Nitrogen Assimilation and Leaf Development in Indeterminate Soybeans as Influenced by Post-Flowering Photoperiod

Guiamét, J. J., Balatti, P. A. and Montaldi, E. R. 1986. Nitrogen assimilation and leaf development in indeterminate soybeans as influenced by post-flowering photoperiod.—J. exp. Bot. 37: 1611–1618. The effects of photoperiod on nitrogen fixation and leaf development in indeterminate soybeans were studied during early reproductive growth. Soybean plants cv. Williams were grown under short days (SD: 8 h-natural daylight (N.D.)+16 h-darkness) or long days (LD: 8 h-N.D. + 8 h-low intensity artificial light+ 8 h-darkness) from full bloom until mid pod filling. Long days greatly increased plant growth, both on the basis of leaf area or weight, mainly due to higher net assimilation rate. Average daily rates of N2-fixation increased under LD; however, average N2-fixation rates on a nodule weight or N basis did not vary, suggesting that changes were not in nodule efficiency but in nodule biomass. As compared to SD, LD reduced N content in vegetative parts (pooled roots, stems and leaves), individual leaf blades and fruits. This seemed to be due to greater dry matter accumulation relative to N2-fixation. The 2nd and 5th trifoliolate leaves showed larger specific leaf weight (SLW) under LD. Soluble protein content on a dry weight basis was higher in the 5th (younger) leaf than in the 2nd, but did not vary due to photoperiod. On the other hand, chlorophyll and Fraction I protein content decreased in terms of dry weight under LD. A larger proportion of leaf N was allocated to soluble proteins under LD, thus compensating for the lower N content. On the whole, growth enhancement by LD seemed unrelated to increased availability of N or to greater leaf soluble protein or Fraction I content.