Zn-inefficient Genotypes Research Articles

Alleviation of zinc deficiency in wheat inoculated with root endophytic fungus Piriformospora indica and rhizobacterium Pseudomonas putida

Plant growth, grain yield, and the nutritional grain quality reduce in Zinc (Zn)-deficient soils. As a practical approach, the utilization of plant growth-promoting microorganisms offers a promising approach to cope with Zinc deficiency. This study was carried out to determine the effects of endophytic fungus Piriformospora indica and rhizobacterium Pseudomonas putida on growth, nutrient status, and antioxidant enzyme activities of two wheat (Triticum aestivum L.) genotypes under Zn deficiency. The experiment was conducted using a completely randomized factorial design with three replications and three treatments: wheat genotypes [Zn-efficient (H-Zn) genotype, Azadi and Zn-inefficient (L-Zn) genotype, Niknejad]), Zn concentrations (0 and 2 μmol Zn L−1) and microbial inoculations (P. indica and P. putida). Under Zn deficiency, P. indica, P. putida, and dual inoculation of P. indica with P. putida significantly increased the shoot dry weight of L-Zn genotype up to 10.3%, 25.6% and, 15.4% compared to control plants, respectively. Inoculation of P. indica led to an increase in thinner and highly-branched roots and subsequently increased Zn uptake in the shoot of L-Zn genotype under Zn deficiency. Under such condition, individual inoculation of L-Zn genotype with P. putida and P. indica increased peroxidase activity by 85.7% and 40% compared with non-inoculated plants. Malondialdehyde content of both wheat genotypes inoculated with P. indica was significantly decreased compared to the non-inoculated plants. Generally, depending on the wheat genotype, P. indica and P. putida can increase plant tolerance by affecting only part of the tolerance mechanisms (including antioxidant and non-antioxidant mechanisms) under Zn deficiency stress. In summary, P. indica and P. putida could improve the fitness of the L-Zn genotype under Zn-deficiency stress.

Zinc Application Enhances Superoxide Dismutase and Carbonic Anhydrase Activities in Zinc-Efficient and Zinc-Inefficient Wheat Genotypes

Deficiency of zinc (Zn) in soils and crops of the world is established fact. There is need for application of Zn application and growing Zn-efficient crop genotypes to tide over the situation. However, information pertaining to application of Zn to Zn-efficient and Zn-inefficient crop genotypes grown in field condition on plant enzyme (wherein Zn is a co-factor) activities is limited. To investigate the influence of different Zn regimes on superoxide dismutase (SOD) and carbonic anhydrase (CA) activities of Zn-efficient and Zn-inefficient wheat genotypes, the present study was carried out comprising three each of Zn-efficient and Zn-inefficient genotypes of wheat grown under field experiment with four Zn treatments such as no Zn, soil Zn, foliar Zn, and both soil and foliar Zn. Application of Zn (soil/foliar/both) enhanced SOD and CA activities of both Zn-efficient and Zn-inefficient genotypes at pre- and post-anthesis growth stages of wheat compared to no Zn. Under no Zn, SOD activities were higher in both Zn-efficient and Zn-inefficient genotypes at post-anthesis stage; however, reverse was true for CA activities. Application Zn enhanced Zn concentration in leaves, stem, and grain of both Zn-efficient and Zn-inefficient genotypes. Grain Zn concentration increased by 25.1, 35.7, and 38.2% with soil, foliar, and both soil and foliar applications of Zn, respectively in Zn-inefficient genotypes and by 7.2, 21.1, and 30.6% with soil, foliar, and both and foliar applications of Zn, respectively in Zn-efficient genotypes, compared to no Zn. In Zn-efficient genotypes, SOD and CA activities contributed about 63 and 77% towards grain Zn concentration, respectively, whereas SOD and CA activities contributed about 50 and 66% towards grain Zn concentration, respectively in Zn-inefficient genotypes. The results indicated that both soil and foliar applications are needed for enhanced SOD and CA activities and plant Zn concentration in wheat. Physiological utilization of Zn plays an important role in Zn efficiency of wheat genotypes.

Patterns of stress response and tolerance based on transcriptome profiling of rice crown tissue under zinc deficiency.

