Christof Engels

Nitrate uptake ability by maize roots during and after drought stress

The effects of different intensities and durations of soil drought and re-watering on the nitrate uptake ability of maize roots were studied. Plants were grown in split-root containers with one part of the root system subjected to different intensities and durations of soil drought and re-watering while the other part of the root system was continuously watered to 23% (w/w) soil water content (70% water capacity). Experiments were performed in split-root containers to maintain a high growth rate, thus ensuring high nutrient demand of the shoot irrespective of the soil water regime. To avoid limitation of nitrate uptake by transport processes in the dry soil, and to ensure a uniform 14N/15N ratio at the root surface, 15N was applied to the roots by placing them into an aerated nutrient solution with 0.5 mM Ca(15NO3)2. Shoot elongation and biomass were only slightly affected by drought in one root compartment when the soil in the other root compartment was kept wet. Therefore, the growth-related nutrient demand of the shoot remained at a high level. At moderate levels of soil drought (10% w/w water content) the ability of the roots for N-uptake was not affected even after 10 d of drought. N-uptake ability was reduced to about 20% of the well-watered control only when the soil water content was decreased to 5%. Total soluble sugar content of the roots increased with increasing soil drought, indicating that low N-uptake ability of roots subjected to severe soil drought was not caused by low assimilate supply from the shoot. Nitrate uptake ability of roots maintained in very dry soil (5% soil water content w/w) even for a prolonged period of 8 d, recovered within 3 d following re-watering. Root growth increased one day after re-watering. A short-term experiment with excised roots formerly subjected to severe soil drought showed that nitrate uptake ability recovered in old and young root segments after 2 d of re-watering. Obviously, the increase in N-uptake ability after re-watering was caused not only by new root growth but also by recovery of the uptake ability of formerly stressed roots.

Response of shoot and root growth to supply of different nitrogen forms is not related to carbohydrate and nitrogen status of tobacco plants

In the present study, we investigated effects of homogeneous or localized supply of different nitrogen (N) forms on shoot and root growth of tobacco. While homogeneous supply of NH4+ and N deprivation inhibited shoot growth compared with application of NO3—, the N form had no significant effect on root growth. In contrast, in a split-root experiment, application of NH4+ or N deprivation in one half of the root system repressed root growth compared with the other part of the root, which was supplied with NO3—. However, shoot growth was not affected by localized NH4+ application or local N deprivation. Inhibitory effects on shoot and root growth by variations of N supply could not be related to limitations in N or C status of the plants or to NH4+ toxicity. A possible involvement of NO3— as a signal compound including of phytohormones is discussed. n n n nReaktionen des Spross- und Wurzel-wachstums auf verschiedene N-Formen bei Tabak sind unabhangig vom C- und N-Status der Pflanzen n n n nIn der vorliegenden Arbeit wurde der Einfluss von homogenem und lokalem Angebot verschiedener N-Formen auf das Spross- und Wurzelwachstum bei Tabak untersucht. Wahrend homogenes NH4+-Angebot oder N-Mangel im Vergleich zu NO3—-Ernahrung das Sprosswachstum verminderte, wurde die Wurzelbiomasse von der Form des N-Angebotes nicht beeinflusst. Im Gegensatz dazu wurde in einem Split-Root Experiment das Wurzelwachstum durch halbseitiges NH4+-Angebot oder einseitigen N-Entzug im Vergleich zur Wurzelhalfte mit NO3—-Angebot gehemmt. Das Sprosswachs-tum dagegen wurde bei lokalem Angebot von NH4+- oder einseitigem N-Entzug kaum beeintrachtigt. Die beschriebenen Hemmwirkungen auf das Spross- und Wurzelwachstum bei unterschiedlichem N-Angebot konnten nicht auf einen limitierten C- oder N-Status der Pflanzen oder auf Ammoniumtoxizitat zuruckgefuhrt werden. Eine mogliche Beteiligung von NO3— als Signalsubstanz und von Phytohormonen wird diskutiert.

Assessing the ability of roots for nutrient acquisition

Nutrient acquisition by roots from soil is a complex process which is dependent on several root features: (1) morphological root characteristics, including mycorrhizal associations, which determine the extent of the interface between plant and soil (2) ability to modify the nutrient availability in the rhizosphere, and (3) ability for nutrient uptake through the plasma membranes (for reviews see Barber 1984; Clarkson 1985; Marschner 1995). The relative importance of these factors for nutrient acquisition is dependent on environmental conditions and the specific nutrient, particularly its chemical availability and mobility in the soil.

