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

Abstract

Short-term (31 h 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 [G. 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 h of vegetative apex removal, and remained higher than for the controls during the 31-h 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 vs. 116 mg/plant) over the 31-h sampling period, more nitrate (1.31 vs. 0.79 mg/plant) and more reduced-N (3.96 vs. 3.45 mg/plant) accumulated in treated plants during the same interval. Vegetative apex removal had little effect on overall net photosynthesis of soybean seedlings during the 31-h treatment period, but it altered 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.