Richard Shibles

Photosynthesis, Rubisco activity and amount, and their regulation by transcription in senescing soybean leaves

Senescence is a phase of leaf ontogeny marked by declining photosynthetic activity that, in soybean (Glycine max [L.] Merr.), is paralleled by a decline in chloroplast function. Soybean leaves have different patterns of decline in photosynthetic capacity and chloroplast function associated with nodal position and sink activity. The objective of this work was to determine whether leaves from nodes 3 and 6 of soybean, which show these different patterns, are similarly regulated with respect to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity and content and also to ascertain the degree of regulation of Rubisco content by transcription. Leaves from nodes 3 and 6 of field-grown soybean plants were sampled periodically from the time of their unfolding until near death. In situ CO2-exchange rate (CER) increased to a maximal level in both leaves and then declined slowly. For node 3 leaves the decline was progressive, but for node 6 leaves the decline was arrested at about 75% of maximum CER for a period of about 20 d, coincident with the onset of rapid seed growth, before a short period of very rapid decline immediately preceding leaf death. Rubisco activities and Rubisco content were directly correlated with CER in the leaves exhibiting the two different patterns. Rubisco activation ratio was similar for the two leaves and did not change throughout development. The primary regulator of photosynthesis at the physiological level, thus, was the amount of Rubisco protein. Decreases in Rubisco holoenzyme during senescence of both leaves were accompanied by coordinate decreases in the levels of mRNAs for the small and large subunits of Rubisco, suggesting that the decrease in Rubisco enzyme amounts during soybean leaf senescence is due to slower transcription rates and that levels of these mRNAs are coordinately controlled during senescence as they are during chloroplast development. However, plastid DNA template availability and posttranscriptional controls may also influence Rubisco content during senescence of these leaves. We conclude that soybean leaf photosynthesis likely unfolds according to a single developmental program but that modifications can be superimposed upon this program to maximize photosynthetic rates.

Influence of spatial arrangements of maize on performance of an associated soybean intercrop

Abstract Various spatial arrangements of maize ( Zea mays L.) were tried in attempt to improve grain yield of a soybean [ Glycine max (L.) Merr.] intercrop, sown in the same row, without substantially reducing the maize yield. The experiment was conducted in 1980 and 1981 in Iowa, U.S.A. Intercropping delayed maize tasseling and silking by up to 2 days, but did not influence rate of soybean development. Intercropping reduced soybean yields by 87% compared with sole cropping, principally because of reduced plant growth and pod set. Harvest index of soybeans was not altered. Compared with a 70 cm × 30 cm (row × intra-row) plant arrangement, grouping maize plants two or three to a hill in wider intra-row spacings (70 cm × 60 cm, 70 cm × 90 cm) improved soybean growth and pod set, reduced its lodging, and allowed greater seed yield. Widening rows to 87 and 105 cm did not improve soybean performance. Improved maize growth, as a consequence of a 135 kg/ha N application, reduced intercropped soybean growth and yield. Intercropping without applied N did not reduce maize yield compared with monocropping without N. Hence, the additional soybean yield from intercropping was supplemental. When 135 kg/ha N was applied, however, intercropping tended to reduce maize yield slightly, though not significantly, resulting in a total maize plus soybean yield about the same as for maize monocropping with N. Thus, when N was applied, there was no supplementary legume yield from intercropping.

EFFECTS OF NITROGEN SOURCE AND TIMING OF SULFUR DEFICIENCY ON SEED YIELD AND EXPRESSION OF 11S AND 7S SEED STORAGE PROTEINS OF SOYBEAN

