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Assimilate Distribution in Soybeans as Affected by Photoperiod During Seed Development¹

The duration of the seed filling period for soybean [Glycine max (L.) Merr.] may be limited by rapid nutrient diversion from leaves to seeds resulting in reduced leaf assimilation capacity. In this study, the effect of short-day and long-day photoperiods on the duration and rate of seed filling and on nutrient remobilization were examined in two determinate genotypes, ‘Ransom’ and ‘D72-8126’. The photoperiod treatments were imposed during the seed development phase in controlled-environment chambers. Dry matter and N distributions throughout reproductive development were measured.

Under long days, both genotypes maintained far greater leaf area and attained greater total and vegetative growth relative to short days. Reproductive growth was decreased 11% for Ransom but was increased 16% for D72-8126 under long day conditions. The rates of growth and N accumulation per seed also were reduced by long days for Ransom but were unaffected for D72-8126. The duration of the seed filling period was not extended for either genotype. Seed number was increased 5 and 18% by long days for Ransom and D72-8126, respectively.

Ransom plants grown under long days had higher N concentrations in vegetative parts and lower seed N concentrations than short day plants as the reproductive phase progressed. Long-day photoperiods caused a similar increase in vegetative N concentrations for D72-8126, but by final harvest reproductive N concentration under long days was equal to that under short days. Nitrogen remobilization from leaves to seeds under short days did not appear to limit the dry matter or N assimilation efficiency of either genotype.

Key Words: Glycine max (L.) Merr. • Nitrogen remobilization • Source/sink relationships • Seed yield

² Former graduate research assistant, Dep. of Crop Science, professor, Dep. of Crop Science, and professors, Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27650.

Received for publication November 16, 1981.

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