Economy of Carbon and Nitrogen in a Nodulated and Nonnodulated (NO₃-grown) Legume

Abstract

Partitioning and utilization of assimilated C and N were compared in nonnodulated, NO(3)-fed and nodulated, N(2)-fed plants of white lupin (Lupinus albus L.). The NO(3) regime used (5 millimolar NO(3)) promoted closely similar rates of growth and N assimilation as in the symbiotic plants. Over 90% of the N absorbed by the NO(3)-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. NO(3)-fed and N(2)-fed plants transported similar types and proportions of organic solutes in xylem and phloem. Withdrawal of NO(3) supply from NO(3)-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 NO(3)-fed plants (69%), and their stems were only half as effective as those of NO(3)-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 NO(3)-fed roots (50%), thus reflecting a higher CO(2) loss per unit of N assimilated (10.2 milligrams C/milligram N) by the nodulated root than by the root of the NO(3)-fed plant (8.1 milligrams C/milligram N). Theoretical considerations indicated that the greater CO(2) output of the nodulated root involved a slightly greater expenditure for N(2) than for NO(3) 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 NO(3)-fed plant.