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.     

Rice genotype differences in nutrient status under excessive ferric‐iron conditions

To investigate the relationship between rice genotypic variation in tolerance to iron (Fe) toxicity and nutrient element status, 10 rice genotypes with different growing performances under Fe toxicity were grown under normal culture solution and with excessive ferrous (Fe²⁺)‐Fe concentrations of 250 and 500 mg Fe²⁺ L^‐1. A close relationship was obtained between the relative ratio of symptomatic leaf numbers to total leaf numbers (SLN/TLN) and a relative decrease in dry matter under Fe²⁺‐toxicity conditions. The genotypic variations in nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) uptake were evaluated by the relative decrease in the N, P, K, and Mg content in the plants. Remarkable genotypic variation in tolerance to excessive Fe²⁺ was observed. The results indicated that excessive Fe²⁺ reduced N, P, K, and Mg uptake. The nutrient element concentrations, however, were still higher above deficient criteria even in severely affected plants, suggesting that the retardation of growth may not be intirely due to the deficiency of these elements in plants at the seedling stage. Significant correlations were found between the genotypic variation and the decrease in N, P, K, and Mg uptake and in their tolerance to Fe²⁺ toxicity, which suggests that the ability to maintain higher nutrient element uptake under a Fe²⁺‐toxic condition contributes the tolerance to Fe²⁺ toxicity.     

Effect of transpiration on iron uptake and translocation in lowland rice

We evaluated six lowland rice (Oryza sativa L.) genotypes with contrasting responses to increasing Fe²⁺ concentrations under conditions of both low (0.3 kPa) and high (2.4 kPa) vapor pressure deficit. Dry atmospheric conditions generally enhanced transpiration with concomitant increases in Fe uptake and leaf bronzing. Some resistant genotypes were able to limit the water loss by transpiration under higher Fe concentrations thus attenuating negative effects associated with increased Fe²⁺ translocation at high vapor pressure deficit.     

Physiological Disorder of Rice Associated with High Levels of Iron in Growth Medium

Three rice (Oryza sativa L.) varieties viz. ‘CR 683‘, ‘Budumoni’ “Budumoni”, and ‘Akisali’ were grown in sand culture in a greenhouse with three levels of iron (Fe) in nutrient solutions viz., 0.045 (control), 5.34, and 7.12 mM Fe to study the effects of iron on physiology of rice seedling growth. Shoot length, root, and shoot dry weights were reduced significantly by higher levels of Fe in the medium. Results of leaf bronzing have revealed higher bronzing score in the seedlings grown at 7.12 mM Fe in the growth medium. Occurrence of bronzing was severe in varieties ‘CR683’ and ‘Akisali’. Variety ‘Budumoni'maintained higher leaf chlorophyll content, nitrate reductase activity and total soluble protein in the leaves at 5.34 and 7.12 mM Fe. Higher concentration of iron in the nutrient medium exerted an inhibiting effect on the concentration and content of almost all the macro and micronutrients in the root and shoot. Higher Fe and nitrogen (N) contents and lower phosphorus (P), potassium (K), manganese (Mn), copper (Cu), and zinc (Zn) were determined in roots and shoots in plants grown in medium supplied with 7.12 mM Fe. The variety ‘Budumoni’ “Budumoni” performed relatively better in comparison to other tested varieties at 7.12 mM Fe in the growth medium. ‘Budumoni’ “Budumoni” can be considered a suitable rice variety to use in the rice-breeding programme for Fe toxicity tolerance in acid soils of Assam.     

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.     

Elemental composition of the rice plant as affected by iron toxicity under field conditions

