| Popis: |
Atmospheric CO2 enrichment is expected to benefit plant growth, despite causing global warming and nitrogen (N) dilution in plants. Most plants procure most of their N as two inorganic forms, nitrate (NO3-) or ammonium (NH4+), using membrane-localized transport proteins in roots which are key targets of improving N-use efficiency in plants. Though it is expected that there will be interactive effects of elevated CO2, chronic warming, and N form on root N relations, this has not been studied. In this study, tomato (Solanum lycopersicum L. cv. Big Boy) plants were grown at two levels of CO2 (ambient=400 ppm vs. elevated=700 ppm) and under two temperature regimes (30oC vs. 37oC), while providing either NO3- (1.5 mM) or NH4+ (1.5 mM) as the sole N source. To assess plant N relations under these conditions, plant biomass (shoot, root, total), %N (shoot, root, total), root N-uptake rate, the concentration of total root proteins, and the concentration of N uptake and assimilatory proteins in roots per unit total root protein and per gram of dry root mass were measured. Elevated CO2 and chronic warming acted synergistically to severely inhibit the growth of plants, regardless of N form, while individually, these factors slightly increased growth. Although %N in roots among all treatments was similar, the combination of elevated CO2 and warming decreased (1) the rate of N uptake by roots, (2) the concentration of total protein in roots, indicating an inhibition of N assimilation (especially in NH4+-treated plants), and (3) shoot %N, indicating a possible negative effect on N translocation from roots to shoots (especially in NO3--treated plants). However, for the most part, the concentrations in roots of the main N-uptake and N-assimilation proteins were little affected by elevated CO2 and warming, and when affected, were not correlated with root total protein or N-uptake rate; hence, N uptake and assimilation in roots were controlled by the activities of these proteins rather than their concentrations. Taken together, these results indicate that elevated CO2 and chronic warming will act synergistically to decrease plant N uptake and assimilation, thereby decreasing growth and protein concentration; thus, future global warming may potentially decrease both crop production and its food quality. |
