adaptation of these enzyme activities in the leaves of N2-fixing plants, and this contributed to an increase in starch accumulation. Another major causal factor associated with increased starch accumulation was the elevation in foliar levels of fructose-6-phosphate, glucose-6-phosphate, and glucose-i-phosphate (G 1 P), which had risen to chloroplast concentrations considerably in excess of the Km values for their respective target enzymes associated with starch synthesis, e.g. elevated GI P with respect to adenosine diphosphate glucose pyrophosphorylase (ADPG-PPiase) binding sites. The cofactor glucose-1,6-bisphosphate (G1,6BP) was found to be obligate for maximal PGM activity in soybean leaf extracts of N2-fixing as well as N-supplemented plants, and G1,6BP levels in N2-fixing plant leaves was twice that of levels in N-supplied treatments. However the concentration of chloroplastic G1,6BP in illuminated leaves was computed to be saturating with respect to PGM in both N2-fixing and N-supplemented plants. This Soybean plants coexist symbiotically with Bradyrhizobium japonicum, the bacteria from which those plants can derive all of their reduced nitrogen through N2-fixation processes. Symbiotic soybeans often display very large accumulations of starch and/or sucrose in their source leaves, while concurrently, these same leaves exhibit much reduced soluble protein levels (3, 4). When N2-fixing plants are supplemented with N03-and NH4', or they are grown bacteria free from emergence with a sufficient supply of N,2 their source leaves exhibit a reallocation of carbon photosynthate skeletons to support amino acid and soluble leaf protein accompanied by a diminution of sucrose and starch (3, 4). Further, N2-fixing plants often possess foliar net CO2 assimilatory rates equivalent to those observed in leaves of plants supplemented with growth sufficient levels of N (4, 24). This has been interpreted to mean that approximately the same amount of newly synthesized carbon photosynthate (e.g. TP) that normally was channeled into the syntheses of amino acids and soluble leaf protein in N-supplemented plants, instead was directed to support the syntheses of starch and sucrose within N2-fixing soybean plant leaves (4). Excessive foliar starch accumulation in N2-fixing plants was similar to that observed in N-deficient plants, although N2-fixing soybeans were relatively healthy (compare refs. 3, 4, and 16 with 23 and 25). 2Abbreviations: N, inorganic nitrogen, i.e., N03-and/or NH4+; ADPG-PPiase, ADP-glucose pyrophosphorylase; FBPase, fructose-1,6-bisP phosphatase; PGM, phosphoglucomutase; PHI, phosphohexoisomerase; HK, hexokinase; ALD, aldolase; PFK, phosphofructokinase; SP, starch phosphorylase; AMY, amylase; TP, triose phosphate (glyceraldehyde-3-P and dihydroxyacetone-P); PVM, paraveinal mesophyll; TEM, transmission electron microscopy. NS plants, soybean plants propagated in vermiculite from seedling emergence through experiment termination (4 weeks), and supplied daily with nutrient solution containing 6 mm NH4NO3. Pri...