co 3 > Co 4 > KM > Paiyur 1. Co 3 was the most Al‐tolerant genotype which performed extremely well in the presence of Al, while Paiyur 1was the most Al‐susceptible genotype. Therefore, the Al‐tolerant genotype can be used for future breeding programmes to produce Al‐tolerant genotypes, subsequently, can be recommended for acidic infertile soils in the tropics.     

Phosphorus‐use efficiency by corn genotypes

Phosphorus (P) deficiency is a principal yield‐limiting factor for annual crop production in acid soils of temperate as well as tropical regions. The objective of this study was to screen nine corn (Zea mays L.) genotypes at low (0 mg P kg^‐1), medium (75 mg P kg^‐1), and high (150 mg P kg^‐1) levels of P applied in an Oxisol. Plant height, root length, shoot dry weight, root dry weight, shoot‐root ratio, P concentration in shoot and root, P uptake in root and shoot, and P‐use efficiency parameters were significantly (P<0.01) influenced by P treatments. Significant genotype differences were found in plant height, shoot and root dry weight, P uptake in root and shoot, and P‐use efficiency. Based on dry matter production and P‐use efficiency, genotypes were classified as efficient and responsive, efficient and nonresponsive, nonefficient and responsive, and nonefficient and nonresponsive.     

Effect of Glomus etunicatum inoculation on aluminum,phosphorus, calcium,and magnesium uptake of two barley genotypes with different aluminum tolerance

Abstract

This study was conducted to evaluate the effect of vesicular‐arbuscular mycorrhizal (VAM) fungus Glomus etunicatum on growth, absorption, and distribution of calcium (Ca), magnesium (Mg), phosphorus (P), and aluminum (Al) in one Al‐tolerant and one Al‐sensitive barley cultivar. The plants were grown in sand daily irrigated with nutrient solution containing 0 or 600 μM Al at pH 4.8. Significant interaction (P=0.05) among variety, mycorrhiza, and aluminum (VxMxAl) were noted for both shoot and root dry matter (DM); shoot concentration and content of Al, P, Ca, and Mg; root concentration of Al, P, and Mg; and root content of Al, P, Ca, and Mg. With VAM inoculation: i) root colonization degree was about 50% in all treatment, ii) shoot DM yield increased between 30 and 70%, iii) Al concentration and content decrease down to a half both in shoots and roots of sensitive barley, iv) Ca concentration in shoots of sensitive barley showed a high increase at 600 μM Al, and v) P concentration and content in shoots of both varieties increased significantly.     

Investigative approaches associated with plausible chemical and biochemical markers for screening wheat genotypes under salinity stress

Twenty genotypes of wheat resulting from different crossings between some wheat parental lines were compared for salt stress (control and gradually increasing salinity). Ion content in root, shoot, and flag leaves and also the root and shoot dry weights were measured. Based on these results, eight genotypes among the twenty were selected as susceptible, semi-tolerant, and tolerant genotypes for evaluating their biochemical characteristics. Results indicated that concentration of sodium (Na⁺) and potassium (K⁺) in shoot, root, and flag leaves of stressed plants were, respectively, higher and lower than that in the non-stressed plants. Overall, salinity stress caused reductions in root and shoot dry weights and relative water content (RWC), but enhancement in pigments content. Concentrations of the total carbohydrate, total protein, and soluble proline were higher in plants under salt stress condition. Salinity stress induced higher production in hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) and also higher activity of catalase (CAT) and ascorbic peroxidase (APX) as antioxidant enzymes, but lower activity of peroxidase (POD). Genotypes 4s, Arg, and 386dh had generally higher enzymatic activity and other tolerant indices, and hence they can be introduced as tolerant genotypes for more study by the plant breeders. On the other hand, genotype 278s was most susceptible based on the most results.     

