Phytosiderophore release in bread and durum wheat genotypes differing in zinc efficiency

The effect of the zinc (Zn) nutritional status on the rate of phytosiderophore release was studied in nutrient solution over 20 days in four bread wheat (Triticum aestivum cvs. Kiraç-66, Gerek-79, Aroona and Kirkpinar) and four durum wheat (Triticum durum cvs. BDMM-19, Kunduru-1149, Kiziltan-91 and Durati) genotypes differing in Zn efficiency.Visual Zn deficiency symptoms, such as whitish-brown necrosis on leaves and reduction in plant height appeared first and more severe in Zn-inefficient durum wheat genotypes Kiziltan-91, Durati and Kunduru-1149. Compared to the bread wheat genotypes, all durum wheat genotypes were more sensitive to Zn deficiency. BDMM-19 was the least affected durum wheat genotype. Among the bread wheat genotypes, Kirkpinar was the most sensitive genotype. In all genotypes well supplied with Zn, the rate of phytosiderophore release was very low and did not exceed 1 mol 32 plants^-1 3h^-1, or 0.5 mol g^-1 root dry wt 3h^-1. However, under Zn deficiency, with the onset of visual Zn deficiency symptoms, the release of phytosiderophores was enhanced in bread wheat genotypes up to 7.5 mol 32 plants^-1 3h^-1, or 9 mol g^-1 root dry wt 3h^-1, particularly in Zn-efficient Kiraç-66, Gerek-79 and Aroona. In contrast to bread wheat genotypes, phytosiderophore release in Zn-deficient durum wheat genotypes remained at a very low rate. Among the durum wheat genotypes BDMM-19 had highest rate of phytosiderophore release. HPLC analysis of root exudates showed that 2-deoxymugineic acid (DMA) is the dominating phytosiderophore released from roots of Zn-efficient genotypes. In root extracts concentration of DMA was also much higher in Zn-efficient than in inefficient genotypes. The results demonstrate that enhanced synthesis and release of phytosiderophores at deficient Zn supply is involved in Zn efficiency in wheat genotypes. It is suggested that the expression of Zn efficiency mechanism is causally related to phytosiderophore-mediated enhanced mobilization of Zn from sparingly soluble Zn pools and from adsorption sites, both in the rhizosphere and plants.     

Root exudation and Fe uptake and transport in wheat genotypes differing in tolerance to Zn deficiency

Tolerance to Zn deficiency in wheat germplasm may be inversely related to uptake and transport of Fe to shoots. The present study examined eight bread (Triticum aestivum) and two durum (T. turgidum L. conv. durum) wheat genotypes for their capacity to take up and transport Fe when grown under either Fe or Zn deficiency. Bread wheat genotypes Aroona, Excalibur and Stilleto showed tolerance to Zn and Fe deficiency, while durum wheat genotypes are clearly less tolerant to either deficiency. Roots of bread wheats tolerant to Zn deficiency exuded more phytosiderophores than sensitive bread and durum genotypes. Greater amounts of phytosideophores were exuded by roots grown under Fe than Zn deficiency. A relatively poor relationship existed between phytosiderophore exudation or the Fe uptake rate and relative shoot growth under Fe deficiency. At advanced stages of Zn deficiency, genotypes tolerant to Zn deficiency (Aroona and Stilleto) had a greater rate of Fe uptake than other genotypes. Zinc deficiency depressed the rate of Fe transport to shoots in all genotypes in early stages, while advanced Zn deficiency had the opposite effect. Compared with Zn-sufficient plants, 17-day-old Zn-deficient plants of genotypes tolerant to Zn deficiency had a lower rate of Fe transport to shoots, while genotypes sensitive to Zn deficiency (Durati, Yallaroi) had the Fe transport rate increased by Zn deficiency. A proportion of total amount of Fe taken up that was transported to shoots increased with duration of either Fe or Zn deficiency. It is concluded that greater tolerance to Zn deficiency among wheat genotypes is associated with the increased exudation of phytosiderophores, an increased Fe uptake rate and decreased transport of Fe to shoots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Differential response of rye,triticale, bread and durum wheats to zinc deficiency in calcareous soils