Abstract

Iron (Fe) toxicity is a major nutrient disorder affecting the production of wetland rice in the humid zone of West Africa. Little attention has been given to determining the macro‐ and micronutrient composition of rice plants grown on wetland soils where Fe toxicity is present although results from such study could provide useful information about the involvement of other nutrients in the occurrence of Fe toxicity. A field experiment was conducted in the 1997 dry season (January‐May) at an Fe toxic site in Korhogo, Ivory Coast, to determine the elemental composition of Fe tolerant (CK 4) and susceptible (Bouake 189) lowland rice varieties without and with application of nitrogen (N), phosphorus (P), potassium (K), and zinc (Zn). For both Fe‐tolerant and susceptible varieties, there were no differences in elemental composition of the whole plant rice tops, sampled at 30 and 60 days after transplanting rice seedlings, except for Fe. All the other nutrient element concentrations were adequate. Both Fe‐tolerant and susceptible cultivars had a high Fe content, well above the critical limit (300 mg Fe kg^‐1 plant dry wt). These results along with our observations on the elemental composition of rice plant samples collected from several wetland swamp soils with Fe toxicity in West Africa suggest that “real”; iron toxicity is a single nutrient (Fe) toxicity and not a multiple nutrient deficiency stress.     

Catalase and ascorbate peroxidase activities are not directly involveded in the silicon‐mediated alleviation of ferrous iron toxicity in rice

Silicon (Si) is the second most abundant element in the soil and can alleviate several abiotic stresses in many plant species. However, the mechanisms involved in alleviating ferrous iron (Fe²⁺) toxicity by Si are still largely unknown, and no study has investigated the role of Si on the Fe²⁺‐induced oxidative stress and antioxidant system in rice. Four cultivars of Asian and African rice (Oryza sativa L. and Oryza glaberrima Steud) were grown for 4 weeks under hydroponic conditions with or without Fe²⁺ (250 mg Fe²⁺ L^?1) and with or without Si (250 mg SiO₂ L^?1). The plants that were treated with Fe²⁺ suffered Fe²⁺ toxicity, and Si helped to alleviate the toxicity symptoms. The bronzing index and the Fe concentration in the foliar tissue increased in the presence of Fe²⁺ but decreased significantly with the application of 250 mg SiO₂ L^?1. The concentration of malonyldialdehyde, that is commonly used as an indicator of oxidative stress, increased in the foliar tissue in the presence of 250 mg Fe²⁺ L^?1 in the nutrient solution. The application of 250 mg SiO₂ L^?1 in the plant nutrient solution treated with Fe²⁺ considerably limited the increase of malonyldialdehyde. However, no significant effect of Si application on the activities of antioxidant enzymes (catalase and ascorbate peroxidase) and non‐enzymatic antioxidants (total ascorbate, reduced ascorbate, oxidized ascorbate, and the ratio of the reduced to oxidized forms) was observed in the rice plants that were grown in the presence of Fe²⁺. These results suggest that Si does not act directly on the antioxidant defense system of rice but reduces the plant Fe²⁺ concentration, which reduces the oxidative stress.     

An agar nutrient solution technique as a screening tool for tolerance to zinc deficiency and iron toxicity in rice

Abstract

Studies examining iron (Fe) toxicity and zinc (Zn) deficiency in rice have shown that screening experiments in nutrient solutions are of limited use because the rankings of genotypes as tolerant or intolerant can be very different from the results obtained in field-screening experiments. A possible reason for such deviation is that crucial rhizosphere processes cannot be reproduced in nutrient solutions. The objective of the present study was to evaluate the suitability of low-concentration agar nutrient solutions (ANS) as an alternative screening tool. Agar was dissolved in boiling water and mixed with nutrient solution to achieve a final agar concentration of 0.1% (w/v). Zinc deficiency was induced by supplying Zn at a low concentration (0.1 × 10^?3 µmol L^?1), while Fe toxicity was induced by supplying excess Fe²⁺ (200 mg L^?1). Three-week-old seedlings were transplanted into this medium. Symptoms of Zn deficiency and Fe toxicity developed more rapidly in ANS compared with conventional nutrient solutions (CNS). For Zn deficiency this was probably because of the development of Zn depletion zones as a result of the reduced convection in the viscous agar medium. In the case of Fe toxicity we observed far less Fe precipitation in ANS compared with CNS. Genotypic comparisons showed that the tolerance rankings obtained in ANS were very similar to the field tolerance rankings, whereas this was not the case in CNS. This was particularly evident with regard to the considerable root growth inhibition detected in intolerant genotypes when stress treatments were imposed in ANS.     

