Differential manganese tolerances of cotton genotypes in nutrient solution

Cotton genotypes [Gossypium hirsutum (L.)] C‐310–73,‐307 (307) and C‐Sgl, 70–517 (517), shown previously to differ in tolerance to an acid (pH 5.1), high manganese (Mn) Grenada soil from Arkansas, were grown in nutrient solutions containing variable concentrations of excess Mn to confirm and characterize their postulated differences in Mn tolerance. Based on crinkle leaf symptoms and leaf dry weights, the 307 genotype was significantly more tolerant than 517 to 4, 8, or 16 mg Mn/L at a maintained pH of 4.6 (Experiment 1) and also to 4 or 8 mg Mn/L at an initial pH of 5.0, not subsequently adjusted (Experiment 2). In Experiment 1, the relative leaf dry weight (wt. with no Mn/wt. with 8 mg Mn/L × 100) was 94% for genotype 307 and only 27% for 517. In Experiment 2, the corresponding relative leaf weights were 75% and 26% for 307 and 517, respectively. Plant analytical results indicated that the 307 genotype tolerates a higher concentration of Mn in its leaves than does 517. This failure to correlate Mn tolerance with Mn concentrations in plant shoots agrees with previous findings when these two genotypes were grown in acid Grenada soil. Iron (Fe) concentrations, Fe/Mn ratios, and magnesium (Mg) concentrations were higher in the Mn‐tolerant 307 than in the Mn‐sensitive 517, but concentrations of phosphorus (P), potassium (K), calcium (Ca), copper (Cu), and zinc (Zn) were not related to Mn tolerance. Because differential Mn tolerance in these two genotypes is associated with differential internal tolerance to excess Mn, rather than differential Mn uptake, studies are needed to determine the chemical forms of Mn in tolerant and sensitive plants whose leaves contain comparable concentrations of total Mn. Because both Mn and Fe (closely related elements in the Mn toxicity syndrome) have spin resonances, electron paramagnetic resonance (EPR) offers promise in attacking the problem of differential Mn tolerance in plants.     

Sorghum genotypic differences in tolerance to excess manganese

Manganese (Mn) toxicity can be a growth limiting constraint for many plants grown on acid soil. Plant species/genotypes tolerant to Mn could help overcome detrimental Mn toxicity effects on plants grown on high Mn soils. Thirty‐seven sorghum [Sorghum bicolor (L.) Moench] genotypes from a broad germplasm base were grown in solution culture (pH 4.5) with 0, 3.0, and 6.0 mM of added Mn above the basic solution concentration (18 μM) to determine genotypic differences in tolerance to excess Mn. Dry matter (DM) was used to evaluate 24‐day‐old plants (10 days in Mn treatments) for Mn toxicity responses. Wide variability among genotypes for differential DM was noted at 3.0 and 6.0 mM Mn. Sorghum generally tolerated high levels of Mn. Genotypes showing relatively high tolerance to excess Mn in solution were NB 9040, Wheatland, IS 7180, IS 7755, and IS 7809. Those genotypes showing relatively low tolerance to high Mn were ICA‐Nataima, Martin, IS 7173c (SC 283), IS 7321, IS 9187, IS 9785, and IS 9828. IS 7173c, an aluminum (Al)‐tolerant standard genotype, was sensitive to high Mn. Wide variability was noted among tissue culture generated lines derived from a common parent. Laboratory screening for tolerance to Mn toxicity was effective with sorghum, but results need to be verified in the field.     

