Down-regulation by stems and sheaths of grain filling with mobilized nitrogen in wheat
Résumé
A major challenge for modern agriculture is to limit the excessive use of chemicals while improving wheat grain quality, mainly protein concentration, but without affecting yield. This paper focuses on the regulation of N mobilization from vegetative organs to fill the grains. Wheat crops were grown at low N uptake post anthesis. Ten days after anthesis, the second and third upper leaf blades were either cut at the ligule or wrapped in aluminium foil. The evolution of chlorophyll, green area, dry matter and N was studied in all organs from anthesis to maturity. N related metabolites (proteins, amino acids, nitrate) and enzymatic activities (glutamine synthetase, glutamate dehydrogenase, nitrate reductase, endoprotease) were also assessed in the blades. The grain N filling immediately slowed down following the defoliation by the same extent as the N mobilization rate from cut blades, while no modification of senescence or N metabolism occurred in other vegetative parts. In contrast, all indices for N metabolism indicated that wrapped blades exhibited strongly accelerated senescence. N was then completely released from these organs within two weeks, thus temporarily increasing N availability for the rest of the plant. However, no difference in grain N was observed between shoots with wrapped leaves and controls, whereas stem and sheaths stored the extra N released by wrapped blades. Grains did not adjust their N filling rate to N mobilization rate. Instead, the rate for N filling was asymmetrically regulated by the stem and/or sheaths. Lowering the N mobilization rate immediately resulted in a lower rate for grain N filling, whereas increasing N mobilization did not change the rate of grain N filling, due to temporary storage.