Nitrogen Assimilation Efficiency Research Articles

Efficiency of Nitrogen Assimilation by N2-Fixing and Nitrate-Grown Soybean Plants (Glycine max [L.] Merr.)

Nodulated and non-nodulated (not inoculated) soybeans (Glycine max [L.] Merr. cv Wells) were grown in controlled environments with N(2) or nonlimiting levels of NO(3) (-), respectively, serving as sole source of nitrogen. The efficiency of the N(2)-fixing plants was compared with that of the nitrate-supplied plants on the basis of both plant age and plant size. Efficiency evaluations of the plants were expressed as the ratio of moles of carbon respired by the whole plant to the moles of nitrogen incorporated into plant material.Continuous 24-hour CO(2) exchange measurements on shoot and root systems made at the beginning of flowering (28 days after planting) indicated that N(2)-fixing plants respired 8.28 moles of carbon per mole of N, fixed from dinitrogen, while nitrate-supplied plants respired only 4.99 moles of carbon per mole of nitrate reduced. Twenty-one-day-old nitrate-supplied plants were even more efficient, respiring only 3.18 moles of carbon per mole of nitrate reduced. The decreased efficiency of the N(2)-fixing plants was not due to plant size since, on a dry weight basis, the 28-day-old N(2)-fixing plants were intermediate between the 28- and 21-day-old nitrate-supplied plants.The calculated efficiencies were predominantly a reflection of root-system respiration. N(2)-fixing plants lost 25% of their daily net photosynthetic input of carbon through root-system respiration, compared with 16% for 28-day-old nitrate-supplied plants and 12% for 21-day-old nitrate-supplied plants. Shoot dark respiration was similar for all three plant groups, varying between 7.9% and 9.0% of the apparent photosynthate.The increased respiratory loss by the roots of the N(2)-fixing plants was not compensated for by increased net photosynthetic effectiveness. Canopy photosynthesis expressed on a leaf area basis was similar for 28-day-old N(2)-fixing plants (15.5 milligrams CO(2) square decimeter per hour) and 21-day-old nitrate-supplied plants (14.5 milligrams CO(2) square decimeter per hour). Both were similar in total canopy leaf area. The larger nitrate-supplied plants (28-day-old) had lower photosynthetic rates (12.5 milligrams CO(2) square decimeter per hour), presumably due to self-shading of the leaves.These data indicate that, during the early stages of plant development, dependence solely on N(2)-fixation is an expensive process compared to nitrate reduction in nitrate-supplied plants, since the N(2)-fixing plants retained 8% to 12% less of their photosynthate as dry matter.

EFFECTS OF DIETARY FIBER ON GROWTH, ASSIMILATION AND CELLULASE ACTIVITY OF THE PRAWN (Macrobrachium rosenbergii)1

ABSTRACTCellulose fiber comprising up to 30% of an isonitrogenous series of diets (0, 5, 15 and 30% fiber concentration) did not suppress growth of prawn having average initial weights of 0.08 ± 0.002 g over a 12‐week period. In addition, dietary fiber concentrations of 5 and 20% were found to stimulate growth over an 8‐week period among prawn having initial average weights of 1.48 ± 0.054 g, as opposed to 0% fiber concentration, which had no significant effect on growth.Cellulase activity measured in hepatopancreas tissues by viscometric techniques was shown to be positive with a six‐fold increase in activity occurring in adult prawn (11 ± 1.16 g) when compared to smaller prawn (1 ± 0.06 g). However, activity in both sizes of prawn was not found to be proportional to increases in dietary fiber concentrations. Total organic carbon assimilation decreased while nitrogen assimilation efficiency increased in response to increasing dietary fiber concentration. Results from the present study indicated that use of dietary fiber may make a significant contribution to commercial formulations of cost‐effective crustacean diets.