Zinc (Zn) deficiency is the most prevalent micronutrient disorder in rice and leads to delayed development and decreased yield. Several studies have investigated how rice plants respond to Zn deficiency and examined the differences between Zn-efficient (ZE) and Zn-inefficient (ZI) genotypes. ZE genotypes reallocate more Zn to roots and are better at maintaining crown root development than ZI genotypes in response to Zn deficiency. However, little is known about the molecular mechanisms controlling these differences. Moreover, the role of the crown, the part of the stem from which crown roots emerge, has yet to be examined. In this study we highlight the molecular mechanisms triggered by early Zn deficiency in crown tissue through RNA sequencing of two contrasting groups of several ZE and ZI genotypes. This method allowed us to (i) identify several novel and well-known Zn transporters involved in Zn retranslocation from the crown to the shoot and roots in response to Zn deficiency; (ii) determine that Zn deficiency triggers the conversion of soluble sugars into starch; and (iii) detect several candidate genes possibly conferring Zn efficiency, including a monosaccharide transporter, a Zn finger domain-containing protein, a gibberellin-stimulated family protein and a plasma membrane polypeptide family protein.

Genotypic Divergence in Per Cent Zinc Derived from Variable Sources and on Zinc Uptake in Rice [Oryza sativa (L.)] as Established by Radiotracer Techniques

A pot culture experiment was conducted at Tamil Nadu Agricultural University, Coimbatore to establish the genotypic divergence in per cent zinc (Zn) derived from fertilizer (% Zndff) or from soil (% Zndfs) and on zinc uptake of two rice genotypes Zn -efficient Norungan and Zn –inefficient PMK 3. To assess the partitioning of Zn in the shoot and root of the genotypes, radiotracer technique was employed, in which graded levels of Zn (0.0, 12.5, 25.0, 37.5 and 50.0 kg ha-1 of ZnSO4) as 65Zn labeled ZnSO4 were applied. Per cent Zndff showed an increase by 50 % for Zn-inefficient PMK 3 than that for Norungan while reverse was the trend for % Zndfs implying the fact that Zn inefficient genotypes have the ability to use only readily available sources of Zn. Zinc uptake from fertilizer increased with increasing dose of applied zinc for PMK 3 while in Norungan it showed a peak at 25 kg ha-1 of ZnSO4 and thereafter it declined. Irrespective of the sources of zinc, root zinc accumulation was higher in PMK 3 than that in Norungan. The Zn-efficient genotype Norungan had better translocation of Zn from root to shoot than PMK 3.

Effect of root morphological traits on zinc efficiency in Iranian bread wheat genotypes

ABSTRACTZinc (Zn) has a vast number of functions in plant metabolism, the lack of which had dramatic effects on growth and yield of plants. Plants have morphological and biochemical responses to enhance mineral solubility in the soil and facilitate uptake, such as root plasticity, secretion processes and symbioses. Root architecture modification is an important plant response to nutrient availability. The aim of this study was to identify root morphological reactions to Zn efficiency in Iranian bread wheat genotypes. Soil and solution cultures were used to survey Zn efficiency. In soil culture, six and seven genotypes with high and low Zn contents were selected among 110 Iranian bread wheat genotypes, respectively. The solution culture experiments were set up in a completely randomized block design and plants fed with Johnson’s grass solution. All traits were assessed at 30 and 60 DAPs (days after planting). Our results showed a significant difference between two groups of efficient and inefficient genotypes only at 60 DAP, and Zn-efficient genotypes showed 1.63-, 1.50-, 1.69- and 1.92-fold increases in root diameter, surface area density, shoot and root dry weight, respectively, compared to inefficient genotypes. In contrast, Zn-inefficient genotypes had 1.20- and 2.62-fold more root length and fineness, respectively, than efficient genotypes. The positive significant correlations were observed between shoot and Zn uptake as well as root dry weight and Zn uptake at both stages. Furthermore, shoot and root dry weight showed a significant correlation with root fineness, diameter and surface area density at both stages. The path analysis showed indirect effects on Zn uptake through root traits. Our results showed that roots have a major role in Zn efficiency. Therefore, the better growth and greater Zn uptake in efficient genotypes, compared to inefficient ones, can be attributed to greater root diameter and surface area density, and lower root fineness in these genotypes.