Aluminum,manganese, and iron tolerance improves performance of wheat genotypes in waterlogged acidic soils

Waterlogging results in high shoot concentrations of iron (Fe), aluminum (Al), and manganese (Mn) in wheat grown in acidic soil. The verification of this observation in several acidic soils, development of screening techniques, and identification of genotypes differing in tolerance made it possible to test whether tolerance of ion toxicities improves performance of wheat in waterlogged acid soils. Six wheat varieties selected for tolerance/intolerance of Al, Mn, and Fe were grown in three acidic soils (pH_CaCl2 4.1–4.3) with or without waterlogging for 40 d. In terms of relative shoot dry weight, Al‐, Mn‐, and Fe‐tolerant genotypes tolerated waterlogging better, outperforming intolerant genotypes by 35%, 53%, and 32%, respectively, across the soils. The Al‐tolerant genotype had up to 1.8‐fold better root growth than the intolerant genotype under waterlogging. Waterlogging increased DTPA‐extractable soil Mn (71%) and Fe (89%), and increased shoot Fe (up to 7.6‐fold) and Al (up to 5.9‐fold) for different genotypes and soils. The Al‐tolerant genotype maintained lower tissue concentrations of Al as compared to intolerant genotypes during waterlogging. Waterlogging delayed crop development but distinctly less so in the tolerant than in the intolerant genotypes, thus jeopardizing the capacity of intolerant genotypes to produce yield in Mediterranean climates with dry finish of the season. Pyramiding multiple ion tolerances into current wheat varieties with desirable agronomic and quality characteristics to enhance their performance under waterlogged acid soils should be considered.     

Evaluation of common bean genotypes for phosphorus use efficiency

A greenhouse experiment was conducted to evaluate phosphorus (P)‐use efficiency of 10 promising genotypes of common bean (Phaseoius vulgaris L.) with short and normal growth duration. The genotypes were grown on an Oxisol at 25 mg P kg^‐1 (low P) and 150 mg P kg^‐1 (high P) of soil. Shoot and root dry weight, root length, P concentration in the shoot, and P uptake in the shoot were significantly (P<0.01) affected by soil P concentration and genotype. However, P level did not effect root length and genotype had no effect on root dry weight. On the basis of P‐use efficiency (mg dry weight of shoot/mg P accumulated in the shoot) genotypes were classified as efficient and responsive (ER), efficient and nonresponsive (ENR), nonefficient and responsive (NER), and nonefficient and nonresponsive (NENR). From a practical point of view, genotypes which produce a lot of dry matter in a soil with a low P level, and respond well to added P are the most desirable because they are able to express their high yield potential in a wide range of P environments. Novo Jalo and Pérola genotypes fall into this group. Genotypes Irai, Jalo Precoce and L93300166 fall into the ENR group. Genotypes Carioca, Rosinha G‐2, and Xamengo were classified NER, whereas, genotypes L93300176 and Diamante Negro were classified as NENR. There were no differences between short and normal growth duration genotypes in P‐use efficiency.     

Differences in Root Exudation Among Phosphorus‐Starved Genotypes of Maize and Green Gram and Its Relationship with Phosphorus Uptake

Abstract

Availability of phosphorus (P) in soil and its acquisition by plants is affected by the release of high and low molecular weight root exudates. A study was carried out to ascertain the qualitative and quantitative differences in root exudation among the genotypes of maize (Zea mays L.) and green gram (Vigna radiata L.) under P‐stress. Results showed that both inter‐ and intra‐species differences do exist among maize and green gram in terms of root exudation, P uptake, and shoot and root P content. In general, green gram, a legume crop, had greater root exudation compared to maize. However, the amino acid content of the total root exudates in maize was two‐fold as compared to green gram. The maize and green gram genotypes possessed genetic variability in root exudation. Irrespective of the species or genotypes, a positive relationship was found among P uptake rates, total root exudation, and shoot and root ³²P content. The amount of sugars and amino acid present in the root exudates of P‐starved seedlings also add to the variation in P uptake efficiency of genotypes.     

Response of two sorghum genotypes to soil lime and sodium chloride amendments

Abstract

Salinity affects plants by interaction between sodium (Na) and calcium (Ca). Two sorghum (Sorghum bicolor) genotypes ('Hegari’ and ‘NB‐9040') were studied for the Na x Ca interaction in a soil amended with 2% calcium carbonate (CaCO₃) and with 0, 12.3, 24.6, and 36.9 mmol sodium chloride (NaCl)/kg soil. The two genotypes were similar in their response to soil NaCl in their shoot and root growth but differed in response to lime. The salinity‐tolerant Hegari was suppressed by high Ca concentration in the soil, mainly in the low‐NaCl treatments, and responded by a lower concentration of potassium (K) and magnesium (Mg) in the leaves, which was associated with leaf‐chlorosis. Since Na uptake was reduced by Ca, the main effect of salinity on plant growth was by the accumulation of chloride (Cl) in the leaves.     

