Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Diurnal trends in leaf carbohydrate partitioning and nodule activity in soybeans under natural conditions and the irradiance level required to allocate sufficient carbohydrate to obtain maximum rates of N(,2)(C(,2)H(,2)) reduction were studied. Soybeans grown outdoors maintained constant levels of soluble carbohydrates in the leaves and constant rates of N(,2) fixation and root + nodule respiration when root temperature was kept constant but shoot temperature varied. When plants were subjected to a 40-hour dark period, then exposed to 200 to 1000 (mu)E m('-2) sec('-1), 200 (mu)E m('-2) sec('-1) resulted in maximum leaf soluble carbohydrate and nodule activity. Results suggest that nodule activity is controlled by carbohydrate partitioning in the shoot and support the concept of an environment-mediated programming of carbohydrate distribution. Carbon and nitrogen limitations to growth of symbiotically-grown soybean plants were assessed by examining growth characteristics of plants grown under low irradiance in a greenhouse and high irradiance outdoors and provided 0.0, 2.0, 6.0 or 12.0 mM NO(,3). Under low irradiance, supplementing N(,2) fixation with 2.0 mM NO(,3) resulted in relative growth rates (RGR), leaf area ratios (LAR) and net assimilation rates (NAR) very similar to plants supplied 12.0 mM NO(,3). As a result, total plant dry weight and leaf area of these two treatments were equivalent in 6-week-old plants despite a significantly lower N content in the 2.0 mM treatment. Under high irradiance, plants supplied 6.0 or 12.0 mM NO(,3) manifested greater relative growth rates and net assimilation rates during growth. Total plant dry weight and N content were also greater compared to the 0.0 and 2.0 mM treatments at six weeks. Leaf N content and area were equivalent in all treatments at this time. Results suggest that growth limitations to nodulated soybeans are primarily due to an inability to arrive at a functional balance between C and N accumulation prior to establishment of a fully functional N(,2) fixation system. Once N(,2) fixation is established, the increased input of N is used preferentially to increase both the photosynthetic efficiency and area of leaf tissue.