Rapid Crown Root Development Confers Tolerance to Zinc Deficiency in Rice.

Zinc (Zn) deficiency is one of the leading nutrient disorders in rice (Oryza sativa). Many studies have identified Zn-efficient rice genotypes, but causal mechanisms for Zn deficiency tolerance remain poorly understood. Here, we report a detailed study of the impact of Zn deficiency on crown root development of rice genotypes, differing in their tolerance to this stress. Zn deficiency delayed crown root development and plant biomass accumulation in both Zn-efficient and inefficient genotypes, with the effects being much stronger in the latter. Zn-efficient genotypes had developed new crown roots as early as 3 days after transplanting (DAT) to a Zn deficient field and that was followed by a significant increase in total biomass by 7 DAT. Zn-inefficient genotypes developed few new crown roots and did not increase biomass during the first 7 days following transplanting. This correlated with Zn-efficient genotypes retranslocating a higher proportion of shoot-Zn to their roots, compared to Zn-inefficient genotypes. These latter genotypes were furthermore not efficient in utilizing the limited Zn for root development. Histological analyses indicated no anomalies in crown tissue of Zn-efficient or inefficient genotypes that would have suggested crown root emergence was impeded. We therefore conclude that the rate of crown root initiation was differentially affected by Zn deficiency between genotypes. Rapid crown root development, following transplanting, was identified as a main causative trait for tolerance to Zn deficiency and better Zn retranslocation from shoot to root was a key attribute of Zn-efficient genotypes.

Genotypic Divergence in Expression of Zinc-Deficiency Symptoms of Rice (Oryza sativa L.) in Sand Culture

Zinc (Zn) deficiency is a ubiquitous disorder constraining rice production worldwide. A sand culture experiment was conducted in factorial randomized block design with three replications to study the relative performance of 18 rice (Oryza sativa L.) genotypes experiencing Zn stress. Modified Hoagland’s solution was employed as the nutrient medium and the experiment was maintained for 60 days from sowing. The genotypes were categorized as efficient (Norungan, ASD 16, Pokkali, Pusa Vikas, TRY 1, and Triveni), moderately efficient (White Ponni, CO 47, ADT 36, IR 8, ASD 20, and TRY 2), and inefficient (ADT 38, PMK 3, CO 43, CSR 10, ADTRH 1, and MDU 5) to Zn stress based on the preliminary solution culture experiment (Sudhalakshmi 2007). Growth performance, dry-matter production, biochemical constituents, and enzymatic activity of rice were measured employing graded levels of Zn (at 0, 0.025, 0.05, 0.1, and 0.2 mg L−1) and were statistically analyzed using the method of Panse and Sukhatme (1978). The efficient rice genotypes showed minimum reduction in dry-matter production in Zn stress conditions while the inefficient rice genotypes suffered severe reduction. Zinc-efficient rice genotypes were not responsive to fertilizer. Dry-matter production, chlorophyll a, chlorophyll b, total chlorophyll, soluble protein, and indole-3-acetic acid (IAA) oxidase activity can be employed as diagnostic tools to differentiate Zn-efficient and Zn-inefficient rice genotypes in sand culture.

Grain Phytic Acid Accumulation of Domestic and Exotic Rice Genotypes in Zinc-Deficient Soil