Field and greenhouse experiments were carried out to study the response of rye (Secale cereale L. cv. Aslim), triticale (× Triticosecale Wittmark. cv. Presto), two bread wheats (Triticum aestivum L, cvs. Bezostaja-1 and Atay-85) and two durum wheats (Triticum durum L. cvs. Kunduru-1149 and C-1252) to zinc (Zn) deficiency and Zn fertilization in severely Zn-deficient calcareus soils (DTPA-Zn=0.09 mg kg^-1 soil). The first visible symptom of Zn deficiency was a reduction in shoot elongation followed by the appearance of whitish-brown necrotic patches on the leaf blades. These symptoms were either absent or only slight in rye and triticale, but occurred more rapidly and severely in wheats, particularly in durum wheats. The same was true for the decrease in shoot dry matter production and grain yield. For example, in field experiments at the milk stage, decreases in shoot dry matter production due to Zn deficiency were absent in rye, and were on average 5% in triticale, 34% in bread wheats and 70%, in durum wheats. Zinc fertilization had no effect on grain yield in rye but enhanced grain yield of the other cereals. Zinc efficiency of cereals, expressed as the ratio of yield (shoot dry matter or grain) produced under Zn deficiency compared to Zn fertilization were, on average, 99% for rye, 74% for triticale, 59% for bread wheats and 25% for durum wheats.These distinct differences among and within the cereal species in susceptibility to Zn deficiency were closely related to the total amount (content) of Zn per shoot, but not with the Zn concentrations in shoot dry matter. For example, the most Zn-efficient rye and the Zn-inefficient durum wheat cultivar C-1252 did not differ in shoot Zn concentration under Zn deficiency, but the total amount of Zn per whole shoot was approximately 6-fold higher in rye than the durum wheat. When Zn was applied, rye and triticale accumulated markedly more Zn both per whole shoot and per unit shoot dry matter in comparison to wheats.The results demonstrate an exceptionally high Zn efficiency of rye and show that among the cereals studied Zn efficiency declines in the order rye>triticale>bread wheat>durum wheat. The differences in expression of Zn efficiency are possibly related to a greater capacity of efficient genotypes to acquire Zn from the soil compared to inefficient genotypes.     

Zinc deficiency as a critical problem in wheat production in Central Anatolia

In a soil and plant survey, and in field and greenhouse experiments the nutritional status of wheat plants was evaluated for Zn, Fe, Mn and Cu in Central Anatolia, a semi-arid region and the major wheat growing area of Turkey.All 76 soils sampled in Central Anatolia were highly alkaline with an average pH of 7. 9. More than 90% of soils contained less than 0.5 mg kg^-1 DTPA-extractable Zn, which is widely considered to be the critical deficiency concentration of Zn for plants grown on calcareous soils. About 25% of soils contained less than 2.5 mg kg^-1 DTPA-extractable Fe which is considered to be the critical deficiency concentration of Fe for plants. The concentrations of DTPA-extractable Mn and Cu were in the sufficiency range. Also the Zn concentrations in leaves were very low. More than 80% of the 136 leaf samples contained less than 10 mg Zn kg^–1. By contrast, concentrations of Fe, Mn and Cu in leaves were in the sufficient range.In the field experiments at six locations, application of 23 kg Zn ha^-1 increased grain yield in all locations. Relative increases in grain yield resulting from Zn application ranged between 5% to 554% with a mean of 43%. Significant increases in grain yield (more than 31%) as a result of Zn application were found for the locations where soils contained less than 0.15 mg kg^-1 DTPA-extractable Zn.In pot experirnents with two bread (Triticum aestivum, cvs. Gerek-79 and Kirac-66) and two durum wheats (Triticum durum, cvs. Kiziltan-91 and Kunduru-1149), an application of 10 mg Zn kg^-1 soil enhanced shoot dry matter production by about 3.5-fold in soils containing 0.11 mg kg^-1 and 0.15 mg kg^-1 DTPA-extractable Zn. Results from both field observations and greenhouse experiments showed that durum wheats were more susceptible to Zn deficiency than the bread wheats. On Zn deficient soils, durum wheats as compared to bread wheats developed deficiency symptoms in shoots earlier and to a greater extent, and had lower Zn concentration in shoot tissue and lower Zn content per shoot than the bread wheats.The results presented in this paper demonstrate that (i) Zn deficiency is a critical nutritional problem in Central Anatolia substantially limiting wheat production, (ii) durum wheats possess higher sensitivity to Zn deficient conditions than bread wheats, and (iii) wheat plants grown in calcareous soils containing less than 0.2 mg kg^-1 DTPA-extractable Zn significantly respond to soil Zn applications. The results also indicate that low levels of Zn in soils and plant materials (i.e. grains) could be a major contributing factor for widespread occurrence of Zn deficiency in children in Turkey, whose diets are dominated by cereal-based foods.     