Elevated atmospheric CO2 concentration favors nitrogen partitioning into roots of tobacco plants under nitrogen deficiency by decreasing nitrogen demand of the shoot

Anthropogenic increase of atmospheric CO2 concentration is likely to affect plant growth in natural and agricultural ecosystems. Since nitrogen (N) is one of the major factors limiting agricultural plant production, we investigated the effect of elevated atmospheric CO2 concentration on N partitioning at the whole-plant level and the cellular level at limited N supply. Tobacco was grown at ambient (400 ppm) and elevated (800 ppm) concentrations of atmospheric CO2 under conditions of defined N supply with the same amount of N supplied to all plants, independent of CO2-induced changes of the actual growth rate. Under conditions of N deficiency, high CO2 concentration promoted root growth whereas shoot growth was only slightly increased, which resulted in an increased root/shoot ratio. At low N supply, elevated atmospheric CO2 concentration decreased N concentrations in the shoot tissue, but not in roots. Obviously elevated CO2 supply stimulated N partitioning into roots relative to the shoots, which coincided with relatively stronger root growth. At the cellular level, Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) protein decreased under N deficiency and elevated CO2. This was associated with increasing starch concentrations, while sugar concentrations were not affected. We suggest that root growth under N limitation is restricted by the internal N availability rather than by other factors such as carbon supply. The present results suggest that N partitioning into roots is favored by elevated CO2 supply due to a decreased N demand of the shoot.

Regulation of xylem transport of calcium from roots to shoot of maize by growth-related demand

The aim of the present experiments was to study the effect of growth-related nutrient demand on Ca2+ translocation from roots to shoot of maize (Zea mays L.). The plants were grown under controlled environmental conditions in nutrient solution with constant Ca2+ supply. The growth-related demand for Ca2+ and other nutrients was modified by growing the plants with their apical shoot meristem either at air temperature (24°C/20°C day/night) or at 14°C. Reduction of the shoot meristem temperature (SMT) to 14°C decreased shoot growth without affecting root growth in the first five days, which diminished the growth-related demand of the shoot for nutrients per unit of roots. This decrease in shoot demand led to a reduction not only of Ca2+ translocation rates in intact transpiring plants but also of Ca2+ fluxes in the xylem exudate of decapitated plants. This indicates that the decrease in xylem flux of Ca2+ at low SMT was not only the result of low transpiration-related water flux, and thus possibly low apoplasmic bypass transport of Ca2+ into the stele. In decapitated plants precultured at low SMT, the water flux through the roots was diminished even more than Ca2+ flux, leading to a significant increase in the Ca2+ concentration of the exudate, and thus presumably an increase in the Ca2+ gradient between cytosol and apoplast of stelar parenchyma cells. When the osmotically driven water flux was reduced by addition of mannitol to the nutrient solution, Ca2+ concentration in the exudate markedly increased, whereas Ca2+ translocation was only slightly affected. From these results it is suggested that the decrease in Ca2+ translocation rates at low shoot demand was not related to low water flux but to direct effects on the capacity of Ca2+ transport mechanisms in the roots. n n n nRegulierung des Transportes von Ca2+ im Xylem aus den Wurzeln in den Spross von Mais durch den wachstumsbedingten Sprossbedarf n n n nZiel der vorliegenden Versuche war es, die Auswirkungen des wachstumsbedingten Sprossbedarfes fur Nahrstoffe auf die Verlagerung von Ca2+ aus den Wurzeln in den Spross von Mais (Zea mays L.) zu untersuchen. Dazu wurden Pflanzen unter kontrollierten Umweltbedingungen in Nahrlosung mit konstantem Ca2+-Angebot angezogen. Der wachstumsbedingte Sprossbedarf fur Ca2+ und andere Nahrstoffe wurde variiert, indem das apikale Sprossmeristem der Pflanzen entweder der Lufttemperatur in der Klimakammer (24°C/20°C Tag/Nacht) ausgesetzt wurde oder mithilfe einer schmalen Kuhlmanschette um die Sprossbasis auf 14°C gekuhlt wurde. Die Absenkung der Sprossmeristemtemperatur (SMT) auf 14°C verringerte in den ersten funf Tagen zwar das Sprosswachstum, nicht jedoch das Wurzelwachstum. Dadurch wurde der wachstumsbedingte Bedarf des Sprosses fur Nahrstoffe pro Einheit Wurzeln deutlich verringert. Die Verringerung des Sprossbedarfes fuhrte zu einer Abnahme nicht nur der Ca2+-Translokationsraten von intakten, transpirierenden Pflanzen, sondern auch der Ca2+-Flusse im Xylemexsudat dekapitierter Pflanzen. Dies zeigt, dass die Abnahme des Ca2+-Flusses im Xylem bei tiefer SMT nicht nur auf einen geringeren transpirationsbedingten Wasserfluss und damit moglicherweise geringeren apoplastischen Ca2+-Transport in den Zentralzylinder zuruckzufuhren war. In dekapitierten Pflanzen, die vorher bei tiefen SMT angezogen worden waren, war der Wasserfluss durch die Wurzeln starker vermindert als der Ca2+-Fluss. Dies war mit einem deutlichen Anstieg der Ca2+-Konzentration im Exsudat verbunden und damit vermutlich mit einer Erhohung des Ca2+-Gradienten zwischen Cytosol und Apoplast der Xylemparenchymzellen. Eine Verringe-rung des osmotisch bedingten Wasserflusses durch Zugabe von Mannitol zur Nahrlosung f¨hrte ebenfalls zu einer deutlichen Erhohung der Ca2+-Konzentration im Xylemexsudat, hatte jedoch keine deutliche Verminderung der Ca2+-Translokation zur Folge. Aus diesen Ergebnissen wird gefolgert, das die Abnahme der Ca2+-Translokation bei geringem Sprossbedarf nicht durch einen geringeren Wasserfluss durch die Wurzeln verursacht wurde, sondern auf eine veranderte Kapazitat von Ca2+-Transportmechanismen in den Wurzeln zuruckzufuhren war.

