Allocation of S in Generative Growth of Soybean

Abstract

Soybean plants (Glycine max L. Merr) were grown with 100 p~ S and 15 mM N and studied with respect to S allocation during grain development. The grains accounted for 87% of the S taken up after d 42, the balance coming from internal redistribution of S from leaves and pods. Detailed studies of the leaves, pods, and grains associated with leaf axils 6 and 7 showed that sulfate accumulated in the pods as they expanded to 50% of full length, ahead of grain enlargement, but declined to very low levels as grain growth commenced. Conversely, homoglutathione (hCSH), cysteine, and methionine increased. In developing grains, hCSH accounted for 60 to 90% of the soluble-S but sulfate was barely detectable. The data are consistent with a model in which, under S-limiting conditions, the pods act as sinks for sulfate and grain growth initiates the assimilation of sulfate into hCSH in the pods, and then into developing grains, where it is incorporated into grain proteins.Mature soybean (Glycine max L. Merr) grains contain about 30 to 40 pmol S, almost entirely in the insoluble fraction, where it is associated with the seed-storage proteins (Sunarpi and Anderson, 1995). In generative plants under greenhouse conditions, the pods associated with each leaf axil typically contain about three to six grains in one or more pods, representing a total S demand of about 90 to 240 Fmol of S per axil during grain development. This compares with approximately 6 to 15 pmol of S for a typical mature leaf grown under S-sufficient conditions. In soybean it has long been known that hGSH occurs at a very high concentration in grains, leading to the suggestion that hGSH is an important source of S for grain development (Macnicol and Bergmann, 1984; Klapeck, 1988). However, with this exception, little is known about the internal and externa1 sources of S that are required to satisfy the very large demand for S in generative soybean, and there appears to be no information concerning the allocation of S and internal redistribution of S in generative plants. In this paper we address these questions as they apply to generative soybean. We report that the large demand for S during grain enlargement is associated with net loss of soluble S from the leaves and pods but that most of the demand for grain S is fulfilled by uptake of S during generative growth. Under the experimental conditions ' used, which can be viewed as being S-limiting, the data support a model in which S is imported into the pods mostly as sulfate, and is assimilated into hGSH before incorporation into the developing grains. MATERIALS A N D M E T H O D S Nomenclature for the Plant PartsThe terms L, P, and G are used to refer to leaves, pods, and grains, respectively, and are numbered according to the relevant leaf axil. Thus, L6 refers to leaf 6, P6 refers to the pods (usually two, Table I) in the axil of L6, and G6 refers collectively to the grains in the P6 pod(s). Growing ConditionsSoybean (Glycine max L. var Stephens) plants were raised during the summer in a greenhouse in sand:ver...

Distribution and Mobilization of Sulfur during Soybean Reproduction

“…Similarly, developing soybean seeds may require mobilized reduced-8 for protein production as well (Sunarpi and Anderson, 1997c;and Sexton et al, 1998a). Sulfur, however, may not be mobilized from vegetative tissues with the same efficiency as is N (Sexton et aL, 1998a).…”

“…Soybean vegetative tissues may accumulate ample amounts of sulfur for the production of high quality and high protein seed, while not efficiently mobilizing it from vegetative to reproductive tissues (Sunarpi and Anderson, 1997c;Sexton, et al, 1998a).…”

“…Sunarpi and Anderson (1997c) examined S mobilization in reproductive soybean. Maximum length of leaves formed de novo during their study period seemed to be inhibited by S deficiency.…”

“…Likewise, the harvest index for S may be 15-20% lower than that for N (Sexton et aL, 1998a). Nitrogen to sulfur ratio in leaves drops from 17.2 to 11.5 indicating that about 50% more S relative to N remains in abscised leaves than in healthy ones.Soybean vegetative tissues may accumulate ample amounts of sulfur for the production of high quality and high protein seed, while not efficiently mobilizing it from vegetative to reproductive tissues (Sunarpi and Anderson, 1997c;Sexton, et al, 1998a).Sulfate is first delivered to the aerial portion of plants as S04'^ via xylem (Adiputra and Anderson, 1995;Bell et aL, 1995; Sunarpi and Anderson, 1996). Smith and Lang (1988) reported that S is brought into mature leaves and is quickly exponed, unchanged, via phloem.…”

“…Pods play a large role in 8 metabolism and 8 storage for the developing seed in oilseed rape. Sunarpi and Anderson (1997c) implied that soybean pods were reducing this 8 to hGSH for export to the seed. They could not, however, rule out the possibility that pods export 804'^ to seeds where it is reduced within the cotyledons.…”