Micronutrient malnutrition in humans living in rice growing areas is increasing rapidly due to less absorption of mineral nutrients chelated by phytic acid (anti-nutrients) present in rice grains. A field study was conducted to evaluate the grain phytic acid and zinc (Zn) accumulation of 10 field grown rice (Oryza sativa L.) genotypes on a Zn deficient soil. Both the Zn- efficient (Shua-92, IR-9, Shandar, IR-36, and IR-6) and Zn-inefficient (Sarshar,. UPL-48, Khushboo-95 and RG-120) rice genotypes were included in the study. The two Zn treatments (0 and 15 kg ha-1) were arranged in a two factor randomized complete block design with three replications. Nitrogen (N) and phosphorus (P2O5) were applied at the rate of 120 and 80 kg ha-1. The rice genotypes IR-36, UPL-79, Shandar and Shua-92 were the most Zn accumulators whereas; Sarshar, IR-9 and Khushboo-95 the least accumulator in Zn deficiency. Zinc in-efficient genotype Sarshar was the highest Zn accumulator in response to Zn application. Phytic acid content of rice genotypes was significantly influenced (p < 0.05) by the application of Zn fertilizer. Phosphorus concentration in rice grains decreased with Zn application. Zinc in-efficient genotypes accumulated more phytic acid in their food reserves than Zn-efficient genotypes. Phytic acid: zinc ratio decreased significantly more in Zn-inefficient genotypes as compared to Zn efficient genotypes, with application of Zn fertilizer. Zinc efficient genotype Shua-92 accumulated low concentration of phytic acid. The rice genotypes Shua-92, IR-9, Shandar and IR-36 low accumulators of phytic acid performed successfully and contained higher concentrations of Zn than other genotypes.

Phytosiderophore release by wheat genotypes differing in zinc deficiency tolerance grown with Zn-free nutrient solution as affected by salinity

There is limited information concerning the effect of salinity on phytosiderophores exudation from wheat roots. The aim of this hydroponic experiment was to investigate the effect of salinity on phytosiderophore release by roots of three bread wheat genotypes differing in Zn efficiency (Triticum aestivum L. cvs. Rushan, Kavir, and Cross) under Zn deficiency conditions. Wheat seedlings were transferred to Zn-free nutrient solutions and exposed to three salinity levels (0, 60, and 120mM NaCl). The results indicated that Cross and Rushan genotypes exuded more phytosiderophore than did the Kavir genotype. Our findings suggest that the adaptive capacity of Zn-efficient ‘Cross’ and ‘Rushan’ wheat genotypes to Zn deficiency is due partly to the higher amounts of phytosiderophore release. Only 15 days of Zn deficiency stress was sufficient to distinguish between Zn-efficient (Rushan and Cross) and Zn-inefficient (Kavir) genotypes, with the former genotypes exuding more phytosiderophore than the latter. Higher phytosiderophore exudation under Zn deficiency conditions was accompanied by greater Fe transport from root to shoot. The maximum amount of phytosiderophore was exuded at the third week in ‘Cross’ and at the fourth week in ‘Kavir’ and ‘Rushan’. For all three wheat genotypes, salinity stress resulted in higher amounts of phytosiderophore exuded by the roots. In general, for ‘Kavir’, the largest amount of phytosiderophore was exuded from the roots at the highest salinity level (120mM NaCl), while for ‘Cross’ and ‘Rushan’, no significant difference was found in phytosiderophore exudation between the 60 and 120mM NaCl treatments. More investigation is needed to fully understand the physiology of elevated phytosiderophore release by Zn-deficient wheat plants under salinity conditions.

Revisiting the role of organic acids in the bicarbonate tolerance of zinc-efficient rice genotypes

It has been hypothesised that enhanced organic acid release from the roots of zinc-efficient rice (Oryza sativa L.) genotypes plays a strong role in plant tolerance to both bicarbonate excess and Zn deficiency. To address several uncertainties in the literature surrounding the tolerance of rice to bicarbonate, we initially assessed the tolerance of six rice genotypes to bicarbonate stress under field conditions and in solution culture. The landrace Jalmagna and its recombinant inbred offspring, RIL46, consistently performed better in terms of maintenance of biomass and root length under high bicarbonate concentrations. In the hydroponic experiments, increased root malate (but not citrate) accumulation and efflux were responses to high solution bicarbonate in the short-term (12h) in all genotypes. Although both citrate and malate accumulation and efflux increased after long-term exposure (10 days) to high bicarbonate and Zn deficiency, it coincided with amino acid leakage from the roots. Partial least-squares regression showed that this leakage consistently ranked highly as an indicator of poor plant health under all stress conditions, whereas specific malate efflux (the ratio of malate to amino acid efflux) was an important predictor of good plant health. The root leakage of Zn-inefficient genotypes under bicarbonate and dual stress (bicarbonate with low Zn) was typically higher than in Zn-efficient genotypes, and coincided with higher peroxide concentrations, suggesting that bicarbonate tolerance is related to the ability of Zn-efficient genotypes to overcome oxidative stress, maintain root membrane integrity and minimise root leakage.

