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 indeterminatesoybeans as influenced by post-flowering photoperiod.—J.exp. Bot. 37: 1611–1618. The effects of photoperiod on nitrogen fixation and leaf developmentin indeterminate soybeans were studied during early reproductivegrowth. 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 basisof leaf area or weight, mainly due to higher net assimilationrate. Average daily rates of N₂-fixation increased under LD;however, average N₂-fixation rates on a nodule weight or N basisdid not vary, suggesting that changes were not in nodule efficiencybut in nodule biomass. As compared to SD, LD reduced N contentin vegetative parts (pooled roots, stems and leaves), individualleaf blades and fruits. This seemed to be due to greater drymatter accumulation relative to N₂-fixation. The 2nd and 5th trifoliolate leaves showed larger specific leafweight (SLW) under LD. Soluble protein content on a dry weightbasis was higher in the 5th (younger) leaf than in the 2nd,but did not vary due to photoperiod. On the other hand, chlorophylland Fraction I protein content decreased in terms of dry weightunder LD. A larger proportion of leaf N was allocated to solubleproteins under LD, thus compensating for the lower N content.On the whole, growth enhancement by LD seemed unrelated to increasedavailability of N or to greater leaf soluble protein or FractionI content. Key words: Photoperiod, leaf development, soybean, nitrogen fixation