Accumulation and Distribution of Mineral Nutrients, Carbohydrate, and Dry Matter in Soybean Plants as Influenced by Reproductive Sink Size

The purpose of this experiment was to determine the extent to which the number of pods per node (“sink Size”) controls the accumulation of mineral nutrients, carbohydrate, and total dry matter in soybean plants (Glycine max (L.) Merrill). A quantitative description of the relationship between sink size and nutrient accumulation will help to identify nutrients which might limit yield at higher reproductive loads. Plants were grown in the field and various numbers of pods were removed as they formed to give an average of 0, 0.8, 1.1, 1.9, 2.4, or 2.7 pods. per node. Leaf blades, stems pins petioles, pods, and seeds from mature plants were analyzed for N, P, K, Ca, Mg, and carbohydrate concentration. Increasing the sink size resulted in decreases in N and P concentration in leaves, stems, pods, and seeds and in Mg concentration in stems plus petioles. Conversely, K concentration in all plant parts and Ca concentration in leaf blades increased in response to increasing sink size. These increases in K and Ca concentration appeared to be due to a greater decline in the matter content than in K and Ca content as sink size increased, resulting in concentration of K and Ca. As sink size increased from zero to 2.7 pods per node: 1) carbohydrate content of leaf blades decreased 64%; 2) carbohydrate content of stems plus petioles decreased 40%; 3) mineral nutrient content of leaves plus stems decreased as follows, N — 72%, P — 64%, K — 28%, Ca — 18%, and Mg — 55%; and 4) total dry weight of leaves plus stems decreased 37%. When nutrient content of leaves and stems was plotted against sink size the result was a nearly perfect fit to a straight line for all nutrients except potassium. A comparison among nutrients of the slopes of these curves suggested that maximum reduction of N and P content in leaves and stems would occur at sink sizes smaller than required for maximum reduction of other mineral nutrients. Total Mg and Ca accumulated by above-ground plant parts was not dependent on sink size, whereas total N, P, and K accumulated was strongly dependent on sink size.