Influence of arbuscular mycorrhizal fungi on uptake of Zn and P by two contrasting rice genotypes

There is little experimental evidence about the functional significance of arbuscular mycorrhizal fungi (AMF) colonization in providing nutrients for lowland rice. This study was undertaken to examine whether growth and nutrient deficiencies may affect plants benefit from AMF inoculation. Two contrasting rice (<I>Oryza sativa</I> L.) genotypes and two AMF species (<I>Glomus mosseae</I> and <I>G. intraradices</I>) were used in this experiment. Under P starvation, P uptake in the genotype tolerant to P deficiency (Fajr), declined significantly up to 36% (<I>P</I> < 0.05) in response to AMF inoculation, while it enhanced by about 70% (<I>P</I> < 0.01) in susceptible genotype (Shafagh). Under Zn starvation, Zn uptake of Zn-efficient genotype (Shafagh) increased by about 2 fold (<I>P</I> < 0.01), but a reduction of 52% (<I>P</I> < 0.05) was observed in the Zn-inefficient genotype (Fajr) upon mycorrhization. Greater genotypic differences were observed for –P–Zn plants. Our results imply that genotypic difference in responsiveness to inoculation with AMF is attributable to different contribution of mechanisms for increased nutrient uptake in mycorrhizal plants depending on nutrient, nutritional status and nutrient efficiency of genotypes.

Growth Responses to Varying Zinc Activities in Rice Genotypes Differing in Zn Efficiency and Zn Density

ABSTRACT The relationship between tolerance to Zn-deficiency and Zn-dense in grain was examined. The results showed that the plant height and leaf growth rate (number) decreased more in the Zn-dense genotype (Biyuzaonuo) than in the Zn-efficient genotype (IR8192) when grown at pZn2+ ≥ 11.0. Shoot dry matter yield decreased about 6 times for the dense CV whereas only around 3 times for the efficient CV at Zn activities pZn2+ 9.7–11.4. Zinc uptake by the whole plant was greater in the Zn-dense genotype than in the Zn-efficient genotype, and the lowest in the Zn-inefficient genotype grown at pZn2+ ≥ 11.0. However, both the efficient and the dense cultivars showed higher Zn translocation efficiency at low Zn activity. These results indicate that Zn uptake by rice seedling and translocation might be more suitable than growth parameters for identifying Zn-efficient and Zn-dense genotype at seedling stage.

Quantitative trait loci analysis of zinc efficiency and grain zinc concentration in wheat using whole genome average interval mapping

Zinc (Zn) deficiency is a widespread problem which reduces yield and grain nutritive value in many cereal growing regions of the world. While there is considerable genetic variation in tolerance to Zn deficiency (also known as Zn efficiency), phenotypic selection is difficult and would benefit from the development of molecular markers. A doubled haploid population derived from a cross between the Zn inefficient genotype RAC875-2 and the moderately efficient genotype Cascades was screened in three experiments to identify QTL linked to growth under low Zn and with the concentrations of Zn and iron (Fe) in leaf tissue and in the grain. Two experiments were conducted under controlled conditions while the third examined the response to Zn in the field. QTL were identified using an improved method of analysis, whole genome average interval mapping. Shoot biomass and shoot Zn and Fe concentrations showed significant negative correlations, while there were significant genetic correlations between grain Zn and Fe concentrations. Shoot biomass, tissue and grain Zn concentrations were controlled by a number of genes, many with a minor effect. Depending on the traits and the site, the QTL accounted for 12–81% of the genetic variation. Most of the QTL linked to seedling growth under Zn deficiency and to Zn and Fe concentrations were associated with height genes with greater seedling biomass associated with lower Zn and Fe concentrations. Four QTL for grain Zn concentration and a single QTL for grain Fe concentration were also identified. A cluster of adjacent QTL related to the severity of symptoms of Zn deficiency, shoot Zn concentration and kernel weight was found on chromosome 4A and a cluster of QTL associated with shoot and grain Fe concentrations and kernel weight was found on chromosome 3D. These two regions appear promising areas for further work to develop markers for enhanced growth under low Zn and for Zn and Fe uptake. Although there was no significant difference between the parents, the grain Zn concentration ranged from 29 to 43 mg kg−1 within the population and four QTL associated with grain Zn concentration were identified. These were located on chromosomes 3D, 4B, 6B and 7A and they described 92% of the genetic variation. Each QTL had a relatively small effect on grain Zn concentration but combining the four high Zn alleles increased the grain Zn by 23%. While this illustrates the potential for pyramiding genes to improve grain Zn, breeding for increased grain Zn concentration requires identification of individual QTL with large effects, which in turn requires construction and testing of new mapping populations in the future.