Physiological responses of wheat genotypes grown in chelator-buffered nutrient solutions with increasing concentrations of excess HEDTA

The chelator-buffered nutrient solutions containing excess chelator have been used frequently in the micronutrient research, but potential toxicity of the excess chelator has not been ascertained. The present study was conducted to test effects of four concentrations of excess HEDTA [ N-(2-hydroxyethyl)ethylenedinitrilotriacetic acid] and two levels of total Zn on growth, root exudation, and nutrient uptake and transport by Triticum aestivum L. (cv. Aroona) and Triticum turgidum L. conv. durum (Desf.) MacKey (cv. Durati) genotypes differing in tolerance to Zn deficiency. Excess HEDTA at 50 μM reduced root and shoot growth and caused visual toxicity symptoms (necrotic lesions) on leaves; these effects were generally absent at lower concentrations of excess HEDTA. Root exudation of phytosiderophores increased with increasing concentrations of excess HEDTA at deficient and sufficient Zn levels, and was higher in Zn-deficiency-tolerant Aroona than in Zn-deficiency-sensitive Durati wheat. Shoot and root Zn concentrations showed a saturable response to increasing Zn²⁺ activities in solution. Excess HEDTA at 50 μM caused an increase in shoot concentrations of Fe and a decrease in concentrations of Mn and Cu. An average rate of Zn uptake increased with an increase in Zn²⁺ ionic activity in solution, with Zn-deficiency-tolerant Aroona having a higher rate of Zn uptake than Zn-deficiency-sensitive Durati in the deficiency range of Zn²⁺ activities. Average uptake rates of Mn and Cu decreased with an increase in concentration of excess HEDTA. Similar observations were noted for transport of Mn and Cu to shoots, while Zn transport to shoots was proportional to Zn²⁺ activities in solution. It was concluded that excess HEDTA at 50 μM adversely affects wheat growth and physiology, while excess of 25 μM or less does not cause measurable toxicity. This revised version was published online in June 2006 with corrections to the Cover Date.     

Expression of high zinc efficiency of Aegilops tauschii and Triticum monococcum in synthetic hexaploid wheats

The effect of varied zinc (Zn) supply on shoot and root dry matter production, severity of Zn deficiency symptoms and Zn tissue concentrations was studied in two Triticum turgidum (BBAA) genotypes and three synthetic hexaploid wheat genotypes by growing plants in a Zn-deficient calcareous soil under greenhouse conditions with (+Zn=5 mg kg^-1 soil) and without (−Zn) Zn supply. Two synthetic wheats (BBAADD) were derived from two different Aegilops tauschii (DD) accessions using same Triticum turgidum (BBAA), while one synthetic wheat (BBAAAA) was derived from Triticum turgidum (BBAA) and Triticum monococcum (AA). Visible symptoms of Zn deficiency, such as occurrence of necrotic patches on leaves and reduction in shoot elongation developed more rapidly and severely in tetraploid wheats than in synthetic hexaploid wheats. Correspondingly, decreases in shoot and root dry matter production due to Zn deficiency were higher in tetraploid wheats than in synthetic hexaploid wheats. Transfer of the DD genome from Aegilops tauschii or the AA genome from Triticum monococcum to tetraploid wheat greatly improved root and particularly shoot growth under Zn-deficient, but not under Zn-sufficient conditions. Better growth and lesser Zn deficiency symptoms in synthetic hexaploid wheats than in tetraploid wheats were not accompanied by increases in Zn concentration per unit dry weight, but related more to the total amount of Zn per shoot, especially in the case of synthetic wheats derived from Aegilops tauschii. This result indicates higher Zn uptake capacity of synthetic wheats. The results demonstrated that the genes for high Zn efficiency from Aegilops tauschii (DD) and Triticum monococcum (AA) are expressed in the synthetic hexaploid wheats. These wheat relatives can be used as valuable sources of genes for improvement of Zn efficiency in wheat. This revised version was published online in June 2006 with corrections to the Cover Date.     

Uptake of zinc by rye,bread wheat and durum wheat cultivars differing in zinc efficiency