Abstract The nutritional value of soybean (Glycine max [L.] Merr.) seed as a protein source for nonruminants is limited by the amount of the S-containing amino acids, methionine and cysteine, they contain. The nutritional quality of S soybean is known to be influenced by both S and N nutrition. The capacity of the plant for S assimilation during late seed filling is an additional factor that might limit protein quality even when S availability is more than adequate. This paper describes experiments conducted to examine the effects of S and reduced N availability on protein quality, and the capacity of the plant for assimilation of sulfate-S during seed filling. Availability of sulfate-S was varied in the hydroponic culture during various vegetative and reproductive stages and N was supplied as either urea (reduced form) or as nitrate (oxidized form). The ratio of 11S/7S seed storage proteins was used as an indicator of protein quality. The 11S protein is of greater nutritional value than is the 7S protein; therefore, the greater the 11S/7S ratio the better the nutritional quality of the seed protein. Provision of reduced N caused seed yield to increase across S treatments and the 11S/7S ratio to decline among plants that were S-sufficient but not among those that were S-deficient. Seed yield was very sensitive to S-deficiency occurring during vegetative growth, but not to S-deficiency occurring during reproductive growth. The 11S/7S ratio was strongly influenced by S-deficiency occurring during reproductive growth, but was relatively insensitive to S availability during vegetative growth. Provision of S near the middle of the seed filling period to previously S-deficient plants caused 11S/7S ratio to increase threefold over plants that were maintained S-deficient throughout seed filling. We conclude that the plant maintains a substantial capacity for S assimilation late into seed filling, and that mobilization of vegetative S is not a large source of S for developing seeds.

Nutrient mobilization and yield of soybean genotypes

Abstract Mobilization of nitrogen, phosphorus, and potassium from leaf and carpel tissue during seed growth was estimated for 18 diverse, field‐grown soybean lines and cultivars during two years. The objective was to determine whether lines differed in nutrient mobilization and, if so, whether such differences were related to yielding ability. Lines differed in nutrient concentration and tissue mass and in nutrient content, both at beginning of seed growth and at plant maturity. Mobilization from leaves and carpels potentially supplied up to 47, 23, and 29% of seed N, P, and K. Leaves were three to four times more important than carpels as sources of seed N and K and nearly twice as important as sources of P. Only mobilized leaf N exhibited a relationship with seed yield (r = 0.73∗∗ in 1978 and 0.43∗ in 1979). Leaf N mobilization was not heritable (correlation between years for line means was 0.09) in our environment, but might be more so under N‐stress conditions. The major determinant of mobilizable N w...

Differential accumulation of Soybean seed storage protein subunits in response to sulfur and nitrogen nutritional sources

Abstract Soybean (Glycine max [L.] Merr.) seed storage proteins consist of subunits that differ in amino acid profile, the β-subunit of 7S protein being essentially devoid of the S-containing amino acids, methionine and cysteine. Our objective was to examine the interaction of N and S nutrition on the relative abundance of these storage protein subunits in soybean seed. ‘Kenwood’ soybean was grown in hydroponic culture, and during vegetative growth (V2–R4.5) N was provided as 5 mM KNO3 to plants grown under sulfur-deficient (0.004 raM Na2SO4) or sulfur-sufficient (0.4 mM Na2S04) conditions. During seed fill (R4.5–R7) N was supplied as 5 mM KNO3 or 2.5 mM urea. Each N group was given S treatments of 1) no sulfur, 2) 0.4mM Na2SO4, 3) 0.2 mM L-cystine, or 4) 0.4 mM L-methionine. Effects on seed protein quality of S deficiency during vegetative growth were essentially overcome by supplying sulfate as late as R4.5. Total protein and seed storage protein were increased with urea as N source, but urea also increased the β-subunit. Provision of reduced S as methionine essentially suppressed β-subunit production, but cystine did not, suggesting that cystine did not influence methionine level in the seed. We also report the accumulation of two as yet unreported proteins which occur at extremes of S nutrition : (1) a putative β-subunit of 7S protein occurring in the embryonic axis under S-deficiency ; and (2) a ca. 14kD protein in cotyledon tissue under provision of L-methionine. Though S and N did interact to a limited extent to influence seed protein composition, major effects were from S or N acting individually.