Upland rice genotypes evaluation for phosphorus use efficiency

Phosphorus (P) deficiency is one of the most important yield‐limiting factors in acid soils in various parts of the world. The objective of this study was to evaluate the growth and P‐use efficiency of 20 upland rice (Oryza sativa L.) genotypes at low (0 mg P kg^‐1), medium (75 mg P kg^‐1), and high (150 mg P kg^‐1) levels of applied P on an Oxisol. Plant height, tillers, shoot and root dry weight, shoot‐root ratio, P concentration in root and shoot, P uptake in root and shoot, and P‐use efficiency were significantly (P<0.01) affected by level of soil P as well as genotype. Shoot weight and P uptake in shoot were found to be the plant parameters most sensitive to P deficiency, suggesting that these two parameters may be most suitable for screening rice genotypes for P‐use efficiency under greenhouse conditions.     

The Tolerance Index and Translocation Factor were Used to Identify the Barley Genotypes with High Arsenic Stress Tolerance

Ninety-four barley genotypes were used to investigate the genotypic differences in arsenic (As) uptake and translocation and their relationships with As tolerance index (TI) and translocation factor (TF). Two As treatments (300 µM and 500 µM) were applied in the initial screening and the confirmatory experiments, respectively. The results showed significant (p < 0.05) differences in tissue biomass, shoot height, root length, As TI and TF among genotypes. Based on As TI, 11 barley genotypes were selected and divided into 3 groups, i.e. tolerant, mildly tolerant and sensitive. There was more As uptake in the roots of the As tolerant genotypes, while the As sensitive genotypes contained more As in shoots, which was further proved by the greater TF. Significantly negative correlation was observed between shoot and root As concentration. The results showed that As tolerant genotypes are able to restrict the upward movement of As, thus developing their tolerance.     

Differential Growth Response of Wheat Genotypes to Ammonium Phosphate and Rock Phosphate Phosphorus Sources

ABSTRACT

A solution culture experiment was conducted to determine the response of 15 wheat genotypes for growth, phosphorus (P) uptake, and P utilization efficiency, and their adaptability to P stress conditions using adequate [250 μM P in nutrient solution as ammonium phosphate (NH₄H₂PO₄)] and stress (powdered rock phosphate suspended in nutrient solution) P supply levels. Shoot dry matter (SDM) and total plant DM (shoot + root) and P uptake were generally higher for most genotypes in adequate P than stress P level treatment, but the opposite was true for root dry matter (RDM), root: shoot ratio (RSR), and root P uptake. Relative reduction in SDM due to P deficiency stress ranged from none to 54%. Genotypes Kohinoor 83, PB 85, Parvaz 94 and 4770 did not respond to P deficiency stress for SDM production, while genotypes FSD 83, Chakwal 86, Pasban 90, 4072, 4943, 5039, 6529-11, and 6544-6 were highly responsive to P application for SDM. Shoot P uptake in genotypes at adequate P level was about 3-times higher than those genotypes grown at stress P level. Differences in P concentration of shoot ranged between 2.00 to 3.06 mg P g^?1 in stress P level treatment, and had a significant positive correlation with P harvest index (PHI) (r = 0.558?, P < 0.05) and root efficiency ratio (RER) (r = 0.611?, P < 0.05) and negative correlation with P efficiency ratio (PER) (r = ?0.909??, P < 0.01). A significantly positive correlation of P utilization index (PUI) and SDM (r = 0.784??, P < 0.01) and non-significant negative correlation (r = ?0.483) of PUI with P concentration in shoot implies that wheat genotypes with higher PUI may be selected for P deficient milieu. Genotypes with higher PUI (>0.8 g mg^?1 P) in rook phosphate treatment were Inqlab-91, Pak-81, Lu 26s, Parvaz 94, 4072, 4770, 4943, and 5039. There was no interrelationship observed between shoot P uptake and P efficiency in stress P level treatment. However, highly significant and positive correlation (r = 0.720??, P < 0.01) between PHI and RER suggested that shoot P uptake depended upon root efficiency and it increased with the increase in P uptake per unit RDM. Consequently, this resulted in increased SDM which is evident from the significant positive correlation (r = 0.833??, P < 0.01) between SDM and shoot P uptake. In summary, the findings suggest that PUI and RER may be used for selecting P efficient wheat genotypes (e.g., 4072, 4770, 4943, Pak 81, and Inqlab 91) for dry matter production and P use.     

