Metabolism of Carbon and Nitrogen by Soybean Seedlings in Response to Vegetative Apex Removal

Short-term (31-hour diurnal) growth-chamber studies were conducted to determine the effects of removing the vegetative apex (meristem and developing trifoliolate leaves) on net photosynthesis (changes in plant dry weight), on distribution of metabolites among plant parts, and on nitrate metabolism and reduced-N accumulation by soybean [Glycine max (L.) Merr.] seedlings. Roots and stems served as alternate sinks for dry matter accumulation in the absence of the vegetative apex. Sugar concentration in roots increased (42%) within 4 hours of vegetative apex removal, and remained higher than for the controls during the 31-hour experimental period. Nitrate assimilation (nitrate reductase activity and total accumulation of reduced-N) was also enhanced in response to vegetative apex removal. Although dry matter accumulation was similar between treated and control plants (113 versus 116 milligrams per plant) over the 31-hour sampling period, more nitrate (1.31 versus 0.79 milligrams per plant) and more reduced-N (3.96 versus 3.45 milligrams per plant) accumulated in treated plants during the same interval. It was concluded that vegetative apex removal had little effect on overall net photosynthesis of soybean seedlings during the 31-hour treatment period, but did alter partitioning of photosynthate and enhanced uptake, transport, and reduction of nitrate. Implications are that uptake and metabolism of nitrate by soybeans may be limited by flux of carbohydrate to the roots, although hormonal effects due to vegetative apex removal cannot be ruled out.