Dry weights and chemical composition of soybean plant parts throughout the growing season 1

Abstract Three genetic lines of soybeans were grown in field experiments at different soil fertility levels in 1971 and 18 lines were grown at one fertility level in 1978. Plants were sampled at comparable stages of plant development through the entire growing season in 1971 and through the seed‐filling period in 1978. The plant samples were separated into different plant parts, dried, weighed, and chemically analyzed for total N, P, S, K, Ca, Mg, Zn, Cu, Mn, B, Mo, Na, Ba, Sr, Fe, Si, and Al. Seed yields did not vary significantly among genetic lines or soil fertility levels in 1971, but yields varied significantly among genetic lines in 1978. Rates of rapid seed dry weight increase in 1978 were similar at 98 kg/ha per day for all lines during most of the seed‐filling period, but the rapid seed‐fill period was longer for some lines than for others. Concentrations of the different elements in the plants generally were similar in different cultivars. Concentrations of the most phloem‐mobile elements ‐ N, P...

Response of determinate and indeterminate tropical soybean cultivars to water stress

Abstract Field and glasshouse experiments were conducted at the University of Costa Rica Experiment Station to test the hypothesis that indeterminate tropical soybean ( Glycine max (L.) Merr.) cultivars could better tolerate moisture stress than determinate types. In the field (dry season 1982) stress treatments, consisting of withholding water for 15 days, were given before flowering and during the R2 (full bloom) and the R3 (early podding) stages of plant development. In the glasshouse (August–November, 1982) stress treatments, consisting of withholding water for 10 days, were imposed preflowering, R2, R4 (mid-podding), and R6 (mid-seedfilling). Field-grown soybean plants under stress had predawn leaf water potentials that were 0.1–0.2 MPa less than those of well-watered control plants 7 days after witholding water began. Stomata showed the most rapid decline in water vapor conductance at leaf water potentials between −1.2 and −1.4 MPa. Under moderate to severe stress (leaf Ψ ca. −1.3 MPa), stomata on the adaxial surface closed completely. Indetermine cultivars were better able to recover from water stress than were determinate ones, as indicated by their greater seed yield after recovery from stress treatment. This response was associated with more pods per node rather than with other yield components, or with leaf area index or leaf area duration differences. Environmental conditions in the glasshouse induced determinateness in the indeterminate cultivar; consequently, in growth and seed yield it responded to water stress in the same way as the determinate cultivar.

Regulation of photosynthesis in developing leaves of soybean chlorophyll-deficient mutants

In this report we examine the factors that regulate photosynthesis during leaf ontogeny in y3y3 and Y11y11, two chlorophyll-deficient mutants of soybean. Photosynthetic rates were similar during wild type and Y11y11 leaf development, but the senescence decline in photosynthesis was accelerated in y3y3. Photosynthetic rates fell more rapidly than chlorophyll concentrations during senescence in wild type leaves, indicating that light harvesting is not strongly limiting for photosynthesis during this phase of leaf development. Chlorophyll concentrations in Y11y11, though significantly lower than normal, were able to support normal photosynthetic rates throughout leaf ontogeny. Chlorophyll a/b ratios were constant during leaf development in the wild type, but in the mutants they progressively increased (y3y3) or decreased (Y11y11). In all three sets of plants, photosynthetic rates were directly proportional to Rubisco contents and activities, suggesting that Rubisco plays a dominant role in regulating photosynthesis throughout leaf ontogeny in these plants. The expression of some photosynthetic proteins, such as Rubisco activase, was coordinately regulated with that of Rubisco in all three genotypes, i.e. an early increase, coincident with leaf expansion, followed by a senescence decline in the fully-expanded leaf. On the other hand, the light harvesting chlorophyll a/b-binding proteins of PS II (the CAB proteins), while they showed a profile similar to that of Rubisco in the wild type and y3y3, progressively increased in amount during Y11y11 leaf development. We conclude that Y11y11 may be defective in the accumulation of a component required for LHC II assembly or function, while y3y3 has more global effects and may be a regulatory factor that controls the duration of senescence.