Effect of Previous Nitrate Deprivation on 15N-nitrate Absorption and Assimilation by Wheat Seedlings

Nitrate uptake and assimilation were examined in intact 18 days old wheat (Triticum aestivum, cv Capitole) seedlings either permanently grown on nitrate (high-N seedlings) or N-stressed by transfer to an 0 N-solution for the final 7 days (low-N seedlings). The N-stressed seedlings were characterized by a lower organic N content (2.5 mg instead of 4.9 mg per seedling) and an increased root dry weight.The seedlings received ¹⁵NO₃K for 7 h in the light. Nitrate uptake was 2.8 times higher in low-N than in high-N seedlings. The assimilation rate was 35 and 16 μmol NO₃^?·^h?1· g^?1 dry weight respectively. Partitioning of NO₃^? to reduction and assimilation was the very same in both kinds of seedlings. The results support the view that 50 % of the nitrate reduction in Triticum aestivum, cv Capitole could be achieved in the roots.The present observations are interpreted as evidence that factors closely associated with the seedling N-status may have a major role in regulating NO₃^? uptake and assimilation. In low-N seedlings, the high amount of carbohydrates in roots may add its stimulus to the specific inducing effect of nitrate whereas in high-N seedlings, excess of nitrate or amino-acids may set the pace by negative feedback control.     

Competitiveness and dry matter allocation of oilseed rape (Brassica napus L.) and two mustards (Sinapis alba L. and S. arvensis L.) under water stress conditions

In Morocco, oilseed rape is commonly exposed to mustards competition which are not totally controlled by herbicides. To understand the competitiveness of each species, growth parameters should be studied notably dry matter allocation. The objective of this study was to confirm the competitiveness of oilseed rape with regard to Sinapis alba and S. arvensis and to investigate how the dry matter is allocated. A pot experiment was undertaken with a quartz sand as substrate. Two plant densities were tested (one and two plants). The binary density was either a monoculture or a mixture. Half the pots were maintained at field moisture capacity and the other half was irrigated up to 70% of its water holding capacity. Dry matter allocation of each species at density two was compared to that of the same species at density one. Results of replacement series diagrams and those of the relative crowding coefficient (based on total dry matter) showed that Brassica napus was more competitive than S. alba. S. arvensis was the least competitive. Under competition, B. napus irrigated at water holding capacity allocated more dry matter to stem when compared to its dry matter at density one. Under the same condition, when reducing water supply, B. napus allocated more dry matter to leaves. In case of S. alba, dry matter percent in leaves and roots were respectively low and high in normally irrigated plant and inversely under water shortage. S. arvensis allocated high and low dry matter percent respectively to root than to leaves when sufficiently irrigated. But no clear tendency was noticed under water shortage, for this species.     

Effects of inoculation of EPS-producing Pantoea agglomerans on wheat rhizosphere aggregation

We investigated plant and soil nitrogen pools and soil processes in monospecific stands of the C₃ sedge Scirpus olneyi and the C₄ grass Spartina patens grown in the field in open top chambers in a brackish marsh on the Chesapeake Bay. Stands of S. olneyi responded to eight years of elevated CO₂, by increased rates of net ecosystem gas exchange and a large stimulation of net ecosystem production. We conducted our study in the summer of 1994 and 1995 when soil cores were collected and aboveground biomass was estimated. Nitrogen concentration in elevated CO₂ treatments was reduced 15% in stems of S. olneyi and 8% in the upper 10 cm of the soil profile. While total plant nitrogen per unit of land area remained the same between treatments, total soil nitrogen showed a non-significant tendency to decrease in the upper 10 cm of the soil profile in elevated CO₂ both years of study. A significant decrease in soil bulk density largely contributed to the observed decrease in soil nitrogen. Exchangeable nitrogen and potential denitrification rates were also reduced in elevated CO₂, but net nitrogen mineralization was unchanged by elevated CO₂ treatment in S. olneyi both years. Plants and soils in a pure stand of the C₄ grass, S. patens, showed none of these effects of elevated CO₂ treatment. Our data provides evidence of changes in nitrogen dynamics of an ecosystem exposed to elevated CO₂ for eight years; however due to the variability in these data, we cannot say if or how these changes are likely to impact the effect of rising CO₂ on primary production or carbon accumulation in this ecosystem in the future.     

Effect of plant density on competitiveness of Brassica napus, Sinapis alba and S. arvensis under water stress conditions

Under Mediterranean climate, oilseed rape is subjected especially to the competition of weeds with respect to water. Herbicides registered for this crop do not effectively control species of the same family, in particular Sinapis alba and Sinapis arvensis. Moreover, there are no results of the effect of plant density on the competitiveness of these species. The purpose of this experiment was to determine if the competitiveness of the species varies according to the total density. The experiment was carried out in pots under greenhouse conditions, according to a replacement series method. Plant densities tested were 2, 4 and 8 plants per pot. The results of the replacement series diagram and those of relative crowding coefficients showed that Brassica napus was the most competitive, whatever the density is. This classification is explained primarily by leaf area. Indeed, the intraspecific competition due to B. napus has affected more its leaf area than the interspecific competition. Conversely, the intraspecific competition due to S. arvensis has less affected its leaf area than the interspecific competition. Regarding S. alba, the intraspecific competition effect was less severe than the interspecific competition effect due to B. napus and more severe than the interspecific competition effect due to S. arvensis on S. alba