Photoperiod Effects on Soybean Growth during the Onset of Reproductive Development under Various Temperature Regimes

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Abstract
The vegetative growth of soybeans was examined after 21 days of exposure to photoperiods of 10, 12, 14, 15 and 16 h following expansion of the 1st trifolate leaf of plants grown under day/night temeperature regimes of 18.degree./14.degree., 22.degree./18.degree., 26.degree./22.degree., 30.degree./26.degree. and 34.degree./30.degree. C. Photoperiods consisted of a common 9-h period of radiation from fluorescent plus incandescent lamps that provided photosynthetic photon flux density (PPFD) of 700 .+-. 30 .mu.mol s-1 m-2. This common 9-h period coincided with the day temperature and was preceded and followed by 0.5, 1.5, 2.5, 3.0, or 3.5 h of radiation from the incandescent lamps alone, which provided 9-12 W m-2 of photomorphogenic radiation between wavelengths of 700 and 850 nm and PPFD of 69 .mu.mol s-1 m-2. Of all the growth parameters measured, only total branch length and total number of nodes were significantly different among photoperiod treatments and across all temperature regimes. Dry weights of neither leaf, stem, nor root portions varied significantly with photoperiod after 21 days of treatment. The apical dominance ratio tended to increase either as photoperiod was lengthened or as temperature was increased. Results indicate tht photomorphogenetically induced changes in leaf number and leaf area may enhance overall reproductive efficiency of soybeans under long-day photomorphogenic photoperiods by increasing flowers per node.