Ratios of Nutrients in Lowland Rice Grown on Two Iron Toxic Soils in Nigeria

ABSTRACT

Iron (Fe) toxicity is a widespread nutritional soil constraint affecting rice production in the wetland soils of West Africa. Critical levels of total iron in plant causing toxicity is difficult to determine as different rice cultivars respond to excessive Fe^{2 +} in various ways in what is called “bronzing” or “yellowing” symptoms (VBS). An investigation was conducted to evaluate the relationship between plant growth and nutrient ratios at four iron levels (1000, 3000, 4000 μ g L^?1) and control. This involved two rice cultivars (‘ITA 212’ and ‘Suakoko 8’), and two soil types (Aeric Fluvaquent and Aeric Tropaquept). The experimental design was a 2 × 2 × 4 factorial in a completely randomized fashion with four replications. The results showed that nutrient ratios [phosphorus (P)/Fe, potassium (K)/Fe, calcium (Ca)/Fe, magnesium (Mg)/Fe, and manganese (Mn)/Fe), Fe content, and Fe uptake vary widely with the iron levels as well as with the age of the cultivars. The iron toxicity scores expressed as VBS increased with increasing Fe^{2 +} in the soils, resulting in simultaneous reduction of the following variables: plant height, tiller numbers/pot, relationships grain yield (GY) and dry matter yield (DMY). There were no significant difference between nutrient ratios, Fe contents, Fe uptake, the GY and DMY of both rice cultivars on both soil types. Multiple stepwise regression analysis showed that Fe uptake and Fe contents contributed 42% and 17% respectively to the variation in the grain yield of ‘ITA 212’ on Aeric Tropaquept. On both soil types and cultivars, Fe uptake and Fe content contributed between 26 and 68% to the variation in the DMY, while the nutrient ratios (P/Fe, K/Fe, Ca/Fe, and Mn/Fe) contributed between 3% and 13% DMY. Thus, it could be concluded that iron toxicity in rice is more a function of a single nutrient (Fe) rather than nutrient ratios.     

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.     

Iron toxicity in rice—conditions and management concepts

Iron toxicity is a syndrome of disorder associated with large concentrations of reduced iron (Fe²⁺) in the soil solution. It only occurs in flooded soils and hence affects primarily the production of lowland rice. The appearance of iron toxicity symptoms in rice involves an excessive uptake of Fe²⁺ by the rice roots and its acropetal translocation into the leaves where an elevated production of toxic oxygen radicals can damage cell structural components and impair physiological processes. The typical visual symptom associated with these processes is the “bronzing” of the rice leaves and substantial associated yield losses. The circumstances of iron toxicity are quite well established. Thus, the geochemistry, soil microbial processes, and the physiological effects of Fe²⁺ within the plant or cell are documented in a number of reviews and book chapters. However, despite our current knowledge of the processes and mechanisms involved, iron toxicity remains an important constraint to rice production, and together with Zn deficiency, it is the most commonly observed micronutrient disorder in wetland rice. Reported yield losses in farmers' fields usually range between 15% and 30%, but can also reach the level of complete crop failure. A range of agronomic management interventions have been advocated to reduce the Fe²⁺ concentration in the soil or to foster the rice plants' ability to cope with excess iron in either soil or the plant. In addition, the available rice germplasm contains numerous accessions and cultivars which are reportedly tolerant to excess Fe²⁺. However, none of those options is universally applicable or efficient under the diverse environmental conditions where Fe toxicity is expressed. Based on the available literature, this paper categorizes iron‐toxic environments, the steps involved in toxicity expression in rice, and the current knowledge of crop adaptation mechanisms in view of establishing a conceptual framework for future constraint analysis, research approaches, and the targeting of technical options.     

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.     

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.     

