Distribution and mobilization of sulfur during soybean reproduction

Soybean (Glycine max (L.) Merr.) seed is an important source of dietary protein for humans and livestock; however, its protein contains low concentrations of the essential amino acids methionine (met) and cysteine (cys). It is believed that the physiological restriction to more favorable concentrations of met and cys can be attributed to a limited availability of free S-amino acids within developing cotyledons. These experiments were conducted to study resistances to S-amino acid accumulation in seed protein due to restrictions on mobilization of S from vegetative to seed tissues during reproduction;By radiolabeling hydroponically grown soybean with 35S, the effects of time of S uptake and form of S stored in tissues on S mobilization to seed were explored. In addition, N and S were systematically withdrawn from 35S labeled soybean during seed-fill to examine the relative effects that these nutrient stresses may have on the mobilization of vegetative N and S to seed;Mobilization of amino-S from leaf proteins supplied the seed with ca. 20% of its total-S requirement, and up to 80% of the mobilized-S from a single pulse of 35S during reproductive development. Although leaves mobilized sulfur with the same efficiency regardless of the time of sulfur uptake, they acquired a larger portion of the total plant label before rapid seed growth began. The quantity of sulfur mobilized from leaves appears to be reliant most on the quantity stored there, emphasizing the importance of S-reservoirs within growing leaves. Seeds of plants pulsed at R2 (beginning of pod growth), and supplied with sufficient N and S retained ca. 18% of their 35S in the 35SO4-2 fraction. Both N and 35S acquired by plants at R2 were mobilized to mature seed, with the same efficiency irrespective of N, S, or combined N- and S-stresses from R5 (beginning of seed growth) through maturity. Sulfur stress did not affect N or 35S distribution, and a combined N- and S-stress was required to induce or accelerate mobilization of what appeared to be organic-N and -S to developing seed. Neither N- nor S-stress alone effected the nitrogen or sulfur harvest indices