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Sulfur Availability, Rubisco Content, and Photosynthetic Rate of Soybean

Dep. of Agronomy
Dep. of Agricultural and Biosystems Engineering, Iowa State Univ., Ames, IA 50011

^* Corresponding author (email: rshibles{at}iastate.edu).

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.

Received for publication February 4, 1997.

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