Classification of rice genotypes based on their mechanisms of adaptation to iron toxicity

Iron (Fe) toxicity is a nutritional disorder that affects lowland rice (Oryza sativa L.). The occurrence of excessive amounts of reduced Fe(II) in the soil solution, its uptake by the rice roots, and its transpiration‐driven transport result in elevated Fe(II) concentrations in leaf cells that catalyze the formation of reactive oxygen species. The oxidative stress causes rusty brown spots on leaves (bronzing) and the reduction of biomass and yield. While the use of resistant genotypes is the most promising approach to address the problem, the stress appears to differentially affect rice plants as a function of plant age, climatic conditions, stress intensity and duration, and the prevailing adaptation mechanism. We comparatively assessed 21 contrasting 6‐week‐old rice genotypes regarding their response (symptom score, biomass, Fe concentrations and uptake) to a 6 d iron pulse of 1500 mg L^–1 Fe(II). Eight selected genotypes were further compared at different stress intensities (0, 500, 1000, and 1500 mg L^–1 Fe(II)) and at different developmental stages (4‐, 6‐, and 8‐week‐old plants). Based on Fe‐induced biomass reduction and leaf‐bronzing score, the tested spectrum was grouped in resistant and sensitive genotypes. Linking bronzing scores to leaf iron concentrations allowed further differentiation into includer and excluder types. Iron precipitation on roots and organ‐specific iron partitioning permitted to classify the adaptation strategies into root exclusion, stem and leaf sheath retention, and leaf blade tissue tolerance. The effectiveness of these strategies differed with stress intensity and developmental stage. The reported findings improve the understanding of Fe‐stress response and provide a basis for future genotype selection or breeding for enhancing Fe‐toxicity resistance in rice.     

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.     

Tolerance of tropical common bean genotypes to manganese toxicity: Performance under different growing conditions

Manganese (Mn) toxicity is an important constraint to the production of common bean (Phaseolus vulgaris L.) in tropical and subtropical soils. Amelioration of Mn toxicity by soil modification is difficult in Andosols, and liming of acid soils is often not feasible for small farmers. Substantial genetic variation for Mn tolerance exists in bean germplasm, but is difficult to assess in field trials due to interactions with several environmental factors. The objectives of this study were to identify sources of genetic tolerance to Mn toxicity and to compare their performance using three growing conditions. Contrasting genotypes were evaluated for Mn tolerance by 1) biomass accumulation under Mn stress in solution culture, 2) biomass accumulation under Mn stress in silica sand culture, and 3) seed yield of plants grown in Mn‐amended soil. Genotypes varied substantially in Mn tolerance: A‐283, BAT‐795, Dore de Kirundu, IPA‐7419, Carioca, G‐12896a, and NEP BAYO 22 were susceptible, while Argentino, BAT‐271, Calima, EMP‐84, H6 Mulatinho, and Pintado were more tolerant when tested in solution culture. Genotypic tolerance observed in solution culture correlated well with tolerance observed in silica sand. Some genotypes that performed very well in solution culture and in silica sand did suffer severe yield reduction in Mn‐amended mineral soil. Manganese toxicity reduced shoot branching resulting in fewer seeds per plant in soil grown plants. We conclude that screening of genotypes in solution culture is useful to identify sources of tolerance to Mn toxicity, but performance of those genotypes in soil might be confounded by other edaphic stresses.     

Quantification of the confounding effect of seed manganese content in screening for manganese efficiency in durum wheat (triticum turgidum L. var. durum)

Whether due to the genotype or the environment of the mother plant, the nutrient content of seeds vary over a wide range; the amount of the nutrient contributes greatly to seedling vigor, especially on deficient soils and may result in major differences in grain yield. This effect has important implications for breeding programs. This paper examines the impact of seed manganese (Mn) on screening of durum wheats for tolerance to Mn‐deficient soils. Seed stocks with a range of Mn contents (0.4–2.4 μg seed^‐1) were produced, and the effect on expression of Mn efficiency measured as either relative yield or shoot Mn content for two durum wheat (Triticum turgidum L. var. durum) genotypes differing in Mn efficiency. Both genotypes responded to seed Mn content in terms of enhanced root and shoot growth; there was no genotype by seed Mn interaction, so Mn provided in seed was utilized additively by both Mn‐efficient and Mn‐inefficient genotypes. Manganese efficiency, measured as relative yield, was a function of seed Mn content and varied from 40 to 70% in Hazar and 58 to 90% in Stojocri 2, in the same assay using seed of variable Mn content. From the response curves of yield vs. soil Mn added, the Mn required for 90% relative yield was determined for each level of seed Mn content. Seed Mn was regressed against the soil added Mn needed to obtain 90% of maximal growth at each level of seed Mn content (a total of 8 levels) for each of two genotypes. There was an inverse linear relationship between the amount of soil Mn and seed Mn needed for each genotype. Using the Mn‐efficient genotype with high seed Mn content, the soil Mn needed to obtain 90% growth was nil, while inefficient genotypes with low Mn content required 75 mg Mn kg^‐1 soil to produce the same relative yield. This relationship can be used to adjust the levels of soil applied Mn to be used in a pot bioassay when seeds have a certain Mn content, so as to maintain the screening at an optimal overall level of Mn stress.     