Physiological investigations of management and genotype options for adapting rice production to iron toxicity in Madagascar

Iron (Fe) toxicity is one of the major mineral disorders affecting rice (Oryza sativa L.) production in Madagascar. This study aimed at linking physiological and agronomic responses of diverse rice genotypes to Fe resistance mechanisms with different nutrient management practices. Twenty‐three local and exotic rice varieties were grown in Fe‐toxic soil in parallel greenhouse and field experiments and subjected to two treatments: (1) no fertilizer; (2) mineral and organic fertilizer application at recommended rates. Growth, straw and grain yield, symptom formation, and physiological responses including Fe uptake, root plaque formation, and lipid peroxidation were monitored. The application of fertilizer significantly decreased average shoot Fe concentrations partly due to Fe exclusion favored by enhanced root plaque formation. Visual symptoms negatively correlated with straw biomass in both experiments and grain yield in the greenhouse experiment, and positively correlated with lipid peroxidation. However, no plausible correlation occurred with grain yield in the field due to sterility in exotic varieties un‐adapted to local climate. Even though grain Fe concentrations were orders of magnitude lower than in vegetative tissue, some exotic varieties were significantly superior to local checks. Our results provide insight into management and genotype options for adapting rice to Fe toxicity under field conditions.     

Evaluating quantitative screening methods for manganese toxicity in cotton genotypes 1

Identification of cotton genotypes more tolerant of toxic concentrations of soil solution manganese (Mn²⁺) would integrate well with soil ameliorations of that problem. Several quantitative and semi‐quantitative methods to determine the amount of Mn toxicity were evaluated on three genotypes of Gossypium hirsutum (LaDSIS 12513, LaDASS 5175, and Coker gl 79–501) and one genotype of Gossypium barbadense (Pitnas S‐5). Specific leaf weight (SLW) and the semi‐quantitative, ‘percentage of leaves that were damaged’ (PLD) correlated the least with other methods of Mn toxicity determination. Neither SLW or PLD provided more separation between genotypes than area/leaf (AL), peroxidase (POD) activity, and indole‐3‐acetic acid oxidase (IAAO) activity. Similar genotype separations occurred for AL, POD, and IAAO at 10 mg/L Mn in solution, but POD and IAAO produced more genotype separations than AL at 5 mg/L of Mn. There were differences in enzyme activity between genotypes at control (0.25 mg/L) Mn solution concentration, making assessment difficult, especially between species. Barring this caveat, the relatively fast POD activity assay was considered to be the best method since it paralleled activity of IAAO, the functional enzyme of Mn toxicity, which had a relatively slow assay method.     

Physiological basis of differential aluminum tolerance in rice genotypes

Abstract

Two rice genotypes, aluminum (Al)‐tolerant Co 37 and Al‐susceptible ADT 36, were evaluated for their physiological responses in the presence of Al stress in a hydroculture experiment. Two levels of Al (0 and 222 μmol/L) were supplied in the nutrient solution and the two rice genotypes were subjected to Al for two weeks. Root growth parameters, relative growth reduction in roots (RGRR), effects of calcium (Ca²⁺) nitrate metabolism, Al content in roots, and pH shift patterns were recorded. The Al treatments had a lesser effect on Co 37 in terms of reduction in root growth and root dry matter production as compared to ADT 36. While Co 37 did not show significant differences in response to various levels of Ca²⁺ added in the medium under Al stress, ADT 36 registered a dose‐dependent effect in circumventing the injurious effects of Al. Further, reduction in nitrate content and in vivo nitrate reductase (NR) activity in the leaf tissue in Co 37 under Al treatment was less than that in ADT 36. Moreover, Co 37 had a lower content of Al in its root tissue than did ADT 36. Co 37 was also able to shift the pH of the medium more efficiently than ADT 36, thereby minimizing the uptake of Al, and eventually reducing Al toxicity. The higher level of tolerance to Al of Co 37 seems to have emanated from an efficient nitrate metabolism and its capacity to shift the pH of the medium. It is also evident that Al toxicity in ADT 36 can be circumvented by a Ca treatment to a considerable extent. Our results offer a possible physiological basis for Al tolerance in crop plants.     

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.     