Effect of zinc (Zn) nutritional status on uptake of inorganic ⁶⁵Zn was studied in rye (Secale cereale, cv. Aslim), three bread wheat (Triticum aestivum, cvs. Dagdas, Bezostaja, BDME-10) and durum wheat (Triticum durum, cv. Kunduru-1149) cultivars grown for 13 days in nutrient solution under controlled environmental conditions. The cultivars were selected based on their response to Zn deficiency and to Zn fertilization in calcareous soils under field conditions. When grown in Zn-deficient calcareous soil in the field, the rye cultivar had the highest, and the durum wheat the lowest Zn efficiency. Among the bread wheats, BDME-10 showed higher susceptibility to Zn deficiency and Bezostaja and Dagdas were less affected by Zn deficiency. Similarly to field conditions, in nutrient solution visual Zn deficiency symptoms (i.e. necrotic lesions on leaf blade) appeared to be more severe in Kunduru-1149 and BDME-10 and less severe in rye cultivar Aslim. Under Zn deficiency, shoot concentrations of Zn were similar between all cultivars. Cultivars with adequate Zn supply did not differ in uptake and root-to-shoot translocation rate of ⁶⁵Zn, but under Zn deficiency there were distinct differences; rye showed the highest rate of Zn uptake and the durum wheat the lowest. In the case of bread wheat cultivars, ⁶⁵Zn uptake rate was about the same and not related to their differential Zn efficiency. Under Zn deficiency, rye had the highest rate of root-to-shoot translocation of ⁶⁵Zn, while all bread and durum wheat cultivars were similar in their capacity to translocate ⁶⁵Zn from roots to shoots. When Zn²⁺ activity in uptake solution ranged between 117 p M and 34550 pM, Zn-efficient and Zn-inefficient bread wheat genotypes were again similar in uptake and root-to-shoot translocation rate of ⁶⁵Zn. The results indicate that high Zn efficiency of rye can be attributed to its greater Zn uptake capacity from soils. The inability of the durum wheat cultivar Kunduru-1149 to have a high Zn uptake capacity seems to be an important reason for its Zn inefficiency. Differential Zn efficiency between the bread wheat cultivars used in this study is not related to their capacity to take up inorganic Zn. This revised version was published online in June 2006 with corrections to the Cover Date.     

Light-mediated release of phytosiderophores in wheat and barley under iron or zinc deficiency

The effect of varied light intensity (50 – 600 mol m-2 s-1) on the rate of phytosiderophore release was studied under zinc (Zn) deficiency using a bread (Triticum aestivum L. cv. Aroona) and a durum wheat cultivar (Triticum durum Desf. cv. Durati) differing in zinc (Zn) efficiency and under iron (Fe) deficiency using a barley cultivar (Hordeum vulgare L. Europe). Plants were grown under controlled environmental conditions in nutrient solution for 15 days (wheat plants) or 11 days (barley plants). Phytosiderophore release was determined by measuring capacity of root exudates to mobilize copper (Cu) from a Cu-loaded resin.With increasing light intensity visual Zn deficiency symptoms such as whitish-brown lesions on leaf blade developed rapidly and severely in wheat, particularly in the durum cultivar Durati. In wheat plants supplied well with Zn, increases in light intensity from 100 to 600 mol m-2 s-1 did not clearly affect the rate of phytosiderophore release. However, under Zn deficiency increases in light intensity markedly enhanced release of phytosiderophores in Zn-deficient Aroona, but not in Zn-inefficient Durati. When Fe-deficient barley cultivar Europe was grown first at 220 mol m-2 s-1 and then exposed to 600 mol m-2 s-1 for 24 and 48 h, the rate of release of phytosiderophores was enhanced about 4-fold and 7-fold, respectively. Transfer of Fe-deficient plants from 600 to 50 mol m-2 s-1 for 48 h reduced the rate of release of phytosiderophores by a factor of 7. The effect of light on phytosiderophore release was similar regardless of whether the rate of phytosiderophore release was expressed per plant or per unit dry weight of roots.The results demonstrate a particular role of light intensity in phytosiderophore release from roots under both Zn and Fe deficiency. It is suggested that in the studies concerning the role of phytosiderophore release in expression of Zn or Fe efficiency among and within cereals, a special attention should be given to the light conditions.     

Genotypic variation in zinc efficiency and resistance to crown rot disease (Fusarium graminearum Schw. Group 1) in wheat

