Seasonal Distribution of Carbohydrates in Nodules and Stem Exudate from Field-grown Soya Bean Plants*

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Abstract
The concentration of carbohydrates in tap root nodules from field-grown soya bean [Glycine max (L.) Merr.] plants was very similar to the concentration of compounds previously reported in greenhouse-grown nodules during vegetative growth of seedlings. The concentration of D-pinitol, sucrose and starch in nodules declined during rapid fruit growth, but the concentration of other compounds did not decline. The availability of carbohydrate in nodules during fruit growth did not seem likely to be the cause of the decline in nitrogen-fixing activity of nodules which has been reported by others. All compounds except glucose and α, α-trehalose declined to concentrations near zero during a 10-day period of nodule decay. However, the decline in carbohydrate did not appear to cause nodule senescence because it did not precede the period of decay and because decayed nodules contained substantial quantities of glucose and α, α-trehalose. Seasonal mean concentrations (72 samples from 24 dates) of compounds, in mg carbohydrate per g f. wt of nodule, were: sucrose, 2.84; D-pinitol, 1.14; D-chiro-inositol, 1.27; glucose, 1.40; α, α-trehalose, 1.34; myo-inositol, 0.65; maltose, 0.31; and fructose, 0.21. Quantities of sugars and cyclitols in stem exudate collected in the field on 13 dates were small (< 10 percent) relative to the quantity of nitrogenous compounds transported from roots to shoots. The seasonal pattern of pinitol transport in the xylem was very similar to the seasonal pinitol concentration in nodules. A large increase in sugar concentration in stem exudate subsequent to 80 days after planting supports the view that lack of carbohydrate was not a cause of nodule senescence.

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