Response of a hypernodulating soybean mutant to increased photosynthate supply

Growth chamber studies were conducted to determine if increased photoassimilate supply, through light enhancement and CO2 enrichment, could reverse the deleterious plant growth and enhance nodule function traits of NOD1-3, a hypernodulating mutant of Williams. Both light enhancement and CO2 enrichment increased nodule number, acetylene reduction activity plant(-1) (but not specific activity) and dry matter accumulation in all tissues in both genotypes. Total biomass and specific nitrogenase activity were always less in the mutant than in Williams 82, indicating that the inferiority of the mutant may not be reversed by enhanced photoassimilate supply. Under all growth conditions, the mutant allocated relatively more photosynthate to nodules and less photosynthate to roots, compared to the control. Despite this, the decreased growth of the mutant relative to the control was not solely attributable to excessive nodulation of the mutant, since decreased growth was observed even on uninoculated plants. It is suggested that light enhancement and CO2 enrichment may have stimulated nodulation through increased photosynthate supply, independent of the nodulation autoregulatory signal. (C) 1997 Elsevier Science Ireland Ltd.