D. B. Egli

Seed- Fill Duration and Yield of Grain Crops*

Abstract Time, as expressed through seed-fill duration, is an important component of the productivity of grain crops. Seed-fill duration is influenced by environmental conditions and there is genetic variation in most crops. Seed-fill duration is regulated by the leafs ability to supply assimilate to the developing seed (i.e., leaf senescence) and by the ability of the seed to use this assimilate for continued growth. Seed-fill duration in many crops increases as temperature decreases below 30°C. Water stress during seed filling accelerates leaf senescence and shortens the seed-filling period. The response of seed-fill duration to alterations in source–sink ratios is inconsistent and probably depends on whether the supply of assimilate to the seed is affected and if the seed can respond to changes. In some cases increases in harvest index are a result of lengthening the seed-filling period. Environmental variation in seed-fill duration is frequently associated with yield in most crops. Direct selection for long seed-filling periods may increase yield and, conversely, selection for higher yield in many crops resulted in longer seed-filling periods. Seed-fill duration can have a relatively low heritability and is somewhat difficult to measure, and it has not been widely used by plant breeders in cultivar improvement programs. However, given the intractable nature of crop growth rate, lengthening the seed-filling period may be the most promising avenue to higher yields, but new approaches to manipulate it will probably have to be devised.

Cultivar maturity and potential yield of soybean

Abstract Soybean ( Glycine max (L.) Merrill) cultivars from maturity groups 00, I, III, and V were grown in the field to evaluate the relationship between the length of total growth cycle and potential yield. Cultivars from maturity group 00 and I were grown in narrow rows (0.38 m) to obtain maximum insolation interception. The length of the vegetative growth period increased by 35 days from maturity group 00 to V. Plant size (total nodes per plant and maximum vegetative mass in g m −2 ) also increased with increasing maturity group. All cultivars reached maximum insolation interception soon after initial flowering. The crop growth rate of control plots (measured between growth stages R1 and R5) was not related to plant size. Shade (30 and 63%) from growth stage R1 to R7 was used to create variation in crop growth rate within a cultivar. For each cultivar, the number of seeds m −2 increased linearly with increasing crop growth rate. After adjusting for cultivar differences in individual seed growth rate, there were no cultivar differences in seeds m −2 at a constant crop growth rate. Thus, seeds m −2 was related to crop growth rate, not to the size of the plant. The maturity growth 00 cultivar tended to have a shorter seed-filling period but there were no consistent differences among the others. These data suggest that the longer vegetative growth period of later-maturing cultivars does not provide a higher yield potential and that shorter-season cultivars may have equal yield potential if exposed to a similar environment.

Changes in ribulosebisphosphate carboxylase/oxygenase and ribulose 5-phosphate kinase abundances and photosynthetic capacity during leaf senescence.

The abundances of ribulose-1,5-bisphosphate carboxylate/oxygenase (Rubisco) and ribulose-5-phosphate (Ru5P) kinase in field-grown soybean (Glycine max L. Merr.) leaves were quantified by a Western blot technique and related to changes in chlorophyll and photosynthetic capacity during senescence. Even though the leaf content of Rubisco was approximately 80-fold greater than that of Ru5P kinase, the decline in the levels of these two Calvin cycle enzymes occurred in parallel during the senescence of the leaves. Moreover, the decrease in the content of Rubisco was accompanied by parallel decreases of both the large and small subunits of this enzyme but not by an accumulation of altered large or small subunit isoforms. With increasing senescence, decreases in abundances of Rubisco, Ru5P kinase and chlorophyll were closely correlated with the decline in photosynthetic capacity; thus, the specific photosynthetic capacity when expressed per abundance of any of these parameters was rather constant despite an 8-fold decrease in photosynthetic capacity. These results suggest that during senescence of soybean leaves the chloroplast is subject to autolysis by mechanisms causing an approximately 80-fold greater rate of loss of Rubisco than Ru5P kinase.