A crown rot disease in wheat caused by the fungusFusarium graminearum Schw. Group 1 is a widespread problem in chronically Zn-deficient Australian soils. A link between crown rot and Zn deficiency was established by Sparrow and Graham (1988). This paper reports a test of a further hypothesis, that wheat genotypes more efficient at extracting zinc from low-zinc soils are more resistant to infection by this pathogen. Three wheat cultivars (Excalibur, Songlen and Durati) of differential Zn efficiency were tested at three zinc levels (0.05, 0.5 and 2.0 mg Zn kg⁻¹ of soil) and three levels ofF. graminearum S. Group 1 inoculum (0.1 g and 0.3 g kg⁻¹ live chaff-inoculum and control having 0.1 g kg⁻¹ dead chaff inoculum). Six weeks after sowing dry matter production of shoots and roots was decreased byFusarium inoculation at 0.05 mg and 0.5 mg kg⁻¹ applied Zn.Fusarium inoculum at 0.1 g was as effective as 0.3 g kg⁻¹ for infection and decreasing dry matter. The infection at the basal part of culm decreased significantly by increasing the rate of Zn application. Excalibur, a Zn-efficient cultivar (tolerant to Zn deficiency) produced significantly more shoot and root dry matter, and showed less disease infection compared with Zn-inefficient cultivars (Durati and Songlen) at low (0.05 mg Zn kg⁻¹ soil) and medium (0.5 mg Zn kg⁻¹ soil) Zn fertilization rates. Higher rate of Zn fertilization (2.0 mg Zn kg⁻¹ soil) reduced the disease level in Durati to the level of Excalibur but the disease level of Songlen was still high, indicating its high Zn requirement and or sensitivity to crown rot. The data on Zn uptake show that Excalibur, being Zn-efficient, was able to scavenge enough Zn from Zn-deficient soil, we suggest that besides sustaining growth Excalibur was able to build and maintain resistance to the pathogen; inefficient cultivars needed extra Zn fertilization to achieve performance comparable to that of Excalibur. The present study indicates that growing Zn-efficient cultivars of wheat along with judicious use of Zn fertilizer in Zn-deficient areas where crown rot is a problem may sustain wheat production by reducing the severity of the disease as well as by increasing the plant vigour through improved Zn nutrition. ei]Section editor: R Rodriques-Kalana     

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 μM 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. Received: 25 October 1999 / Accepted: 3 December 1999     

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%). This revised version was published online in June 2006 with corrections to the Cover Date.     

Differences in Zinc Efficiency among and within Diploid, Tetraploid and Hexaploid Wheats

Greenhouse experiments were carried out with six diploid, ninetetraploid and seven hexaploid wheats, including wild and primitivegenotypes, to study the influence of varied zinc (Zn) supplyon the severity of Zn deficiency symptoms, shoot dry matterproduction and shoot Zn concentrations. In addition to wildand primitive genotypes, one modern tetraploid cultivar withhigh sensitivity to Zn deficiency and two modern hexaploid cultivars,one highly sensitive to and one resistant to Zn deficiency,were included for comparison. Plants were grown for 44 d ina severely Zn-deficient calcareous soil, with (+Zn; 5 mg Znkg^-1soil) and without (-Zn) Zn fertilization. Visible Zn deficiencysymptoms, including whitish-brown necrotic patches on leaf blades,appeared very rapidly and severely in all tetraploid wheat genotypes.Compared with tetraploid wheats, diploid and hexaploid wheatswere less sensitive to Zn deficiency. With additional Zn, shootdry matter production was higher in tetraploid than diploidand hexaploid wheats. However, under Zn-deficient conditionstetraploid wheats had the lowest shoot dry matter production,indicating the very high sensitivity of tetraploid wheats toZn deficiency. Consequently, Zn efficiency expressed as theratio of shoot dry matter produced under Zn deficiency to Znfertilization, was much lower in tetraploid wheats than in diploidand hexaploid wheats. On average, Zn efficiency ratios were36% for tetraploid, 60% for diploid and 64% for hexaploid wheats.Differences in Zn efficiency among and within diploid, tetraploidand hexaploid wheats were positively related to the amount ofZn per shoot of the genotypes, but not to the amount of Zn perunit dry weight of shoots or seeds used in the experiments.The seeds of the accessions of tetraploid wild wheats containedup to 120 mg Zn kg^-1, but the resulting plants showed very highsensitivity to Zn deficiency. By contrast, hexaploid wheatsand primitive diploid wheats with much lower Zn concentrationsin seeds had higher Zn efficiencies. It is suggested that notonly enhanced Zn uptake capacity but also enhanced internalZn utilization capacity of genotypes play important roles indifferential expression of Zn efficiency. The results of thisstudy also suggest the importance of the A and D genomes asthe possible source of genes determining Zn efficiency in wheat.Copyright 1999 Annals of Botany Company Seeds, Triticum aestivum, Triticum monococcum, Triticum turgidum, zinc concentrations, zinc deficiency, zinc efficiency.     