Manganese in substrate clays—harmful for plants?

Mixtures of peat and substrate clays are commonly used as growth media for horticultural plant production. A quality protocol for substrate clays defines a threshold value of active manganese (Mn_act = sum of exchangeable and easily reducible Mn) in substrate clays of < 500 mg kg^–1 to prevent toxic reactions of plants. This threshold value was tested in experiments with peat‐clay blends under various growth conditions, and nutrient solution experiments were additionally conducted to investigate the effects of silicic acid and dissolved organic matter on the occurrence of Mn toxicity. Common bean (Phaseolus vulgaris L.) and hydrangea (Hydrangea macrophylla) plants were cultivated in different peat‐clay substrates and in peat under different moisture and pH levels. The clays varied in their Mn_act content from 4–2354 mg kg^–1. The results of the substrate experiments reveal that a threshold value for Mn in substrate clays is not justified, as plants grown in all peat‐clay substrates did not develop any Mn toxicity even at high substrate moisture or low pH conditions which are known to increase the Mn availability. The extraction of active Mn did not well reflect the Mn concentrations in plant dry matter and substrate solution. As plants tolerated high Mn concentrations in the substrate solution compared to the nutrient solution without toxicity symptoms, the influence of silicic acid and dissolved organic matter (DOM) on Mn toxicity was characterized in a nutrient‐solution experiment. Manganese toxicity was clearly diminished by silicic acid application, but not by DOM. The former effect probably explains the tolerance of bean plants in peat substrates where high silicon concentrations in the substrate solution were observed. Peat‐clay blends even provided up to five times more silicon to plants than pure peat.     

Silicon modulates the metabolism and utilization of phenolic compounds in cucumber (Cucumis sativus L.) grown at excess manganese

Cucumber plants (Cucumis sativus L. cv. Chinese long) were grown in nutrient solution with increasing manganese (Mn) concentrations (0.5, 50, and 100 µM) with (+Si) or without silicon (–Si) supplied as silicic acid at 1.5 mM. High external Mn supply induced both growth inhibition of the whole plant and the appearance of Mn‐toxicity symptoms in the leaves. The application of Si alleviated Mn toxicity by increasing the biomass production. Although the total Mn concentration in the leaves did not differ significantly between +Si and –Si plants, symptoms of Mn toxicity were not observed in Si‐treated plants. The concentrations of phenolic compounds, particularly in the leaf extracts of cucumber plants grown at high external Mn concentrations, differed from those of plants grown without Si. The increased tissue concentrations of phenols (e.g., coniferyl alcohol, coumaric and ferulic acids) were in agreement with enhanced enzymes activities, i.e., peroxidases (PODs) and polyphenol oxidases (PPO) in the tissues of –Si plants. The activities of both enzymes were kept at a lower level in the tissue extracts of +Si plants grown at high external Mn concentrations. These results suggest that Si nutrition modulates the metabolism and utilization of phenolic compounds mainly at the leaf level, most probably as a consequence of the formation of Si‐polyphenol complexes.     