Genotypic variation of potato for phosphorus efficiency and quantification of phosphorus uptake with respect to root characteristics

Potato (Solanum tuberosum L.), an important food crop, generally requires a high amount of phosphate fertilizer for optimum growth and yield. One option to reduce the need of fertilizer is the use of P‐efficient genotypes. Two efficient and two inefficient genotypes were investigated for P‐efficiency mechanisms. The contribution of root traits to P uptake was quantified using a mechanistic simulation model. For all genotypes, high P supply increased the relative growth rate of shoot, shoot P concentration, and P‐uptake rate of roots but decreased root‐to‐shoot ratio, root‐hair length, and P‐utilization efficiency. Genotypes CGN 17903 and CIP 384321.3 were clearly superior to genotypes CGN 22367 and CGN 18233 in terms of shoot–dry matter yield and relative shoot‐growth rate at low P supply, and therefore can be considered as P‐efficient. Phosphorus efficiency of genotype CGN 17903 was related to higher P‐utilization efficiency and that of CIP 384321.3 to both higher P‐uptake efficiency in terms of root‐to‐shoot ratio and intermediate P‐utilization efficiency. Phosphorus‐efficient genotypes exhibited longer root hairs compared to inefficient genotypes at both P levels. However, this did not significantly affect the uptake rate and the extension of the depletion zone around roots. The P inefficiency of CGN 18233 was related to low P‐utilization efficiency and that of CGN 22367 to a combination of low P uptake and intermediate P‐utilization efficiency. Simulation of P uptake revealed that no other P‐mobilization mechanism was involved since predicted uptake approximated observed uptake indicating that the processes involved in P transport and morphological root characterstics affecting P uptake are well described.     

Manganese availability and microbial populations in the rhizosphere of wheat genotypes differing in tolerance to Mn deficiency

Plant genotypes differ in their capacity to grow in soils with low manganese (Mn) availability. The physiological mechanisms underlying differential tolerance to Mn deficiency are poorly understood. To study the relationship between Mn content in soil, plant genotypes, and rhizosphere microorganisms in differential Mn efficiency, two wheat (Triticum aestivum L.) cultivars, RAC891 (tolerant to Mn deficiency) and Yanac (sensitive), were grown in a Mn‐deficient soil to which 5, 10, 20 or 40 mg Mn kg^–1 were added. The shoot dry matter of both cultivars increased with increasing Mn addition to the soil. At all soil Mn fertilizer levels, the tolerant RAC891 had a greater shoot dry matter and a higher total shoot Mn uptake than the sensitive Yanac. The concentration of DTPA‐extractable Mn in the rhizosphere soil of RAC891 at Mn20 and Mn40 was slightly lower than in the rhizosphere of Yanac. The population density of culturable microorganisms in the rhizosphere soil was low (log 6.8–6.9 cfu (g soil)^–1) in both cultivars and neither Mn oxidation nor reduction were observed in vitro. To assess the non‐culturable fraction of the soil microbial community, the ribosomal intergenetic spacer region of the bacterial DNA in the rhizosphere soil was amplified (RISA) and separated in agarose gels. The RISA banding patterns of the bacterial rhizosphere communities changed markedly with increasing soil Mn level, but there were no differences between the wheat cultivars. The bacterial community structure in the rhizosphere was significantly correlated with the concentration of DPTA‐extractable Mn in the rhizosphere, fertilizer Mn level, shoot dry matter, and total shoot Mn uptake. The results obtained by RISA indicate that differential tolerance to Mn deficiency in wheat may not be related to changes in the composition of the bacterial community in the rhizosphere.