Physiological and mineralogical properties of arsenic-induced chlorosis in rice seedlings grown hydroponically

Abstract

A hydroponic experiment was conducted to observe the effect of arsenic (As) on a number of physiological and mineralogical properties of rice (Oryza sativa L. cv. Akihikari) seedlings. Seedlings were treated with 0, 6.7, 13.4 and 26.8 µmol L^?1 As (0, 0.5, 1.0 and 2.0 mg As L^?1) for 14 days in a greenhouse. Shoot dry matter yield decreased by 23, 56 and 64%; however, the values for roots were 15, 35 and 42% for the 6.7, 13.4 and 26.8 µmol L^?1 As treatments, respectively. Shoot height decreased by 11, 35 and 43%, while that of the roots decreased by 6, 11 and 33%, respectively. These results indicated that the shoot was more sensitive to As than the root in rice. Leaf number and width of leaf blade also decreased with As toxicity. Arsenic toxicity induced chlorosis symptoms in the youngest leaves of rice seedlings by decreasing chlorophyll content. Concentrations and accumulations of K, Mg, Fe, Mn, Zn and Cu decreased significantly in shoots in the 26.8 µmol L^?1 As treatment. However, the concentration of P increased in shoots at 6.7 and 13.4 µmol L^?1 As levels, indicating a cooperative rather than antagonistic relationship. Arsenic and Fe concentration increased in roots at higher As treatments. Arsenic translocation (%) decreased in the 13.4 and 26.8 µmol L^?1 As treatments compared with the 6.7 µmol L^?1 As treatment. Arsenic and Fe were mostly concentrated in the roots of rice seedlings, assuming co-existence of these two elements. Roots contained an almost 8–16-fold higher As concentration than shoots in plants in the As treatments. Considering the concentration of Mn, Zn and Cu, it was suggested that chlorosis resulted from Fe deficiency induced by As and not heavy-metal-induced Fe deficiency.     

The relationship of soil and leaf nutrients to rice leaf oranging

A symptom called leaf‐oranging, indicating a deficiency of many nutrients, occurs in paddy rice (Oryzasativa L.) when production expands into some upland soils. Rice (Gui Chou cv.) was grown in culture pots in a flooded, weathered, upland soil (Nacogdoches) and compared to rice growth in a flooded soil currently used for paddy rice production (Dacosta) in Texas to understand the soil and plant factors involved in leaf‐oranging. Fertilizer rates of 0, 10, and 100 mg N/kg as (NH₄)₂SO₄ were applied to each soil along with phosphorus (P) and potassium (K) fertilizer. The orange Leaf Index (OLI), a measure of leaf‐oranging, was determined weekly and increased to 60–70% for plants grown in the upland soil but its progression was delayed by higher N treatments. No leaf‐oranging was observed in the paddy soil. The soil evoking leaf‐oranging was low in silicon (Si) and high in iron (Fe). In addition, analysis of leaves from these plants showed 19–25% higher leaf ammonium‐nitrogen (NH₄‐N), 9–137% higher manganese (Mn) levels and lower total N:NH₄ concentration compared to normal rice leaves four weeks after transplanting. This inferred that leaf‐oranging probably was associated with some degree of NH₄‐N toxicity and antagonism with K. Leaf‐oranging was also associated with low calcium (Ca) assimilation or Ca uptake inhibition because of the heavy Fe‐oxide coating of the roots of the affected rice plants. In this experiment, leaf‐oranging was not associated with toxic levels of Fe or Mn.     

Effect of commercial amino acids on iron nutrition of tomato plants grown under lime‐induced iron deficiency

The objective of this work was to study the effect of root and foliar application of two commercial products containing amino acids from plant and animal origin on iron (Fe) nutrition of tomato seedlings cultivated in two nutrient media: lime and normal nutrient solutions. In the foliar‐application experiment, each product was sprayed with 0.5 and 0.7 mL L^–1 2, 7, 12, and 17 d after transplanting. In the root application experiment, 0.1 and 0.2 mL L^–1 of amino acids products were added to the nutrient solutions. In both experiments, untreated control plants were included as well. Foliar and root application of the product containing amino acids from animal origin caused severe plant‐growth depression and nonpositive effects on Fe nutrition were found. In contrast, the application of the product from plant origin stimulated plant growth. Furthermore, significantly enhanced root and leaf Fe^III‐chelate reductase activity, chlorophyll concentration, leaf Fe concentration, and Fe^II : Fe ratio were found in tomato seedlings treated with the product from plant origin, especially when the amino acids were directly applied to the roots. These effects were more evident in plants developed under lime‐induced Fe deficiency. The positive results on Fe uptake may be related to the action of glutamic acid, the most abundant amino acid in the formulation of the product from plant origin.