Differential expression of iron deficiency‐induced genes in barley genotypes with differing manganese efficiency

The expression of two barley genes, Ids1 and Ids2, that were induced specifically by iron (Fe) deficiency stress in solution culture, was examined in two barley genotypes differing in manganese (Mn) efficiency. Plants were grown in a calcareous soil supplied with two levels of Mn (15 and 100 mg/kg soil). Ids1 was expressed at equal levels in the roots of both genotypes, and this expression was not affected by Mn supply. These results suggest that the expression of Ids1 probably does not contribute to Mn efficiency. A contrasting result was obtained for Ids2, which was expressed at a higher level in the roots of the Mn‐inefficient genotype than in the Mn‐efficient genotype. However, the expression levels also were not affected by Mn supply. The differential expression of Ids2 may indicate that this gene plays a role both in the Fe deficiency response and in the Mn efficiency mechanism. An interesting observation made on the time course of expression of the two genes. Initially, both genes had low expression in two week old plants and then much higher expression in three week old plants. The timing of this increase probably relates to the exhaustion of the seed Fe reserves. Therefore, our results indicate a need to consider the effect of seed nutrient content in research on the molecular basis for micronutrient acquisition.     

Enhanced Mn accumulation by snapbean cultivars under FE stress

Iron deficiency in dicots is accompanied by an increased potential for Fe uptake and translocation. The mechanisms responsible for these changes in metabolism (Fe‐stress response) provide for the adaptation of Fe‐efficient genotypes to conditions which limit the availability of Fe. Previous studies indicated that when Fe‐stress response is initiated, the uptake of Mn, as well as Fe, is enhanced in Fe efficient species such as sunflower. The present study was conducted to determine the relationship between Fe‐stress response and Mn uptake in snapbean (Phaseolus vulgaris L., cvs. Bush Blue Lake 290, Bush Blue Lake 274). The plants were grown in complete nutrient solutions containing 0.02 to 0.52 mg L^‐1 Mn, at acid or alkaline pH. Iron stress was induced with 0.22 mg l‐¹ Fe(EDDHA) (molar ratio 1:1 or 1:2), high P (14.3 mg L‐¹) and excess CaCO_3. Bush Blue Lake 290 ('BBL 290') was more sensitive than Bush Blue Lake 274 ('BBL 274') to Mn toxicity in acid (pH 5.2) nutrient solutions with adequate Fe. Under alkaline conditions, Mn accumulation by ‘BBL 290’ snapbean was increased dramatically with Fe stress, while a moderate Increase was found for ‘BBL 274’. Foliar symptoms of Mn toxicity, observed on Fe stressed ‘BBL 290’, increased in severity at higher Mn (0.06 to 0.26 mg L^‐1 ) concentrations. It was concluded that the magnitude of the enhanced Mn uptake was related to the intensity of Fe stress response as well as the cultivar sensitivity to Mn.     

Screening Upland Rice Genotypes for Manganese‐use Efficiency

Manganese (Mn) deficiency in upland rice grown after common bean or soybean, which received adequate rate of liming on highly weathered Oxisols, is observed. A greenhouse experiment was conducted to evaluate Mn‐use efficiency of 10 promising upland rice genotypes. The genotypes were grown on an Oxisol at 0 mg Mn kg^?1 (natural soil Mn level) and 20 mg Mn kg^?1 of soil applied as manganese sulfate. Grain yield, panicle number, and grain harvest index (GHI) were significantly (P < 0.01) influenced by genotype. However, shoot dry weight was significantly affected by Mn as well as genotype treatments. Manganese uptake in the shoot as well as in the grain was also affected by genotype treatment. On the basis of Mn‐use efficiency (mg grain weight/mg Mn accumulated in shoot and grain), genotypes were classified as efficient and responsive (ER), efficient and nonresponsive (ENR), nonefficient and responsive (NER), and nonefficient and nonresponsive (NENR). Genotypes Carisma, CNA8540, and IR42 were classified as ER, and genotypes CNA8557 and Maravilha were classified as ENR. Genotype Caipo was in the group NER, and in the NENR group were genotypes Bonança, Canastra, Caraja, and Guarani. From a practical point of view, genotypes that produce high grain yield at a low level of Mn and respond well to Mn additions are the most desirable because they are able to express their high yield potential in a wide range of Mn availability.     

