Effect of Nitrogen Supply on Photosynthate Partitioning, Root Carbohydrate Accumulation, and Acetylene Reduction in Birdsfoot Trefoil1

Abstract

Birdsfoot trefoil (Lotus corniculatus L.) has both low rates of N2 fixation and low levels of root total nonstructural carbohydrate (TNC) during midsummer regrowth. These observations suggest that reduced assimilate transport to roots may limit N2 fixation and consequently total plant growth. The objective of this study was to determine how rate of growth, as influenced by N supply, affected the distribution and accumulation of photosynthate during regrowth of trefoil. Nutrient solution supplied to well‐nodulated plants grown in sand was either 0 or 1 mM N (NH4NO3). Two and 4 weeks after cutting and the start of N treatments, plants were harvested and analyzed. Supplemental N caused a sharp decrease in acetylene reduction rate and a two‐fold increase in shoot dry weight 4 weeks after cutting. However, neither root dry weight nor TNC were significantly affected by supplemental N. Distribution of 14C showed reduced amounts of photosynthate accumulated in N supplemented roots at both 2 and 4 weeks after cutting. These data suggest that shoot growth of trefoil is a dominant sink for photosynthate and that photosynthate accumulation is not readily altered by N source or amount supplied. Numerous significant differences in distribution of 14C among classes of metabolites were evident only at the 2‐week harvest. More rapid metabolism of labeled sugars into other growth intermediates in N‐supplemented plants at this stage indicates that available soil N may be very important for rapid regrowth after cutting. Four weeks after cutting, when N2 fixation rates had increased several fold, distribution of 14C among various metabolites was not influenced by N source indicating little effect of N source on rate of assimilate metabolism in the plant.