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

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.

Key Words: Lotus corniculatus L. • Nitrogen fixation • Total non-structural carbohydrates • Regrowth

² Associate professor of agronomy.

Received for publication January 5, 1979.

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