Growth and photosynthesis of soybean (Glycine max (L.) Merr.) in simulated vegetation shade: influence of the ratio of red to far‐red radiation*

Abstract

Abstract. Glycine max (L.) Merr. was grown under several light conditions to determine the role of red and far‐red radiation in plant adaptation to vegetation shade. Neutral density,‘neutral’ density with elevated far‐red radiation, and green shade treatments were used in a greenhouse, producing calculated phytochrome photostationary state (Pfr/Pr+Pfr) values of 0.68, 0.63 and 0.51, respectively. Cool‐white fluorescent lamps either alone or in conjunction with far‐red fluorescent lamps were used in a growth chamber, providing Pfr/Pr+Pfr of 0.79 and 0.61, respectively. Daily photo‐synthetically active radiation was about 25% of daylight and was approximately equal for both greenhouse (2.15MJ m−2) and growth chamber (2.57MJ m−2). Developmental stage 4 weeks after sowing was similar for all treatments, but axillary growth and rates of leaf area and dry matter accretion differed between plants from greenhouse and growth chamber. Light conditions simulating vegetation shade (i.e. a low ratio of red to far‐red radiation) significantly promoted petiole elongation and retarded the rate of stem elongation in both greenhouse and growth chamber experiments. Other aspects of growth either were not significantly altered by spectral quality or were not modified consistently in both greenhouse and growth chamber environments. Net photosynthetic rates measured under growth conditions for unifoliate and first trifoliolate (TF1) leaves of growth chamber plants between 9 and 21 d after sowing were generally unaffected by spectral quality, but supplemental FR enhanced TF1 leaf area expansion. The latter effect was not correlated with increased dry matter accumulation. The significance of spectral quality for adaptation of soybeans to canopy closure and intercropping is discussed.