Genotypic variation of potassium uptake and use efficiency in cotton (Gossypium hirsutum)

Potassium (K) deficiency is one of the main limiting factors in cotton (Gossypium hirsutum L.) production. To study the mechanism of high K‐use efficiency of cotton, a pot experiment was conducted. The experiment consisted of two cotton genotypes differing in K‐use efficiency (H103 and L122) and two K‐application levels (K₀: 0 g (kg soil)^–1; K₁: 0.40 g (kg soil)^–1). Root‐hair density and length, partitioning of biomass and K in various organs, as well as K‐use efficiency of the two cotton genotypes were examined. The results show that there was no significant difference in K uptake between the two genotypes at both treatments, although the genotype H103 (high K‐use efficiency) exhibited markedly higher root‐hair density than genotype L122 in the K₁ treatment. Correlation analysis indicates that neither root‐hair density nor root‐hair length was correlated with plant K uptake. Furthermore, the boll biomass of genotype H103 was significantly higher than that of genotype L122 in both treatments, and the K accumulation in bolls of genotype H103 was 39%–48% higher than that of genotype L122. On the other hand, the litter index (LI) and the litter K‐partitioning index (LKPI) of genotype H103 were 14%–21% and 22%–27% lower than that of genotype L122. Lastly, the K‐use efficiency of total plant (KUE‐P) of genotype H103 was comparable with that of genotype L122 in both treatments, but the K‐use efficiency in boll yield (KUE‐B) of genotype H103 was 24% and 41% higher than that of genotype L122 in K₀ and K₁ treatments. Pearson correlation analysis indicated that KUE‐P was positively correlated with BKPI and negatively correlated with LKPI, while KUE‐B was positively correlated with BKPI and boll‐harvest index (HI_B), and negatively correlated with LKPI. It is concluded that there were no pronounced effects of root‐hair traits on plant K uptake of the two genotypes. The difference in K‐use efficiency was attributed to different patterns of biomass and K partitioning rather than difference in K uptake of the two genotypes.     

Electron paramagnetic resonance studies of manganese toxicity,tolerance, and amelioration with silicon in snapbean

Plant genotypes within species differ widely in tolerance to excess manganese (Mn) that may occur in acid soils, or in neutral or alkaline soils having poor aeration caused by imperfect drainage or compaction. However, Mn tolerance mechanisms in plants are largely unknown. Silicon (Si) is reported to detoxify Mn within plants, presumably by preventing localized accumulations of Mn associated with lesions on leaves. Because Mn is paramagnetic, electron paramagnetic resonance (EPR) spectroscopy, shows promise as a tool for characterizing toxic and non‐toxic forms of Mn in tolerant and sensitive plants. The objective of our study was to use EPR to: i) determine the chemical/ physical state of Mn in Mn‐tolerant and ‐sensitive snapbean cultivars; and ii) characterize the protective effects of Si against Mn toxicity. Manganese‐sensitive Wonder Crop 2 (WC) and Mn‐tolerant Green Lord (GL) cultivars of snapbean were grown at pH 5.0, in a greenhouse, in a modified Steinberg solution containing: Mn=0.05mg.L^‐1 (optimal); Mn=1.0mgL^‐1 (toxic); Mn=1.0 mg L^‐1 plus Si=4 mg L^‐1; and Mn=0.05 mg L^‐1 plus 4 mg Si L^‐1. All trifoliate leaf samples exhibited a 6‐line EPR signal that is characteristic of hexaaquo Mn²⁺. In both cultivars, a higher EPR Mn²⁺ signal‐intensity generally correlated with lower total leaf mass, higher total Mn concentrations and more pronounced symptoms of toxicity. Tolerance to excess Mn coincided with lower Mn²⁺ signal intensity. Silicon treatments ameliorated Mn toxicity symptoms in both genotypes, decreased total leaf Mn concentrations, and decreased EPR Mn²⁺ signal intensity. Results suggest that Mn toxicity is associated with reduced electron transport and accumulation of oxidation products in leaves. Amelioration of Mn toxicity by Si is regarded as connected with a reduction in this Mn‐induced process. Results indicated that EPR spectroscopy can be useful in investigating the biochemical basis for differential Mn tolerance in plants. The EPR observations might also help plant breeders in developing Mn‐tolerant cultivars.     

