Contributions of root uptake and remobilization to grain zinc accumulation in wheat depending on post-anthesis zinc availability and nitrogen nutrition

Background and aims

Whether root Zn uptake during grain filling or remobilization from pre-anthesis Zn stores contributes more to grain Zn in wheat is subject to an on-going debate. This study investigated the effects of N nutrition and post-anthesis Zn availability on the relative importance of these sources.

Methods

Durum wheat plants were grown in nutrient solution containing adequate Zn (0.5?μM) and three different N levels (0.5; 1.5; 4.5?mM). One third of the plants were harvested when they reached anthesis. One half of the remaining plants were grown to maturity with adequate Zn, whereas the Zn supply to the other half was discontinued at anthesis. Roots, straw and grains were harvested separately and analyzed for Zn and N.

Results

Depending on the N supply, Zn remobilization from pre-anthesis sources provided almost all of grain Zn when the Zn supply was withheld at anthesis; otherwise up to 100?% of grain Zn could be accounted for by Zn taken up post-anthesis. By promoting tillering and grain yield and extending the grain filling, higher N supply favored the contribution of Zn uptake to grain Zn accumulation.

Conclusion

Remobilization is critical for grain Zn accumulation when Zn availability is restricted during grain filling. However, where root uptake can continue, concurrent Zn uptake during grain development, favored by higher N supply, overshadows net remobilization.     

Effect of nitrogen on uptake, remobilization and partitioning of zinc and iron throughout the development of durum wheat

Deficiencies of zinc (Zn) and iron (Fe) are global nutritional problems and caused most often by their limited dietary intake. Increasing Zn and Fe concentrations of staple food crops such as wheat is therefore an important global challenge. This study investigated the effects of varied nitrogen (N) and Zn supply on the total uptake, remobilization and partitioning of Zn, Fe and N in durum wheat throughout its ontogenesis. Plants were grown under greenhouse conditions with high or low supply of N and Zn, and harvested at 8 different developmental stages for analysis of Zn, Fe and N in leaves, stems, husks and grains. The results obtained showed that the Zn and Fe uptake per plant was enhanced up to 4-fold by high N supply while the increases in plant growth by high N supply were much less. When both the Zn and N supplies were high, approximately 50% of grain Zn and 80% of grain Fe were provided by post-anthesis shoot uptake, indicating that the contribution of remobilization to grain accumulation was higher for Zn than for Fe. At the high N and Zn application, about 60% of Zn, but only 40% of Fe initially stored in vegetative parts were retranslocated to grains, and nearly 80% of total shoot Zn and 60% of total shoot Fe were harvested with grains. All these values were significantly lower at the low N treatment. Results indicate that N nutrition is a critical factor in both the acquisition and grain allocation of Zn and Fe in wheat.     

Overexpression of AtHMA4 enhances root-to-shoot translocation of zinc and cadmium and plant metal tolerance

AtHMA4 is an Arabidopsis thaliana P1B-ATPase which transports Zn and Cd. Here, we demonstrate that AtHMA4 is localized at the plasma membrane and expressed in tissues surrounding the root vascular vessels. The ectopic overexpression of AtHMA4 improved the root growth in the presence of toxic concentrations of Zn, Cd and Co. A null mutant exhibited a lower translocation of Zn and Cd from the roots to shoot. In contrast, the AtHMA4 overexpressing lines displayed an increase in the zinc and cadmium shoot content. Altogether, these results strongly indicate that AtHMA4 plays a role in metal loading in the xylem.     

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.     

