Effect of Localized Nitrogen Availability to Soybean Half-Root Systems on Photosynthate Partitioning to Roots and Nodules

摘要

Soybean (Glycine max [L.] Merr. cv Davis) was grown in a split-root growth system designed to maintain control of the root atmosphere. Two experiments were conducted to examine how 80% Ar:20% O2 (Ar:O2) and air (Air) atmospheres affected N assimilation (NH4NO3 and N2 fixation) and the partitioning of photosynthate to roots and nodules. Application of NH4NO3 to nonnodulated half-root systems enhanced root growth and root respiration at the site of application. A second experiment applied Ar:O2 or air to the two sides of nodulated soybean half-root systems for 11 days in the following combinations: (a) Air to both sides (Air/Air); (b) Air to one side, Ar:O2 to the other (Air/Ar:O2), and (c) Ar:O2 to both sides (Ar:O2/Ar:O2). Results indicated that dry matter and current photosynthate (¹⁴C) were selectively partitioned to nodules and roots where N2 was available. Both root and nodule growth on the Air side of Air/Ar:O2 plants was significantly greater than the Ar:O2 side. The relative partitioning of carbon and current photosynthate between roots and nodules on a half-root system was also affected by N₂ availability. The Ar:O₂ sides partitioned relatively more current photosynthate to roots (57%) than nodules (43%), while N₂-fixing root systems partitioned 36 and 64% of the carbon to roots and nodules, respectively. The Ar:O₂ atmosphere decreased root and nodule respiration by 80% and nitrogenase activity by 85% compared to half-root systems in Air while specific nitrogenase activity of nodules in Ar:O₂ was 50% of nodules supplied Air. Results indicated that nitrogen assimilation, whether from N₂ fixation or inorganic sources, had a localized effect on root development. Nodule development accounted for the major decrease in total photosynthate partitioning to non-N₂-fixing nodules. Soybean compensates for ineffective nodulation by controlling the flux of carbon to ineffective nodules and their associated roots.