Sulfur Availability, Rubisco Content, and Photosynthetic Rate of Soybean

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.