Effect of source-sink relations and nitrogen nutrition on senescence and N remobilization in the flag leaf of wheat

The relation between the source-sink ratio and nitrogen nutrition on grain yield of wheat ( Triticum aestivum L. cv. Klein Chamaco) was studied in a greenhouse experiment. Plants were grown until anthesis in pots with soil fertilized with 0.16 mmol N per plant twice a week. At anthesis, all leaves but the flag leaf were excised in a group of plants. In another group the treatment consisted in a similar defoliation plus the longitudinal excision of half the ear, while a third group was left untouched as a control. At the same time, the N supply to half of the plants in each group was interrupted, while the other half continued receiving 16 m M N. The defoliated plants showed a longer functional life of the flag leaf than the control, retaining the chlorophyll, soluble proteins and total reduced nitrogen for a longer time. The ear-excised plants showed an intermediate behavior. The plants with the interrupted N supply showed a faster leaf senescence than the N supplied ones, and this coincided with an increase in the proteolytic activity and nitrogen transport to the ear. However there were no differences in ear weight between the two nitrogen treatments. It is concluded that leaves and ear compete for the nitrogen, and that a low level of carbohydrates in the flag leaf, due to a low source-sink ratio, delays leaf senescence.     

Restricted root-to-shoot translocation and decreased sink size are responsible for limited nitrogen uptake in three grass species under water deficit

Earlier studies showed that water deficit reduces nitrogen (N) uptake and N nutrition index of grasses. So far, the main effect of water deficit on N uptake and N nutrition status was ascribed to the alteration of the transpiration-dependent transport of mineral N in the soil solution. A split-root experiment was performed to determine whether plant and/or solution water potential could alter N uptake and allocation, independently of N fluxes in the soil solution. The split-root experiment allowed to manipulate separately the water and the N status of the root environment and of the plant, by various combinations of addition of polyethylene glycol 6000 (PEG) and mineral N on half or on the entire root system. Tall fescue, Italian ryegrass and cocksfoot, known for their contrasted sensitivities to water deficit, were studied. The addition of PEG largely reduced water and N uptake of the roots on which it was applied (half or entire root system). A significant accumulation of N was observed on the roots to which PEG was added, particularly in cocksfoot and tall fescue, hypothetically contributing to the alteration of N uptake. Cocksfoot displayed a high plasticity in N allocation to leaves related to sheaths, allowing the maintenance of N concentration and therefore its N status despite the reduction in N uptake. By contrast, leaf N concentration and N status of tall fescue and Italian ryegrass was more sensitive to water deficit, similarly to observations of the effect of drought in field studies. Therefore, it is concluded that the effect of soil solution potential on N uptake and plant N allocation may also contribute to the observed effect of drought on N status of grass crops, additionally to the effect of drought on the transfer of mineral N in the soil solution to the root surface. However, the importance of this physiological effect varies among species.     

Kinetin application to roots and its effect on uptake, translocation and distribution of nitrogen in wheat (Triticum aestivum) grown with a split root system

The root systems of wheat seedlings ( Triticum aestivum L. cv. SUN 9E) were pruned to two seminal roots. One of the roots was supplied with a suboptimal level of NO₃, the other was deprived of N. Different levels of kinetin were supplied to the NO₃-deprived roots. Root respiration and the increment of C and N in the roots were measured to determine the C/N ratio of the phloem sap feeding the NO₃-deprived roots. Thus, it was possible to determine retranslocation of N from the shoots to the roots, as affected by the rate of kinetin application. It was calculated that the C/N ratio of phloem sap feeding roots growing without kinetin was ca 61. Kinetin application increased this ratio to ca 75, partly due to decreased translocation of N from the shoots back to the roots. Kinetin application decreased the proportion of N that was retranslocated to the roots after translocation to the shoots. Kinetin increased the rate of NO₃ uptake per root and the rate of N incorporation in both roots and shoots by ca 60%, but had no effect on shoot dry matter production. In control plants at most 70% of the N incorporated in the NO₃-fed roots could have been imported from the shoots, whilst kinetin application reduced this value to ca 40%. Thus root growth was not fully dependent on a supply of N via the phloem.
It is concluded that cytokinins affect the pattern of N-translocation in wheat plants by increasing incorporation of N in dry matter of the shoot, thus leaving less for export. Cytokinins did not play a major role in the regulation of shoot growth and the shoot to root ratio of the present plants.     

