Redistribution of cobalt and nickel in detached wheat shoots: effects of steam-girdling and of cobalt and nickel supply

Detached wheat shoots (ear with peduncle and flag leaf) were incubated for 4 d in a solution containing 1 mM RbCl and 1 mM SrCl2 as well as 10, 40 or 160 µM NiCl2 and CoCl2. The phloem of some plants was interrupted by steam-girdling the stem below the ear to distinguish between xylem and phloem transport. The phloem-immobile Sr flowed mainly to the leaf lamina and to the glumes via the xylem. The Sr transport was not sensitive to steam-girdling. In contrast, the phloem-mobile Rb accumulated during the incubation time mainly in the stem and the leaf sheath. The Rb transport to the grains was impaired by steam-girdling as well as by elevated Ni and Co concentrations in the incubation solution indicating that Rb was transported via the phloem to the maturing grains and that this transport was affected by the heavy metals. Ni was removed more efficiently from the xylem in the peduncle than Co (but far less efficiently than Rb). It became evident that the two heavy metals can also be transferred from the xylem to the phloem in the stem of wheat and reach the maturing grains via the phloem.

Long-distance transport of cobalt and nickel in maturing wheat

Cobalt, nickel and strontium were introduced via flaps into leaf laminas or into the stem of maturing, intact winter wheat (Triticum aestivum L., cv. `Arina') grown under natural conditions in a field. Long-distance transport of these elements and the influence of the application date and of different application positions were investigated. The dry-matter accumulation in the grains was not markedly affected by the treatments as compared to untreated control plants. The phloem-immobile strontium served as a marker for the distribution of the xylem sap in the plants. After foliar application, nickel accumulated more rapidly and in higher quantities in the grains than cobalt. Therefore, nickel has a slightly better phloem mobility than cobalt. Regardless of the application date, a higher percentage of the two elements was transported from the flag leaf lamina than from the second or third lamina from the top to the grains. These results indicate that the leaf position is highly relevant for the transfer of the heavy metals investigated to the ear. Introduction into the stem led to a higher accumulation of nickel and cobalt in the grains than introduction into one of the leaves. An earlier feeding date caused a higher accumulation of nickel and cobalt in the grains when introduced into the stem. In contrast, no major differences between earlier and later feeding dates were detected when the elements were introduced into the leaves. Losses of the applied elements were detected during maturation and can be explained by leakage in the rain.

Influence of nitrogen deficiency on senescence and the amounts of RNA and proteins in wheat leaves

Senescence-associated coordination in amounts of enzymes localized in different cellular compartments were determined in attached leaves of young wheat (Triticum aestivum L. cv. Arina) plants. Senescence was initiated at the time of full leaf elongation based on declines in total RNA and soluble protein. Removal of N from the growth medium just at the time of full leaf elongation enhanced the rate of senescence. Sustained declines in the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39), and a marked decrease in the rbcS transcripts, just after full leaf elongation indicated that Rubisco synthesis/degradation was very sensitive to the onset of senescence. Rubisco activase amount also declined during senescence but the proportion of rca transcript relative to the total poly A RNA pool increased 3-fold during senescence. Thus, continued synthesis of activase may be required to maintain functional Rubisco throughout senescence. N stress led to declines in the amount of proteins located in the chloroplast, the peroxisome and the cytosol. Transcripts of the Clp protease subunits also declined in response to N stress, indicating that Clp is not a senescence-specific protease. In contrast to the other proteins, mitochondrial NADH-glutamate dehydrogenase (EC 1.4.1.2) was relatively stable during senescence and was not affected by N stress. During natural senescence with adequate plant nitrate supply the amount of nitrite reductase (EC 1.7.7.1) increased, and those of glutamine synthetase (EC 1.4.7.1) and glutamate synthase (EC 6.3.1.2) were stable. These results indicated that N assimilatory capacity can continue or even increase during senescence if the substrate supply is maintained. Differential stabilities of proteins, even within the same cellular compartment, indicate that proteolytic activity during senescence must be highly regulated.

Solute losses from various shoot parts of field-grown wheat by leakage in the rain

Leakage from field-grown wheat was investigated during two seasons differing considerably in their rainfall patterns. For all solutes analyzed, these losses were low from non-senescing plant parts, increased after the onset of senescence and became maximal in fully senesced (dry, brown) organs. The cumulative losses of potassium by leakage in the rain were 65% of the content at anthesis for the flag leaf and 95% for the third leaf from the top, while these relative values were lower for magnesium (50 to 80%) and calcium (around 55%) and extremely low for sodium (<10%). The differences between potassium and sodium may be due to a different compartmentation on the tissue level or on the subcellular level. It became evident that for certain nutrients (e.g. potassium or magnesium) leakage in the rain may represent a major loss from senescing leaves and can be a relevant flux in maturing wheat.

Effect of locally increased zinc contents on zinc transport from the flag leaf lamina to the maturing grains of wheat

From previous experiments, it was evident that the accumulation of zinc in maturing wheat grains is highly regulated, but the regulatory mechanisms involved are not yet identified. In this study, we determined the transfer of radiolabelled zinc (fed directly into a leaf flap) from the flag leaf lamina to the grains. We also determined how this zinc transfer was affected by feeding additional unlabeled zinc (1 μmol per plant) either into the flag leaf sheath or the peduncle. Most of the 65Zn was retained in the feeding flap. A high percentage of the zinc exported from the flap accumulated in the grains with little accumulation of radiolabel in the other parts of the shoot. Unlabeled zinc remained mainly in the feeding flap and in the parts reached by the transpiration stream from the feeding position. The transfer of radiolabelled zinc was essentially not influenced by unlabeled zinc fed into another plant part. Our results suggest that the loading of zinc into the phloem and the mass flow in the sieve tubes might regulate zinc redistribution within the wheat shoot.