Deficiency of manganese is alleviated more by low zinc than low copper in wheat

Abstract

Wheat (Triticum aestivum L.) var. Sonalika was grown in purified sand in complete nutrient solution (normal), deficient manganese (Mn) (0.0055 mg L^‐1), deficient copper (Cu) (0.0065 mg L^‐1), deficient zinc (Zn) (0.0065 mg L^‐1), deficient ?n/deficient Cu, deficient ?n/deficient Zn, deficient Cu/deficient Zn, and deficient ?n/deficient Cu/deficient Zn treatments. The deficiency of Mn decreased the biomass, concentration of Mn, chlorophyll, sugars, Hill reaction activity, acid phosphatase activity, and increased that of peroxidase and polyphenol oxidase. The magnitude of Mn deficiency effects was mitigated to variable extent when Mn was deficient along with deficient Cu and/or deficient Zn. The effects of either Cu or Zn deficiency viz., intensification of foliar symptoms, decrease in biomass, leaf Cu/Zn, seed yield and starch content were increased further in combined deficiency of Cu and Zn. The stimulation in acid phosphatase and decrease in the activity of polyphenol oxidase and carbonic anhydrase in Cu or Zn deficient leaves were further aggravated when both Cu and Zn were deficient together. All these changes reveal a synergism between Cu and Zn in wheat.     

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.     

不同铃重基因型棉花盛花期14C-同化物在“铃-叶系统”中的分配特征

以大铃、中铃和小铃3个不同铃重棉花基因型为材料,通过在盛花期测定棉株中部主茎和果枝叶面积及叶面积指数,并用¹⁴CO₂饲喂中部主茎叶,研究了¹⁴CO₂同化物在棉株不同层次"铃-叶系统"中的分配特征。结果表明,盛花期中部主茎和果枝单叶面积与铃重呈正相关。小铃基因型棉花群体盛花期叶面积指数最大;大铃基因型棉花蕾铃比中、小铃基因型表现出更强的库活性。主茎叶片产生的同化物除主要输送到对应的果枝外,还向上部、下部的库器官及主茎生长点输送;而流向其对应果枝的同化物,主要供应第一果节蕾铃。     

Manganese Supply and pH Influence Growth,Carboxylate Exudation and Peroxidase Activity of Ryegrass and White Clover

ABSTRACT

The effects of differential manganese (Mn) supply (0 to 355 μ M) and pH (4.8 and 6.0) on dry weight (DW), tissue concentrations of Mn, exudation of carboxylates, and the peroxidase activity were studied in ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) grown in nutrient solution. In both plant species, the increase in Mn supply caused a significant reduction in DW due to severe Mn toxicity, especially at pH 4.8. The critical toxicity concentration of Mn in shoots was 421 mg kg^{? 1} for ryegrass and 283 mg kg^{? 1} for white clover. For both plant species, an increase in Mn supply levels stimulated the exudation of carboxylates and the activity of peroxidase, which was related to stress conditions. The highest amount of carboxylates was exuded at pH 4.8. There was no clear effect of carboxylates on the complexation of Mn^{2 +}.     