Distribution and remobilization of Zn and Mn during grain development in wheat

Wheat (Triticum aestivum cv. Aroona) was grown in siliceoussand with essential nutrients for unlimited growth except forthe following treatments: controls (sufficient Zn and Mn), lowMn (sufficient Zn) and low Zn (sufficient Mn) until anthesis.Replicate plants were harvested at anthesis; the remaining plantswere transferred to a chelate-buffered nutrient solution containingall essential nutrients except Zn and Mn to allow monitoringof the remobilization of existing Zn and Mn reserves withinthe plant. These plants were harvested 14 d post-anthesis andat grain maturity. At each harvest plants were separated intoindividual components. There were no growth differences between any of the treatmentsat the three harvests. Large amounts of Zn and Mn found in theroots and stems at anthesis were rapidly depleted during graindevelopment. The Zn content of the leaves increased from anthesisto 14 d post-anthesis, but then declined. The Mn content ofthe leaves increased throughout grain development in the controlswhilst remaining constant in the plants pre-grown at low Mn.The Zn and Mn content of the glumes, palea and lemma rose inthe controls from anthesis to 14 d post-anthesis; thereafterZn content declined but Mn content continued to increase. TheZn and Mn content of the grain rose sharply toward grain maturity.We conclude that Mn was not remobil-ized from the leaves ofwheat during grain development. Zinc was remobilized from theleaves, especially the flag leaf and from the leaves of thelow Zn plants. The post-anthesis accumulation of Zn and Mn withinthe glumes will be discussed in relation to the transport pathwaythat Zn and Mn use to enter the developing seed. Key words: Zinc, manganese, wheat, distribution, remobilization     

Effects of nitrogen nutrition and root medium water potential on growth, nitrogen uptake and osmotic adjustment of rice

The effects of nitrogen (N) nutrition on growth, N uptake and leaf osmotic potential of rice plants (Oryza sativa L. ev. IR 36) during simulated water stress were determined. Twenty-one-day-old seedlings in high (28.6 × 10 ^?4M) and low (7.14 × 10 ⁴M) N levels were exposed to decreased nutrient solution water potentials by addition of polyethylene glycol 6000. The roots were separated from the solution by a semi-permeable membrane. Nutrient solution water potential was ?0.6 × 10⁵ Pa and was lowered stepwise to ?1 × 10⁵, ?2 × 10⁵, ?4 × 10⁵ and ?6 × 10⁵ Pa at 2-day intervals. Plant height, leaf area and shoot dry weight of high and low nitrogen plants were reduced by lower osmotic potentials of the root medium. Osmotic stress caused greater shoot growth reduction in high N than in low N plants. Stressed and unstressed plants in 7.14 × 10⁴M N had more root dry matter than the corresponding plants in 28.6 × 10⁴M N. Dawn leaf water potential of stressed plants was 1 × 10⁵ to 5.5 × 10⁵ Pa lower than nutrient solution water potential. Nitrogen-deficient water-stressed plants, however, maintained higher dawn leaf water potential than high nitrogen water-stressed plants. It is suggested that this was due to higher root-to-shoot ratios of N deficient plants. The osmotic potentials of leaves at full turgor for control plants were about 1.3 × 10⁵ Pa higher in 7.14 × 10^?4M than in 28.6 × 10^?4M N and osmotic adjustment of 2.6 × 10⁵ and 4.3 × 10⁵ Pa was obtained in low and high N plants, respectively. The nitrogen status of plants, therefore, affected the ability of the rice plant to adjust osmotically during water stress. Plant water stress decreased transpiration and total N content in shoots of both N treatments. Reduced shoot growth as a result of water stress caused the decrease in amount of water transpired. Transpiration and N uptake were significantly correlated. Our results show that nitrogen content is reduced in water-stressed plants by the integrated effects of plant water stress per se on accumulation of dry matter and transpiring leaf area as well as the often cited changes in soil physical properties of a drying root medium.     