Species differences in seed water status during seed maturation and germination

Maize ( Zea mays L., cv. B73 × Mo17), wheat ( Triticum aestivum L., cv. Clark), and soybean ( Glycine max L. Merrill, cv. Elgin 87) were grown in the field and seed samples were collected at frequent intervals for the determination of seed fresh and dry weight and water potential of excised embryos (maize and wheat) and axes (soybean). Seed water concentration declined during seed development and the concentration at physiological maturity (PM, maximum seed dry weight) was highest in soybean (550–590 g kg −1 FW), lowest in maize (326–377 g kg −1 ) and intermediate in wheat (437 g kg −1 ). The embryo/axis water potential was relatively constant during much of seed filling before decreasing rapidly as the seeds approached PM and there was little variation among species (soybean −1.52 to −1.63 MPa, maize −1.61 to −1.99 MPa and wheat −1.66 MPa). Seed water concentration when 10% of the seeds germinated (radicle ≥3 mm) was highest in soybean (514 g kg −1 ), lowest in maize (332 g kg −1 ) and intermediate in wheat (345 g kg −1 ) while the water potential of the embryo/axis varied from −2.07 to −2.20 MPa across the three species. There was little variation in the water potential of the embryo/axis among species at the end of seed growth (PM) or at the beginning of germination. This similarity is consistent with the suggestion that the water status of critical seed structures may play a regulatory role in seed growth and germination.

Cultivar maturity and response of soybean to shade stress during seed filling

Abstract Soybean (Glycine max L. Merrill) cultivars with long and short growth cycles frequently exhibit similar yield potentials in nonstress environments. The objective of this experiment was to determine if cultivars with long growth cycles and large maximum vegetative masses were more resistant to stress during seed-filling. Early (Hardin and Kasota, maturity group I) and late (Essex and Hutchinson, maturity group V) cultivars were grown in the field at Lexington, Kentucky (38°N), in 0.38 m rows with irrigation in 1993 to 1995. Average yield of the nonstressed late cultivars was only 6% greater than the early cultivars. Late cultivars had a longer total growth cycle (49 d longer), and their vegetative mass at the beginning of growth stage R6, early in the seed-filling period, was nearly double that of early cultivars. The larger vegetative mass contained more N and starch. Shade cloth placed over the plots at the beginning of growth stage R6 reduced yield (23–26%), seed number (8–16%) and seed size (9–21%). The yield reduction was similar for early and late cultivars. Shade stress did not accelerate leaf senescence, as seen in the decline in leaf N and chlorophyll levels, and had no effect on the timing of physiological maturity. The N and starch levels in leaves ready to abscise from the plant were not affected by cultivar maturity or shade treatment. The large quantity of potentially mobilizable N and starch in the vegetative plant of the late cultivars did not reduce the effect of stress during seed filling on yield.

Species differences in seed growth characteristics

Information on the seed growth characteristics of 13 grain crop species was summarized from the available literature. Characteristics of interest included the rate of growth of the individual seed (mg seed−1 day−1), the duration of seed growth, estimated by the effective filling period (EFP), and maximum seed size (mg seed−1). There was a 100-fold variation in seed growth rate (0.2 to 35.5 mg seed−1 day−1) among the species and cultivars examined and growth rate was significantly correlated (r = 0.81) with maximum seed size. The EFP across all cultivars and species ranged from 7 to 57 days. The EFP was less variable than seed growth rate and was not as closely correlated with maximum seed size (r = 0.50). The variation in growth rate and EFP was not related to known differences in growth characteristics among the species, i.e., C3 vs C4, composition of the seed, or structure of the organ harvested for economic yield. The variation in the length of the EFP among cultivars within a species suggests that it may be possible to increase yields by selecting for longer EFPs.

Fruit removal in soybean induces the formation of an insoluble form of ribulose-1,5-bisphosphate carboxylase/oxygenase in leaf extracts*

In some soybean (Glycine max (L.) Merr.) cultivars, fruit removal does not delay the apparent loss of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) activity and abundance or the decline in photosynthesis. Analysis of leaf extracts from defruited plants indicated a time-dependent increase in both Rubisco activity and abundance in a 30000 · g pellet fraction in cultivars which had been reported to lose all Rubisco protein from the supernatant fraction. Attempts to solubilize the pelleted Rubisco by increasing the buffer volume/tissue ratio or by adding alkylphenoxypolyethoxyethanol (Triton X-100), ethylenediaminetetraacetic acid (EDTA), or NaCl were unsuccessful. However, treatment of the pellets with denaturants such as 8 M urea or 5% (w/v) sodium dodecyl sulfate (SDS) did release Rubisco from the pellet. Redistribution of protein to the pellet fraction appeared to be specific for Rubisco since the amount of ribulose-5-phosphate kinase (EC 2.7.1.19) found in the pellet fraction of leaf extracts of control and defruited plants was small and constant over time. The loss of soluble Rubisco, and the concomitant increase in insoluble Rubisco, in response to fruit removal varied with genotype and was reproducible in both field and greenhouse environments. In addition, the effect was influenced by node position and light; lower and-or shaded leaves exhibited less Rubisco in the pellet fraction than leaves from the top of the plant that was fully exposed to sunlight. When isolated by sucrose-density-gradient centrifugation, the insoluble Rubisco was found to co-purify with a 30-kDa (kilodalton) polypeptide. These results indicate that alteration of the source/sink ratio by removing fruits results in the formation of an insoluble form of Rubisco in leaf extracts of soybean. Whether or not Rubisco exists as an insoluble complex with the 30-kDa polypeptide in intact leaves of defruited plants remains to be determined.