Phytosiderophore release does not relate well with zinc efficiency in different bread wheat genotypes

Using six bread wheat genotypes (Triticum aesttvum L. cvs. Dagdas‐94, Gerek‐79, BDME‐10, SBVD 1–21, SBVD 2–22 and Partizanka Niska) and one durum wheat genotype (Triticum durum L. cv. Kunduru‐1149) experiments were carried out to study the relationship between the rate of phytosiderophore release and susceptibility of genotypes to zinc (Zn) deficiency during 15 days of growth in nutrient solution with (1 μM Zn) and without Zn supply. Among the genotypes, Dagdas‐94 and Gerek‐79 are Zn efficient, while the others are highly susceptible to Zn deficiency, when grown on severely Zn deficient calcareous soils in Turkey. Similar to the field observations, visual Zn deficiency symptoms, such as whitish‐brown lesions on leaf blades occurred first and severely in durum wheat Kunduru‐1149 and bread wheats Partizanka Niska, BDME‐10, SBVD 1–21 and SBVD 2–22. Visual Zn deficiency symptoms were less severe in the bread wheats Gerek‐79 and particularly Dagdas‐94. These genotypic differences in susceptibility to Zn deficiency were not related to the concentrations of Zn in shoots or roots. All bread wheat genotypes contained similar Zn concentration in the dry matter. In all genotypes supplied adequately with Zn, the rate of phytosiderophore release was very low and did not exceed 0.5 μmol/48 plants/ 3 h. However, under Zn deficiency the release of phytosiderophores increased in all bread wheat genotypes, but not in the durum wheat genotype. The corresponding rates of phytosiderophore release in Zn deficient durum wheat genotype were 1.2 umol and in Zn deficient bread wheat genotypes ranged between 8.6 μmol for Partizanka Niska to 17.4 umol for SBVD 2–22. In Dagdas‐94, the most Zn efficient genotype, the highest rate of phytosiderophore release was 14.8 umol. The results indicate that the release rate of phytosiderophores does not relate well with the susceptibility of bread wheat genotypes to Zn deficiency. Root uptake and root‐to‐shoot transport of Zn and particularly internal utilization of Zn may be more important mechanisms involved in expression of Zn efficiency in bread wheat genotypes than release of phytosiderophores.     

Differential genotypic tolerance response to manganese stress in triticale

Abstract

Manganese (Mn) tolerance response in two aluminum (Al)‐tolerant triticale (× Triticosecale Wittmack) varieties was characterized by measurements of growth and dry matter production of seedlings in nutrient solution culture containing 100 mg L^‐1 Mn. Root weight index (RWI) and total weight index (TWI) based on relative plant growth were two indicators of differentiating genotypic Mn tolerance; these two indices were used to make a comparative assessment of the degree of Mn tolerance in a group of eight Australian and South African genotypes which differ in apparent Al tolerance. The G4–95A was more Mn‐tolerant than its Al‐tolerant counterpart Tahara. A wide range of Mn tolerance was found in the eight genotypes, but few were tolerant of both Al and Mn stresses; measurements of RWI at 100 mg L^‐1 Mn stress differentiated them into three response types (i.e., Mn‐tolerant, moderately Mn‐tolerant/Mn‐sensitive, and Mn‐sensitive) at the two critical values of 0.30 and 0.60. Covariation analysis indicated no association between Mn tolerance and Al tolerance; selective breeding for acidic stress tolerance should focus on both stress tolerances.     

Modified procedures for commercial adaptation of root iron‐reducing capacity as a screening technique

Abstract

Genotypic evaluation is critical to development of soybean [Glycine max (L.) Merr.] cultivars with genetic resistance to Fe‐deficiency chlorosis. Root Fe³⁺ reducing activity is correlated with genotypic resistance to Fe chlorosis measured in field nurseries, and thus may be a reliable method for identifying chlorosis‐resistat genotypes. However, to develop methods useful for large‐scale screening, several modifications of the previously published procedure for measuring root Fe³⁺ reducing activity were investigated. Several hydroponic experiments were conducted to test proposed modifications. It was determined that: (a) different genotypes may be grown together in the same nutrient solution without affecting Fe³⁺ reduction, (b) genotype separation is maximized by growth in CaCO₃ buffered solution (37.5 mg L^?1), (c) a labor‐intensive elongation step can be eliminated, and (d) denotype evaluation can be accomplished without introducing Fe into the hydroponic solutions. These refinements to the procedure should allow its adaptation and use in soybean breeding programs.