The effect of rhizoctonia root disease and applied nitrogen on growth,nitrogen uptake and nutrient concentrations in spring wheat

Root disease caused by Rhizoctonia solani is a common problem of spring wheat in South Australia. There are reports that nitrogen applications can reduce the incidence and severity of the disease. A glasshouse trail in pots examined the effects of disease and of applied nitrogen on wheat growth, and evaluated the utility of the basal stem nitrate concentration in diagnosing deficiency in plants with and without root disease. Plants were harvested at the mid-tillering stage. Shoot growth was increased by applied nitrogen until a maximum yield was attained, after which additional N had no effect on shoot yield. Root growth, however, responded positively only to low levels of applied N, after which it declined, and in the highest N treatment root mass was less than in the plants without applied N. Root disease caused severe reductions in plant growth, and both root and shoot mass were affected similarly. Even though growth of diseased plants responded positively to applied nitrogen the response was less than that of disease-free plants. The critical concentration of basal stem nitrate-N did not appear to be affected by root disease, and was estimated at 1200 mg kg^-1, consistent with other glasshouse data. The basal stem nitrate-N concentration, either in fresh or dried tissue, appeared a better diagnostic tool of N stress than did total shoot N concentration or content, because of sharper definition of critical concentrations. Concentrations of other nutrients in shoot tissue were affected differentially by both applied nitrogen and root disease, but generally did not reach critical levels, although phosphorus and magnesium appeared deficient in very disease-stressed plants.     

Iron uptake and translocation by tomato plants as influenced by root temperature and manganese nutrition

The uptake of iron (Fe) by VF-36 tomato plants as influenced by root temperature and manganese (Mn) nutrition was studied. Following a 1-week pretreatment period of various levels of Fe and Mn in the nutrient solution in a controlled temperature room, the uptake of ⁵⁹Fe from ferric ethylenediamine di(O-hydroxyphenylacetate) (FeEDDHA) at 1 μmole per liter was studied for periods of 1 to 5 days.     

Effects of nutrient solution zinc activity on net uptake,translocation, and root export of cadmium and zinc by separated sections of intact durum wheat (Triticum turgidum L. var durum) seedling roots

Cd accumulation in durum wheat presents a potential health risk to consumers. In an effort to understand the physiological mechanisms involved with Cd accumulation, this study examined the effects of Zn on Cd root uptake and phloem translocation in a split– root system. Durum wheat seedlings were grown in chelate-buffered nutrient solution with intact root systems divided into two sections. Each root section grew in a separate 1 l pot, one of which contained 0.2 μM CdSO₄. In addition, each two-pot system contained ZnSO₄ in the following combinations (in μm) (for -cd root system: +cd root system): 1:1, 1:10, 10:1,10:10, 1:19, and 19:1. Harvested plant material was analyzed for Cd and Zn. In addition, rates of Cd and Zn net uptake, translocation to the shoot, and root export (translocation from one root segment to the other) between days 8 and 22 were calculated. Results show that Zn was not translocated from one root section to its connected root section. Uptake rates of Cd increased as solution Zn concentrations increased. Cd translocation from one root section to the other decreased significantly when the Zn concentration in either pot was greater than 1 μM. These results show the potential of Zn to inhibit movement of Cd via the phloem, and suggests that providing adequate Zn levels may limit phloem loading of Cd into wheat grain. Increasing the rhizosphere activity of Zn²⁺ in Cd-containing soils may therefore result in reduced Cd accumulation in grain even while net Cd uptake is slightly enhanced. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cadmium distribution in the root tissues of solanaceous plants with contrasting root-to-shoot Cd translocation efficiencies

Root-to-shoot cadmium (Cd) translocation in Solanum torvum is lower than that of the eggplant Solanum melongena; therefore, grafting S. melongena onto S. torvum rootstock can effectively reduce the Cd concentration in eggplant fruits. We hypothesized that Cd transport in S. torvum roots is restricted in the path between the epidermis and xylem vessel; hence, we investigated the Cd distribution in the roots at the micron-scale. Elemental maps of Cd, Zn and Fe accumulation in S. melongena and S. torvum root sections were obtained by synchrotron micro X-ray fluorescence spectrometry. The Cd was localized in both the stele and the epidermis of the S. melongena root cross sections regardless of the distance from the root apex. In S. torvum root sections taken at 30 and 40 mm above the root apex, a higher abundance of Cd was found within the cells of the endodermis and pericycle. The results suggested that the symplastic uptake and xylem loading of Cd in S. torvum roots were restricted, and thereby, the Cd that was unable to be loaded into the xylem accumulated in the endodermis and in the pericycle. Because symplastic uptake differs only slightly between the two species, the difference in xylem loading would explain the comparatively lower Cd concentration in S. torvum shoots.     

