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This article in CS

  1. Vol. 37 No. 6, p. 1801-1806
     
    Received: Feb 4, 1997
    Published: Nov, 1997


    * Corresponding author(s): rshibles@iastate.edu
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doi:10.2135/cropsci1997.0011183X003700060023x

Sulfur Availability, Rubisco Content, and Photosynthetic Rate of Soybean

  1. P. J. Sexton,
  2. W. D. Batchelor and
  3. Richard Shibles 
  1. Dep. of Agronomy
    Dep. of Agricultural and Biosystems Engineering, Iowa State Univ., Ames, IA 50011

Abstract

Abstract

Understanding the role of S in soybean [Glycine max (L.) Merr.] growth is important because a deficiency of the S-containing amino acids methionine and cysteine limits the nutritional value of soybean protein. As part of a broader study of S uptake and use, our objective was to quantify the effect of S availability on photosynthetic characteristics of soybean. Kenwood soybean was grown in a 3:1 mixture of Ida silt loam [fine-silty, mixed (calcareous), mesic Typic Udorthent] and acid washed sand with varying amounts of S added as gypsum (CaSO4) in two greenhouse trials. Single-leaf CO2-exchange rate (CER), quantum efficiency, dark respiration, leaf S, leaf N, and Rubisco content were determined. There appeared to be a critical level of 35 mg S per kg mixture (about 85 mg S per plant) below which specific leaf S (mg m−2 leaf area) declined linearly with decreases in soil S availability. On average, the most S deficient plants had 70% lower CER, 42% less quantum efficiency, and 50% decline in dark respiration relative to plants receiving adequate S. Total leaf N and Rubisco content were linearly related to leaf S levels, and declined on average 62, and 95%, respectively, as S declined from approximately 120 to 40 mg S m−2 leaf area. There was a linear relation between CER and Rubisco content; however, the amount of nonstructural carbohydrates per unit leaf N increased two-fold under S deficiency, suggesting that limitations on sink strength were greater than those on source capacity. Carbon dioxide exchange rate was linearly related to leaf S content at all sample dates, with a mean relation of 216 (unol CO2 g−1 S s−1. The Rubisco fraction declined linearly from nearly 50 to <10% of soluble protein, as leaf soluble protein content declined under S deficiency. We postulate that the decline in the Rubisco fraction may be a function of increased relative importance of housekeeping enzymes for survival as protein levels decline, and/or down-regulation of Rubisco synthesis in response to a build up of carbohydrates in the source leaves under S deficiency.

Journal Paper no. J-17211 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA. Project no. 2275, a contributing project to NC-142.

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