Contribution of Different Mechanisms to Zinc Efficiency in Bread Wheat During Early Vegetative Stage

Zinc (Zn) has a vast number of functions in plant metabolism and consequently Zn deficiency has a range of effects on plant growth. There are a number of different possible mechanisms by which plants tolerate Zn deficiency (generally expressed as Zn efficiency), such as Zn uptake, translocation to the shoot and physiological efficiency. However, there have been no direct comparisons of the relative importance of these possible mechanisms of Zn efficiency in a large set of genotypes of contrasting Zn efficiency. Soil and solution culture studies were conducted to examine the relative contribution of different mechanisms of Zn efficiency at the whole plant level in bread and durum wheat during early vegetative stage. Zn treatments were 0, 0.05, 0.1 and 1 mg/kg soil in the soil culture, and nil in the solution culture. Visual symptoms of Zn deficiency, dry matter production, Zn uptake, Zn distribution between roots and shoots, Zn utilization in roots and shoots and Zn remobilisation from the seed into growing parts were examined. Significant genotypic differences were observed in most criteria and responses differed with external Zn supply. The results of the present study suggest that while there are a number of different mechanisms contributing to Zn efficiency, uptake is the major mechanism and the effect of this is modified by the physiological efficiency within the shoot. Root:shoot partitioning was not strongly associated with Zn efficiency and seed Zn remobilisation was not linked to Zn efficiency. Visual symptoms of the severity of Zn deficiency was a good predictor of Zn efficiency and was correlated with Zn uptake.     

Domesticated emmer wheat [T. turgidum L. subsp. dicoccon (Schrank) Thell.] as a source for improvement of zinc efficiency in durum wheat

Modern durum wheat (AABB) is more sensitive to zinc (Zn) deficiency than bread wheat (AABBDD). One strategy to increase productivity and expansion of durum wheat industry in Zn-deficient soils is to improve its ability to grow and yield in such soils. This ability is termed Zn efficiency. In a growth room experiment using soil culture, we assessed the potential of Triticum turgidum L. subsp. dicoccon (Shrank) Thell. (domesticated emmer wheat, AABB) as a genetic resource for further improvement of Zn efficiency in modern durum wheat. Twenty four accessions of domesticated emmer wheat, four durum landraces/cultivars, and two bread wheat cultivars/ advanced breeders lines of known Zn efficiency were tested under Zn deficiency and Zn sufficiency. Significant variation was observed among genotypes in Zn deficiency symptoms, dry matter production, shoot Zn concentration, shoot Zn content and Zn utilisation efficiency (physiological efficiency). We identified domesticated emmer wheat accessions with greater Zn efficiency than modern durum wheat and even bread wheat genotypes. These accessions could be used in breeding programs to improve Zn efficiency of durum wheat. The results suggest that Zn efficiency of durum or bread wheat is likely to be determined collectively by its progenitors.     

Role of rye chromosomes in improvement of zinc efficiency in wheat and triticale

Using the disomic wheat-rye addition lines (Triticum aestivum L., cv. Holdfast-Secale cereale L., cv. King-II) and an octoploid triticale line (xTriticosecale Wittmark L. "PlutoxFakon") as well as the respective wheat and rye parents, greenhouse experiments were carried out to study the role of rye chromosomes on the severity of Zn deficiency symptoms, shoot dry matter production, Zn efficiency, shoot Zn concentration and Zn content. Plants were grown in a Zn-deficient calcareous soil with (10 mg Zn kg-1 soil) and without Zn supply. Zinc efficiency was calculated as the ratio of dry weight produced under Zn deficiency to the dry weight produced under Zn fertilization. In the experiments with addition lines, visual Zn deficiency symptoms were slight in the rye cultivar King-II, but were severe in the wheat cultivar Holdfast. The addition of rye chromosomes, particularly 1R, 2R and 7R, into Holdfast reduced the severity of deficiency symptoms. Holdfast showed higher decreases in shoot dry matter production by Zn deficiency and thus had a low Zn efficiency (53 %), while King-II was less affected by Zn deficiency and had a higher Zn efficiency (89 %). With the exception of the 3R line, all addition lines had higher Zn efficiency than their wheat parent: the 1R line had the highest Zn efficiency (80 %). In the experiment with the triticale cultivar and its parents, rye cv. Pluto and wheat cv. Fakon, Zn deficiency symptoms were absent in Pluto, slight in triticale and very severe in Fakon. Zinc efficiency was 88 % for Pluto, 73 % for triticale and 64% for Fakon. Such differences in Zn efficiency were better related to the total amount of Zn per shoot than to the amount of Zn per unit dry weight of shoot. Only in the rye cultivars, Zn efficiency was closely related with Zn concentration. Triticale was more similar to rye than wheat regarding Zn concentration and Zn accumulation per shoot under both Zn-deficient and Zn-sufficient conditions.The results presented in this study show that rye has an exceptionally high Zn efficiency, and the rye chromosomes, particularly 1R and 7R carry the genes controlling Zn efficiency. To our knowledge, the result with triticale and its rye parents is the first report showing that the genes controlling Zn efficiency in rye are transferable into wheat and can be used for development of new wheat varieties with high Zn efficiency for severely Zn-deficient conditions.     