Late foliar diseases in wheat crops decrease nitrogen yield through N uptake rather than through variations in N remobilization

Background and Aims

French wheat grains may be of little value on world markets because they have low and highly variable grain protein concentrations (GPC). This nitrogen-yield to yield ratio depends on crop nitrogen (N) fertilization as well as on crop capacity to use N, which is known to vary with climate and disease severity. Here an examination is made of the respective roles that N remobilization and post-anthesis N uptake play in N yield variations; in particular, when wheat crops (Triticum aestivum) are affected by leaf rust (Puccinia triticina) and Septoria tritici blotch (teleomorph Mycosphaerella graminicola).

Methods

Data from a 4-year field experiment was used to analyse N yield variations in wheat crops grown either with a third or no late N fertilization. Natural aerial epidemics ensured a range of disease severity, and fungicide ensured disease-free control plots. The data set of Gooding et al. (2005, Journal of Agricultural Science 143: 503–518) was incorporated in order to enlarge the range of conditions.

Key Results

Post-anthesis N uptake accounted for a third of N yield whilst N remobilization accounted for two-thirds in all crops whether affected by diseases or not. However, variations in N yield were highly correlated with post-anthesis N uptake, more than with N remobilization, in diseased and also healthy crops. Furthermore, N remobilization did not significantly correlate with N yield in healthy crops. These findings matched data from studies using various wheat genotypes under various management and climatic conditions. Leaf area duration (LAD) accurately predicted N remobilization whether or not crops were diseased; in diseased crops, LAD also accurately predicted N uptake.

Conclusions

Under the experimental conditions, N yield variations were closely associated with post-anthesis N uptake in diseased but also in healthy crops. Understanding the respective roles of N uptake and N remobilization in the case of diseased and healthy crops holds the promise of better modelling of variations in N yield, and thus in GPC.Key words: Triticum aestivum, Puccinia triticina, leaf rust, Mycosphaerella graminicola, Septoria tritici blotch, N uptake, N remobilization, N yield, Leaf area duration     

Zinc fluxes into developing barley grains: use of stable Zn isotopes to separate root uptake from remobilization in plants with contrasting Zn status

Background and Aims

Zn imported into developing cereal grains originates from either de novo Zn uptake by the roots or remobilization of Zn from vegetative tissues. The present study was focused on revealing the quantitative importance of the two pathways for grain Zn loading and how their relative contribution varies with the overall plant Zn status.

Methods

The stable isotope ⁶⁷Zn was used to trace Zn uptake and remobilization fluxes in barley (Hordeum vulgare L.) plants growing in hydroponics at 0.1?μM (low Zn), 1.5?μM (medium Zn) or 5?μM Zn (high Zn). When grain development reached 15?days after pollination the Zn source was changed to an enriched ⁶⁷Zn isotope and plants were harvested after 6 to 48?h. Zn concentrations and isotope ratios were determined using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS).

Results

Plants with low Zn status absorbed 3-fold more Zn than plants with medium or high Zn status when roots were exposed to an external concentration of 1.5?μM ⁶⁷Zn. Stems and ears were the primary recipients of the de novo incorporated Zn with preferential allocation to the developing grains over time. The leaves received in all cases a very small proportion (<5?%) of the newly absorbed Zn and the proportion did not increase over time. Zn fluxes derived from uptake and remobilization were almost equal in plants with low Zn status, while at high Zn status remobilization delivered 4 times more Zn to the developing grains than did root Zn uptake.

Conclusions

Stable isotopes in combination with ICP-MS provided a strong tool for quantification of Zn fluxes in intact plants. The importance of Zn remobilization compared to de novo root absorption of Zn increased with increasing plant Zn status. Very little de novo absorbed Zn was translocated to the leaves during generative growth stages.