Effect of nitrogen supply on individual kernel weight and 14C partitioning in kernels of maize during lag phase and grain filling.

In maize, N deficiency reduces grain yield by decreasing kernel weight and kernel number. In the present study, we investigated the effects of different rates of N supply on sugar concentrations and incorporation of recently assimilated 14C into sucrose, hexoses and ethanol-insoluble compounds of pedicels and kernels during different stages of kernel development. Low individual kernel weight in N deficient plants at maturity was related to a decreased production of total biomass rather than to low biomass partitioning to the ear. In the first five to ten days after pollination, i.e. during the early lag phase of kernel development, the ratio of sucrose to total sugars as well as 14C label ratios of sucrose to total sugars in the pedicels of N deficient plants were higher than in plants with optimal N supply, suggesting lower sucrose cleavage capacity in the pedicels of N deficient plants. In N deficient plants, the concentrations of soluble sugars were generally higher in the pedicels than in the kernels indicating some barrier for sugar transport into the kernels. In contrast, in N sufficient plants sugar concentrations were higher in the kernels than in the pedicel suggesting the involvement of an active mechanism for sugar import into the kernel. During later stages of kernel development (grain filling period), the rate of N supply had no effect on sugar partitioning between pedicels and kernels, and 14C incorporation into various chemical fractions.

Pflanzenartenunterschiede im Wurzelwachstum bei Verschiedener N-Ernährung: N-Form-Effekt und/oder pH-Effekt?

The form of N supply may influence root growth not only by physiological effects within the plants such as carbohydrate demand for N assimilation, but also indirectly by changes in rhizosphere pH. The aim of the present study was to distinguish between these mechanisms in various plant species (tobacco, potato, rice) by growing the plants in nutrient solution culture at pH 4.5 or 7 with either NO3 − or NH4 + supply. In tobacco and potato, the impairment of root extension growth in NO3 − fed plants was caused by the increase in the rhizosphere pH, and accordingly was disposed by growing the plants at pH 4.5. Reversely, roots of NH4 +-fed plants which showed good extension growth at low pH became stunted when the pH was raised to 7. In contrast to tobacco and potato, root growth of rice was neither influenced by the form of N supply nor by pH. The ability of rice for good root extrusion at pH 7 even when supplied with NO3 − was attributed to the fact that the rizosphere of nodal roots was acidified under these conditions.