Is the release of phytosiderophores in zinc-deficient wheat plants a response to impaired iron utilization?

The effect of zinc nutritional status on the time course of phytosiderophore release, and uptake of iron and translocation of iron to the shoot, was studied in nutrient solution cultures for two cultivars of wheat ( Triticum aestivum . cv. Aroona: T. durum , cv. Duratit) differing in their susceptibility to zinc deficiency. In the zinc-efficient cultivar Aroona, under zinc deficiency translocation of iron from roots to shoot was significantly decreased in 13- and 15-day-old plants, whereas release of phytosiderophores was enhanced when the plants were 16 days old. As zinc deficiency became more severe in older plains, translocation of iron to the shoot was further decreased and release of phytosiderophores was further enhanced. Resupplying zinc in nutrient solution to zinc-deficient plants significantly increased the translocation of iron to the shoot after 48 and 72 h. Concomitantly the release of phytosiderophores was repressed. The other cultivar Durati classified as zinc-inefficient in field observations differed from cv. Aroona by showing a lower rate of phytosiderophore release under Zinc deficiency, and a less impaired translocation of iron to the shoot. Foliar application of iron citrate to zinc-deficient Aroona plants repressed the release of phytosiderophores and increased iron concentrations in shoot and roots. Application of ⁵⁵Fe to the leaves demonstrated that retranslocation of iron from the shoot to the roots was not affected by the zinc nutritional status. It is concluded that enhanced release of phytosiderophores in zinc-deficient wheat plants was induced primarily by impaired trans-location of iron lo the shoot.     

Uptake and transport of foliar applied zinc (65Zn) in bread and durum wheat cultivars differing in zinc efficiency

Using two bread wheat (Triticum aestivum) and two durum wheat (Triticum durum) cultivars differing in zinc (Zn) efficiency, uptake and translocation of foliar-applied ⁶⁵Zn were studied to characterize the role of Zn nutritional status of plants on the extent of phloem mobility of Zn and to determine the relationship between phloem mobility of Zn and Zn efficiency of the used wheat cultivars. Irrespective of leaf age and Zn nutritional status of plants, all cultivars showed similar Zn uptake rates with application of ⁶⁵ZnSO₄ to leaf strips in a short-term experiment. Also with supply of ⁶⁵ZnSO₄ by immersing the tip (3 cm) of the oldest leaf of intact plants, no differences in Zn uptake were observed among and within both wheat species. Further, Zn nutritional status did not affect total uptake of foliar applied Zn. However, Zn-deficient plants translocated more ⁶⁵Zn from the treated leaf to the roots and remainder parts of shoots. In Zn-deficient plants about 40% of the total absorbed ⁶⁵Zn was translocated from the treated leaf to the roots and remainder parts of shoots within 8 days while in Zn-sufficient plants the proportion of the translocated ⁶⁵Zn of the total absorbed ⁶⁵Zn was about 25%. Although differences in Zn efficiency existed between the cultivars did not affect the translocation and distribution of ⁶⁵Zn between roots and shoots. Bread wheats compared to durum wheats, tended to accumulate more ⁶⁵Zn in shoots and less ⁶⁵Zn in roots, particularly under Zn-deficient conditions. The results indicate that differences in expression of Zn efficiency between and within durum and bread wheats are not related to translocation or distribution of foliar-applied ⁶⁵Zn within plants. Differential compartementation of Zn at the cellular levels is discussed as a possible factor determining genotypic variation in Zn efficiency within wheat.     

Variation in phosphorus efficiency among 73 bread and durum wheat genotypes grown in a phosphorus-deficient calcareous soil

