Abstract
Partitioning and utilization of assimilated C and N were compared in nonnodulated, NO3-fed and nodulated, N2-fed plants of white lupin (Lupinus albus L.). The NO3 regime used (5 millimolar NO3) promoted closely similar rates of growth and N assimilation as in the symbiotic plants. Over 90% of the N absorbed by the NO3-fed plants was judged to be reduced in roots. Empirically based models of C and N flow demonstrated that patterns of incorporation of C and N into dry matter and exchange of C and N among plant parts were essentially similar in the two forms of nutrition. NO3-fed and N2-fed plants transported similar types and proportions of organic solutes in xylem and phloem. Withdrawal of NO3 supply from NO3-fed plants led to substantial changes in assimilate partitioning, particularly in increased translocation of N from shoot to root. Nodulated plants showed a lower (57%) conversion of C or net photosynthate to dry matter than did NO3-fed plants (69%), and their stems were only half as effective as those of NO3-fed plants in xylem to phloem transfer of N supplied from the root. Below-ground parts of symbiotic plants consumed a larger share (58%) of the plants' net photosynthate than did NO3-fed roots (50%), thus reflecting a higher CO2 loss per unit of N assimilated (10.2 milligrams C/milligram N) by the nodulated root than by the root of the NO3-fed plant (8.1 milligrams C/milligram N). Theoretical considerations indicated that the greater CO2 output of the nodulated root involved a slightly greater expenditure for N2 than for NO3 assimilation, a small extra cost due to growth and maintenance of nodule tissue, and a considerably greater nonassimilatory component of respiration in root tissue of the symbiotic plant than in the root of the NO3-fed plant.
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