Genetic Analysis on Agronomic Traits Related to Zinc Efficiency in Lowland Rice

The hereditary characteristics of plant traits related to zinc (Zn) efficiency in lowland rice (Oryza sativa L.) were analyzed in this study. The plant traits include the relative values of the total plant dry matter, shoot dry matter, root dry matter, leaf number, plant height, and root length of rice seedlings grown at low Zn2+ activity compared to those at sufficient Zn2+ activity. The F1 progeny was obtained by using a diallel cross between Zn-efficient and Zn-inefficient rice genotypes. The results revealed that the variances of dominant effect of all the traits studied were significant or very significant with the ratio being more than 65.4% of total variance. The additive effect variances of other traits except for the root length were also significant or very significant, with the average ratio being 21.85% of total variance. The variance of epistatic effect for root length was significant, with a ratio of 48% of total variance. The relative values of root length at low Zn2+ activity to sufficient Zn2+ activity was found to be governed by dominant and additive effects of hereditary genes. The relative values of leaf number, plant height and dry matter weight of rice seedlings were mainly governed by dominant effects of genes, secondarily by additive effects of genes. As the ability of rice tolerance to Zn deficiency was closely related to these agronomic traits, zinc efficiency in rice was mainly governed by dominant effects of genes, secondarily by additive effects of genes, and might be influenced by its epistatic effects of genes.

Effect of bicarbonate on elongation and distribution of organic acids in root and root zone of Zn-efficient and Zn-inefficient rice ( Oryza sativa L.) genotypes

Bicarbonate content in growth medium has been considered to be one of the major factors in inducing zinc (Zn) deficiency in lowland rice. In this study, the effects of bicarbonate on root elongation, distribution and exudation of organic acids were examined in the Zn-efficient and Zn-inefficient rice genotypes. Bicarbonate significantly increased the growth of fine roots in the Zn-efficient genotype but inhibited their growth in the Zn-inefficient genotype. In fine root zones of 2–8 mm from the tip, of plants grown with bicarbonate, malate, citrate, and fumarate accumulation was greater in the Zn-inefficient genotype, but not the Zn-efficient genotype, than the control. Enhanced exudation of malate at fine root zones of 1–2 cm from the tip by bicarbonate was noted in the Zn-efficient genotype, but not in the Zn-inefficient genotype. Bicarbonate induced greater exudation of citrate at fine root zones of 1–2 cm from the tip in the Zn-efficient than in the Zn-inefficient genotypes. The increase in the concentrations of malate and citrate in xylem sap by bicarbonate were found to be greater for the Zn-efficient genotype than that for the Zn-inefficient genotype. The results indicated that bicarbonate-mediated inhibition of fine root elongation in the Zn-inefficient genotype might result from excessive accumulation of organic acids, particularly malate, in root elongation zones. In the Zn-inefficient rice genotype the increased accumulation of malate and citrate in fine roots by bicarbonate appeared to result from less exudation of them to the rhizosphere and transport up to the shoots.

The role of shoot-localized processes in the mechanism of Zn efficiency in common bean.

Zn efficiency (ZE) is the ability of plants to maintain high yield under Zn-deficiency stress in the soil. Two bean ( Phaseolus vulgaris L.) genotypes that differed in ZE, Voyager (Zn-efficient) and Avanti (Zn-inefficient), were used for this investigation. Plants were grown under controlled-environment conditions in chelate-buffered nutrient solution where Zn(2+) activities were controlled at low (0.1 pM) or sufficient (150 pM) levels. To investigate the relative contribution of the root versus the shoot to ZE, observations of Zn-deficiency symptoms in reciprocal grafts of the two genotypes were made. After growth under low-Zn conditions, plants of nongrafted Avanti, self-grafted Avanti and reciprocal grafts that had the Avanti shoot scion exhibited Zn-deficiency symptoms. However nongrafted and self-grafted Voyager, as well as reciprocal grafts with the Voyager shoot scion, were healthy with no visible Zn-deficiency symptoms under the same growth conditions. More detailed investigations into putative shoot-localized ZE mechanisms involved determinations of leaf biomass production and Zn accumulation, measurements of subcellular Zn compartmentation, activities of two Zn-requiring enzymes, carbonic anhydrase and Cu/Zn-dependent superoxide dismutase (Co/ZnSOD), as well as the non-Zn-requiring enzyme nitrate reductase. There were no differences in shoot tissue Zn concentrations between the Zn-inefficient and Zn-efficient genotypes grown under the low-Zn conditions where differences in ZE were exhibited. Shoot Zn compartmentation was investigated using radiotracer ((65)Zn) efflux analysis and suggested that the Zn-efficient genotype maintains higher cytoplasmic Zn concentrations and less Zn in the leaf-cell vacuole, compared to leaves from the Zn-inefficient genotype under Zn deficiency. Analysis of Zn-requiring enzymes in bean leaves revealed that the Zn-efficient genotype maintains significantly higher levels of carbonic anhydrase and Cu/ZnSOD activity under Zn deficiency. While these data are not sufficient to allow us to determine the specific mechanisms underlying ZE, they certainly point to the shoot as a key site where ZE mechanisms are functioning, and could involve processes associated with Zn compartmentation and biochemical Zn utilization.

Effects of bicarbonate and high pH on growth of Zn-efficient and Zn-inefficient genotypes of rice, wheat and rye

A range of Zn efficient and inefficient genotypes of rice (Oryza sativa L.), wheat (Triticum aestivum L. and Triticum durum L.) and rye (Secale cereale L.) were investigated to examine shoot and root growth response to bicarbonate and high pH at different Zn levels. The treatment of bicarbonate was 10 mM as NaHCO3, high pH was 8.0 buffered with HEPES, and three Zn levels included without addition of Zn (deficient Zn), with 0.5 and 1.0 μM Zn (moderate Zn), and 1.5 and 3.5 μM (high Zn) for rice and wheat or rye, respectively. For rice, shoot and root growth of Zn-inefficient genotypes was strongly inhibited, whereas root length of Zn-efficient genotypes was considerably enhanced by bicarbonate. High pH had much less effect on reduced root growth of the Zn-inefficient rice genotypes and enhanced root length of the Zn-efficient genotypes, as compared with bicarbonate. Responses of plants to bicarbonate at different Zn levels showed that under deficient or moderate Zn supply conditions, root dry weight and length were significantly increased for the Zn-efficient rice genotypes, but were decreased for the Zn-inefficient rice genotypes grown with bicarbonate. At high Zn supply, however, root elongation of the Zn-efficient rice genotypes and of the Zn-inefficient genotypes was enhanced. In contrast to rice, either bicarbonate or high pH had little effects on shoot and root growth for all the Zn-efficient and Zn-inefficient wheat genotypes except for Dagdas (Zn-efficient) at moderate Zn levels. Decreased root growth due to bicarbonate treatment was observed only in the Zn-efficient rye. Root lengths of all the wheat and rye genotypes except for Kunduru (Zn-inefficient) were reduced by high pH but not affected by bicarbonate. At deficient Zn levels, shoot growth was reduced by bicarbonate for both Zn-efficient rye and inefficient wheat genotypes. Root lengths was reduced due to bicarbonate for the Zn-inefficient wheat genotype at deficient and high Zn, but not at moderate Zn levels. The results imply that Zn efficiency in rice is closely associated with plant tolerance to bicarbonate relative to root growth, and bicarbonate tolerance and Zn efficiency might be developed simultaneously in lowland rice.

Genotypic differences in Zn efficiency of Medicago species

Nineteen annual Medicago genotypes from eight species were grown in Mt. Compass sand at three levels of soil Zn application (0, 0.1 and 0.9 mg Zn kg-1) to achieve Zn status from deficiency to adequacy. Genotypes differed in growth response: under Zn deficiency, those classified as Zn-efficient had less reduction in shoot growth, a higher root mass, greater concentration of Zn in the shoot and total Zn content per plant, and a stable shoot/root ratio compared with Zn-inefficient genotypes. While seed Zn content did not influence the Zn-efficiency ranking, it did affect yield, and so it plays an important role in yield response and Zn accumulation as Zn supply decreases.

Zinc deficiency-induced phytosiderophore release by the Triticaceae is not consistently expressed in solution culture.

The effects of zinc (Zn) and iron (Fe) deficiencies on phytosiderophore (PS) exudation by three barley (Hordeum vulgare L.) cultivars differing in Zn efficiency were assessed using chelator-buffered nutrient solutions. A similar study was carried out with four wheat (Triticum aestivum L. and T. durum Desf.) cultivars, including the Zn-efficient Aroona and Zn-inefficient Durati. Despite severe Zn deficiency, none of the barley or wheat cultivars studied exhibited significantly elevated PS release rates, although there was significantly enhanced PS exudation under Fe deficiency. Aroona and Durati wheats were grown in a further study of the effects of phosphate (P) supply on PS release, using 100 microM KH2PO4 as high P, or solid hydroxyapatite as a supply of low-release P. Phytosiderophore exudation was not increased due to P treatment under control or Zn-deficient conditions, but was increased by Fe deficiency. Accumulation of P in shoots of Zn- and Fe-deficient plants was seen in both P treatments, somewhat more so under the KH2PO4 treatment. Zinc-efficient wheats and grasses have been previously shown to exude more PS under Zn deficiency than Zn-inefficient genotypes. We did not observe Zn-deficiency-induced PS release and were unable to replicate the results of previous researchers. The tendency for Zn deficiency to induce PS release seems to be method dependent, and we suggest that all reported instances may be explained by an induced physiological deficiency of Fe.

Alfalfa genotypes differ in their ability to tolerate zinc deficiency

Response of 13 alfalfa (Medicago sativa L.) genotypes to varied Zn supply (+Zn: 2 mg kg−1 soil, −Zn: no added Zn) was studied in a pot experiment under controlled environmental conditions. Plants were grown for four weeks in a Zn-deficient siliceous sandy soil. Plants grown at no added Zn showed typical Zn deficiency symptoms i.e. interveinal chlorosis of leaves, yellowish-white necrotic lesions on leaf blades, necrosis of leaf margins, smaller leaves and a marked reduction in growth. There was solute leakage from the leaves of Zn-deficient plants, while no solute leakage from Zn-sufficient plants. The ratios of P:Zn, Fe:Zn, Cu:Zn and Mn:Zn in Zn-deficient plants were extremely high compared with Zn-sufficient plants indicating disturbance of P:Zn, Fe:Zn, Cu:Zn and Mn:Zn balance within plant system by Zn deficiency. Genotypes differed markedly in Zn efficiency based on shoot dry matter production. Alfalfa genotypes also differed markedly in P:Zn ratio, Cu:Zn ratio and Fe:Zn ratio under —Zn treatment. The shoot dry weight, shoot:root ratio, chlorophyll content of fresh leaf tissue, solute leakage from the leaves, Zn uptake and distribution of Zn in shoots and roots were the most sensitive parameters of Zn efficiency. Zn-efficient genotypes had less solute leakage but higher shoot:root ratio and higher Zn uptake compared with Zn-inefficient genotypes. Under —Zn treatment, Zn-inefficient genotypes had less Zn partitioning to shoots (33–37%) and more Zn retained in roots (63–67%), while Zn-efficient genotypes had about equal proportions of Zn in roots (50%) and shoots (50%).