A greenhouse experiment was carried out to study the severity of phosphorus (P) deficiency symptoms on leaves, shoot dry matter production, and shoot concentration and content (the total amount per shoot) of P in 39 bread wheat (Triticum aestivum L.) and 34 durum wheat (Triticum durum L.) genotypes grown in a severely P-deficient calcareous soil with low (20mgPkg⁻¹ soil) and adequate (80mgPkg⁻¹ soil) P supply for 39 days. As the seed P concentration or content can affect plant performance under P-deficient conditions, the seeds of the genotypes used in the present study were also analyzed for P concentration. Phosphorus efficiency (relative shoot growth) of genotypes, calculated by the ratio of shoot dry matter production under low P to that under adequate P supply, significantly differed among the genotypes, and varied between 46.7% and 78.6%. Phosphorus efficiency ranged from 51% to 71% with an average of 61% for bread and from 47% to 79% with an average of 66% for durum wheat genotypes. There was no correlation between P efficiency ratio and P concentration of plants (R ²=0.0001), but P efficiency of all bread and durum wheat genotypes showed a very significant correlation with the P content (the total amount of P per shoot) (R ²=0.333^***). The relationship between the P efficiency and total amount of P per shoot was much more significant in bread (R ²=0.341^***) than in durum wheat (R ²=0.135^*). Like shoot P concentrations, also severity of visible leaf symptoms of P deficiency on older leaves, including leaf chlorosis and necrosis, did not correlate with P efficiency. In most cases, genotypes showing higher P efficiency had higher absolute shoot dry weight under P deficient conditions. Under P deficient conditions, the absolute shoot dry weight very significantly correlated with shoot P content (R ²=0.665^***), but the correlation between the absolute shoot dry weight and shoot P concentration tended to be negative. There was also variation in native seed P reserve of the genotypes, but this variation had no influence on the P efficiency. The results indicate that the total amount of P per shoot and shoot dry matter production at low P supply are most reliable parameters in ranking genotypes for P efficiency at early growth stage. In wheat germplasm tested in the present study, several wheat genotypes are available showing both very high P efficiency and very high shoot content and concentration of P suggesting that P acquisition ability should be most important mechanism for high P efficiency in such genotypes. On the other hand, there are also genotypes in the germplasm having more or less same P concentration or P content in shoot but differing substantially in P efficiency, indicating importance of P utilization at cellular level in P efficiency. All these results suggest that P efficiency mechanisms can be different from one genotype to other within a given plant species.     

Sulfhydryl groups in root-cell plasma membranes of wheat genotypes differing in Zn efficiency

Sulfhydryl groups were quantified in root-cell plasma membranes of two genotypes of wheat ( Triticum aestivum cv. Warigal and T. turgidum conv. durum cv. Durati) differing in Zn efficiency. Smaller amounts of 5,5'-dithio-bis(2-nitrobenzoic acid)-reactive sulfhydryl groups were found in Zn-deficient than in Zn-sufficient roots; and also in Zn-inefficient genotype Durati compared to Zn-efficient Warigal, regardless of Zn supply. Upon transfer of 15-day-old Zn-deficient plants into solutions containing various Zn²⁺ activities, a Zn-dependent increase in the amount of reactive sulfhydryl groups was evident in roots of both genotypes, but occurred only in Warigal when 20-day-old plants were used, indicating irreversible physiological damage in Durati plants due to prolonged Zn deficiency. Upon transfer into solutions of increasing Zn²⁺ activities, the increase in total Zn concentration in roots was about an order of magnitude smaller than the increase in amounts of reactive sulfhydryl groups in the roots of both genotypes, suggesting that, in wheat roots, a relatively small amount of Zn is required for preventing oxidation of sulfhydryl groups into disulfides. The amount of reactive sulfhydryl groups in the roots is positively related to Zn efficiency of wheat genotypes and may be one of the mechanisms that, under conditions of Zn deficiency, allow better growth and productivity of Zn-efficient genotypes in comparison to Zninefficient ones.     

In vitro regeneration of cereals based on multiple shoot induction from mature embryos in response to thidiazuron

The in vitro competency of mature cereal embryos (winter, spring and durum wheats, oat, barley and triticale) was assessed for direct multiple shoot production on culture media containing the plant growth regulators, thidiazuron (TDZ) and/or 6–benzylaminopurine (BAP). Mature embryos of CDC Dancer oat showed the best response, with 69 shoots per explant on culture medium containing a combination of 4.5 μM TDZ and 4.4 μM BAP. TDZ alone induced about 16 shoots per explant from the oat. Among the wheat genotypes, durum wheat showed the most number of shoots (35) per explant on culture medium containing 4.5 μM of TDZ and 4.4 μM of BAP. With TDZ alone, shoot regeneration for durum wheat ranged from 27–32 shoots per explant. The regeneration frequency from the three winter wheat genotypes ranged from 11–25 shoots per explant and was highest on culture medium containing 9.1 μM TDZ and 4.4 μM BAP. The latter culture medium was also effective for a triticale genotype, inducing 34 shoots per explant. The regeneration from mature embryos of barley genotypes ranged from 5–9 shoots per explant. The mature embryos of all the cereals tested could be used for in vitro regeneration with TDZ and TDZ+BAP combinations.