Leaf starch accumulation and seed set at phloem-isolated nodes in soybean.

Abstract There is usually a linear relationship between pod and seed number per unit area and assimilate supply in soybean (Glycine max L. Merrill) communities. In contrast, the relationship is curvilinear at single phloem-isolated nodes. To further investigate this curvilinear relationship, we evaluated photosynthesis, leaf starch levels and pod and seed number at phloem-isolated nodes of cultivar Elgin 87 in three greenhouse experiments. The main stem was girdled between the fifth and sixth nodes (unifioliolate node was node one) when the first flowers opened at the sixth node. The main stem above the sixth node was removed and defoliation (five or six levels from 0 to 100%) created a range in assimilate supply. Girdling increased leaf starch levels 3- to 7-fold over non-girdled plants within 7 days. In two experiments starch decreased to control levels within 14–27 days after girdling, but in a third experiment the increase was maintained for 28 days. Defoliation reduced leaf starch levels, with 66–91% defoliation lowering it to the level in the non-girdled controls. Pod and seed number were directly related to assimilate supplies at low levels of assimilate availability when there was no accumulation of starch. There were only relatively small increases in pod and seed number at high levels of assimilate availability, but there were large accumulations of starch in the leaves. Flower and pod abortion was always above 50%, so pod set was not limited by flower availability. The failure of pod and seed number to respond to high levels of assimilate availability suggests that there may be other processes involved in determining pod and seed number at isolated nodes in soybean.

Increasing sink size does not increase photosynthesis during seed filling in soybean

Sink size at phloem-isolated nodes in soybean (Glycine max L. Merrill) was increased to determine if a larger sink would stimulate photosynthesis. A larger sink was created by increasing the number of isolated nodes fed by a single leaf from one to three. Greenhouse grown plants (cultivar Elgin 87) were girdled below node 7 (one-node treatment) or below node 5 (three-node treatment) when the first flowers appeared at node 7. The stem above node 7 was removed at the time of girdling as were the leaves and petioles from nodes 5 and 6 on the three-node treatment. The three-node treatment produced approximately 40% more pods and seeds than the one-node treatment, but this increase in sink size had no effect on carbon exchange rate (CER) or leaf chlorophyll levels (estimated with a SPAD chlorophyll meter) during seed filling. There was no treatment effect on leaf soluble sugars, but starch levels tended to be slightly higher in the three-node treatment. These data provide no evidence that increasing the size of the sink exerts any affect on source activity. Mature weight per seed of the three-node treatment was reduced, but yield was the same as the one-node treatment. Our data are consistent with the concept that the seed sink is soybean is relatively passive and increasing sink size above its normal level does not exert a direct influence on leaf photosynthesis.

Temperature during soybean seed storage and the amount of electrolytes of soaked seeds solution

ABSTRACT: The electrical conductivity test measures the electrolytes that leach out of seeds whenthey are immersed in water and this leakage is an indication of seed vigor. The level of standardizationreached by the procedures of this test is such that the test is recommended for pea seeds andsuggested for other large seeded legumes, including soybean Glycine max (L.) Merrill]. This study[was conducted to contribute to the standardization of this test for soybean seeds by verifying whetherthe seed storage temperature influences the composition of the leachate from soaked seeds solution.Two soybean seed lots of distinct physiological potential were stored in moisture-proof containerseither at constant temperatures of 10°C and 20°C or at the temperature of 20°C during the first sevenmonths of storage followed by a change to 10°C for the rest of the storage time (nine months). Thechemical composition of the soaked water was evaluated every three months from January to October1998. The highest amount of leakage was observed for potassium, followed by calcium and magnesium,iron and sodium regardless of temperature and storage period. The amount of electrolytes in thesoaked water increased as the period of time and the temperature of storage increased. On the otherhand the amount of leakage decrease along the time for those seeds stored at 10°C or transferred fromthe temperature of 20 to that of 10°C. The temperature at which soybean seeds remain during storagemay affect the amount of electrolytes in the soaked water and consequently the results of the electricalconductivity test.Key words: