Seed Nitrogen Content Research Articles

Non-destructive Limits to Seed Growth and Leaf Protease Activities in Nodulating and Non-nodulating Soybean Isolines

Leaf senescence leads to a progressive decline in the photosynthetic competence of the leaf. This paper describes some effects of source:sink imbalance on leaf protein catabolism and senescence in soybean. We manipulated pod growth by restricting 100 or 50 % (PR-100 or PR-50, respectively) of young pods at the R4 stage in plastic drinking straws. This effectively reduces final seed mass without interrupting the vascular connections of pods. Nodulating (NOD+) and non-nodulating (NOD−) isolines of the ‘Clay’ soybean were grown in drainage lysimeters and three pod-restriction (PR) treatments were compared. Pod restriction decreased seed biomass per plant as a result of lower individual seed mass, which was only partially balanced by the increase in seed number. The nitrogen concentration in seeds remained unchanged in NOD+ plants, while it increased with the degree of sink restriction in seeds of NOD− plants. Leaf soluble protein, CO2 exchange rate and seed nitrogen content were consistently lower in NOD− plants; the leaf protein level remained stable with time in PR-100 plants, decreased for PR-50 and dropped for controls. Endoprotease (HBase) and carboxypeptidase (CPase) activities were significantly lower in leaves from PR-100 plants, while aminopeptidase activity was enhanced, indicating a de novo synthesis of leaf protein. This is consistent with the reported accumulation of vegetative storage proteins (VSPs) in soybean and other legumes after moderate or severe sink reduction. Thus, small modifications of the source:sink ratio such as those obtained by the non-destructive PR technique have an impact on leaf protein catabolism. Nodulating and non-nodulating soybean isolines showed similar responses to PR in terms of leaf senescence initiation and progression, but the rate of the processes appear to be largely influenced by plant N status.

Effect of extrusion cooking on structure and functional properties of pea and kidney bean proteins

Pea (Pisum sativum L cv Ballet) and kidney bean (Phaseolus vulgaris L cv Pinto) seeds were extruded at 148 and 156 °C respectively. Protein solubility at various pH values and in various solvents was determined and analysis of protein fractions was carried out by SDS-PAGE. Also, sulphhydryl and disulphide groups, water-holding capacity (WHC), water solubility index (WSI) and oil absorption capacity (OAC) were determined. No changes in total nitrogen content of pea and kidney bean seeds occurred as a result of thermal treatment. Protein solubility from raw and extruded legumes was significantly higher in saline solutions than in water in the pH range 2–10. The solubility of proteins from extruded pea and kidney bean flours was greatly decreased with respect to native flours when extraction was in buffer (pH 7.0) alone. Extraction with buffer containing 2-mercaptoethanol (2-ME) or sodium dodecyl sulphate (SDS), alone or in combination, greatly increased protein extractability. As a result, the relative solubility was nearly 100% in buffer with SDS and 2-ME for both raw and extruded samples. Total and free sulphhydryl group and disulphide contents decreased significantly (P < 0.05) after extrusion cooking. Moreover, extrusion treatment caused major changes in the band patterns of the albumin and globulin fractions obtained by SDS-PAGE. WHC and WSI of extrudates increased significantly in both peas and kidney beans. A significant reduction in OAC was observed in extruded kidney bean flour. © 2000 Society of Chemical Industry

Influence of Nitrogen Availability on Seed Nitrogen Accumulation in Pea

ABSTRACTThe final seed nitrogen concentration (the ratio of seed N and dry matter content) is highly variable in pea (Pisum sativum L.) and N remobilization during seed filling may limit yield by restricting the seed filling period. This study was conducted to determine how seed N accumulation is regulated in pea. The effect of N availability and distribution on individual seed N accumulation rate at different nodes was investigated in three genotypes grown in the field and glasshouse under various levels of N fertilizer, depodding, and defoliation. The N content of vegetative plants parts (stems, leaves, podwalls) and seeds from three mainstem nodes were regularly recorded. Plant N available to the seeds at a given time was assessed as the sum of the amount of N still available for remobilization in vegetative parts and the amount of N accumulated by the plant. The results indicate that the N available in a plant at a given time can be considered as one common pool accessible to all seeds and equitably divided among them. Thus, the rate of individual seed N accumulation was unaffected by intra‐plant position of seeds. This rate increased with the amount of N available per seed until a maximum rate of individual seed N accumulation (43 μg seed−1 degree‐day−1) was reached. The relationship established between the rate of individual seed N accumulation and the plant N available per seed will be useful to improve models simulating yield and final seed N concentration in legumes.

Dependence of seed nitrogen concentration on plant nitrogen availability during the seed filling in pea

The final seed nitrogen (N) concentration of the pea ( Pisum sativum L.) varies greatly with environment. These variations seem to be related to N availability in the plant. To understand such an effect, the pattern of seed N concentration during seed filling was analysed at a given node as the ratio of seed N accumulation rate and seed dry matter accumulation rate. Three experiments were conducted in the field and glasshouse. Pea genotypes ‘Solara’, ‘Frisson’ and its non-nodulating mutant ‘P2’ were grown and different treatments were applied to manipulate N availability during seed filling. Treatments included N supply (addition of N fertiliser), depodding (removal of pods) and defoliation (removal of leaves). Changing the N availability during the seed filling affected the seed N concentration in all experiments. Seed N concentration increased or decreased when plant N availability was enhanced or reduced respectively. The response of seed N concentration to N availability was not caused by variations in the seed dry matter accumulation rate, as this rate is fixed at the beginning of the seed filling. Changes in seed N concentrations were mainly caused by variations in the seed N accumulation rates according to N availability in the plant. The results demonstrate that seed N and dry matter accumulations are not similarly controlled during seed filling. Unlike the seed dry matter accumulation rate, the seed N accumulation rate at a given node can vary with time and depends mainly on N availability in the plant regardless of genotype. However, seeds seem to have minimal N requirements (approximately 2.6 mg g −1 of N) whatever the genotype.

Evaluation of determinate and indeterminate pinto bean cultivars under an available soil moisture gradient

Determinate dry bean (Phaseolus vulgaris L.) cultivars are preferred under irrigation in western Canada. Expansion of dry bean production into dryland areas in western Canada requires re-evaluation of the best growth habit. The objective of this study was to compare determinate and indeterminate pinto bean cultivars for seed yield and other agronomic traits under a soil moisture gradient. Six pinto bean cultivars, three determinate and three indeterminate, were grown under an available soil moisture gradient in Saskatoon SK, Canada at one environment in 1993 and at two environments in 1994. An increase in available soil moisture, induced by growing rows of pinto bean plants at increasing distances from a spring-seeded winter barley (Hordeum vulgare L.) strip, increased number of pods per plant, plant height, seed weight, seed yield and seed nitrogen concentration for both determinate and indeterminate cultivars of pinto bean. Indeterminate cultivars, however, were taller and higher yielding than determinate cultivars at every level of soil moisture under these dryland conditions. Key words: Phaseolus vulgaris, dry bean, pinto bean, soil moisture, drought stress

Nutritional Effects of Seed Fall during Mast Years in Boreal Forest

Several hypotheses have been advanced to explain why masting (i.e. episodic. synchronized seed production) occurs in coniferous trees, as well as aspects of its evolution and importance for reproductive success. However, the nutritional effects of the large number of seeds that simultaneously fall to the ground, die and decompose in the soil during mast years have not previously been considered. Norway spruce (Picea abies) has a typical masting behavior and may add 5-10 kg N ha -1 yr -1 through seed fall in mast years in the boreal zone of Northern Sweden. We performed a field experiment in a strongly nitrogen-limited forest in N Sweden where pre-killed Norway spruce seeds were added to plots at an amount comparable to 7 kg N ha -1 yr - '. Seeds were added to an understory vegetation dominated by ericaceous shrubs and two different bottom layer components, Pleurozium schreberi and Cladina spp. Seedlings of Scots pine (Pinus sylvestris) were then planted in the same plots to evaluate growth response to and availability of nutrients from the dead Norway spruce seeds that were added to the two plant communities. Seedlings of Scots pine grew larger and acquired more nitrogen when dead seeds were added in plots dominated by Cladina spp. However, in plots dominated by P. schreberi, uptake of nutrients from dead seeds by Scots pine was blocked by P. schreberi except in treatments where the P. schreberi moss carpet was experimentally disturbed. Analysis of nutrients in field- and bottom-layer vegetation showed that P. schreberi acquired a large proportion (80%) of N released from dead seeds while the ericaceous dwarf shrub Vaccinium vitis-idaea and the Cladina spp. macrolichens gained much less nitrogen. The rates of nutrient release and decomposition of dead Norway spruce seeds were compared with Norway spruce needle litter in a litter bag experiment. Dead Norway spruce seeds decomposed at much higher rates than needle litter, and 65-75% of the seed nitrogen content was lost during the first growing season. We hypothesize that, if germination conditions (type of bottom-layer vegetation) allow tree seedlings to emerge, their survival, growth and nutrition may be enhanced by the nutrient flush from dead seeds and germlings in seed mast years and that this may aid in establishment of a new seedling cohort. Nutrient pulses from dead seeds during mast years may help to explain the strongly pulsed natural tree regeneration patterns that have been found in mixed late successional coniferous forests.

Costs of Reproduction in Subarctic Ranunculus acris: A Five-Year Field Experiment

A fundamental prediction of life-history theory is that current reproduction reduces chances of future reproductive activity and success. In plants, however, short-term manipulative experiments often fail to support this. One reason may be the within-species variation in reproductive frequency and success among habitats and years. In this study, Ranunculus acris in three subarctic habitat types (a meadow forest, a fen woodland, and an alpine meadow) was prevented from seed production by flower bud removal during four years. The objective was to test if this treatment increased future reproductive activity relative to naturally reproducing plants. In the fifth year, all plants were left intact to test if natural, as compared to previously prevented reproduction, entailed negative effects on future fruiting success, seed set, fecundity and/or seed quality. In all habitat types, there were costs of reproduction in terms of reduced future reproductive activity, as naturally reproducing plants produced sexual shoots less frequently than plants for which flower buds were removed. In terms of other traits, fecundity costs of reproduction were detected at two habitats. In the meadow forest, naturally reproducing plants had fewer seeds, due to fewer flowers, compared to previously bud-removed plants. At the alpine meadow, reproduction entailed reduced future seed nitrogen content, and thus, potentially lower future offspring quality. In contrast, at the fen woodland, seeds of naturally reproducing plants weighed more than those of previously bud-removed plants. No costs were found in terms of fruiting success (the proportion of all flowers that set at least one seed), ovules per flower or seed set at any habitat. These results show that costs of reproduction varied among habitat types and were detectable in some, but not all selected traits. We should therefore not expect reproductive costs to be equally expressed in all environments.

Seed nitrogen accelerates the rates of germination, emergence, and establishment of rice plants

We examined the effect of seed nitrogen content on the rates of germination, emergence, and establishment of rice plants. Several seed lots with various nitrogen contents were obtained from the parent plants grown under 3 planting densities and 4 rates of nitrogen application. There was a clear negative correlation between the seed nitrogen content and germination time (R =0.88), whereas the correlation between the seed dry weight and germination time was very low (R=0.23). The seed lot with a high nitrogen content absorbed water faster than the seed lot with a low nitrogen content, especially on the first day after soaking, and also showed a faster emergence, exsertion of the fourth leave, and a more uniform germination. Moreover, the increase of seed nitrogen content as well as the prolongation of soaking time resulted in a uniform emergence. Therefore, seed vigor is likely to be enhanced by the increase of seed nitrogen content, which may be achieved by an adequate application of a large amount of nitrogen to parent plants. Consequently, it may be possible to improve and stabilize the establishment of directly sown rice plants by the sowing of vigorous seeds with a high nitrogen content.

Ammonium nitrogen in soil solution and seed nitrogen affect the percentage of establishment of rice plants in submerged soil

The effect of ammonium nitrogen concentration in soil solution on the establishment of rice plants was examined. The increase of the concentration decreased the percentage of establishment of seeds sown in submerged soil, although most of seeds sown on submerged soil became established. Therefore, the increase of ammonium nitrogen concentration in soil solution may impair the establishment of seeds sown in submerged soil, which would occur presumably because the increase delays the spear growth and emergence without the decline of soil redox potential. Several seed lots with various nitrogen contents were obtained from rice plants grown under various conditions. The percentage of establishment of low-nitrogen seeds sown in submerged soil was much lower than that of high-nitrogen seeds, especially in soils whose solution contained a large amount of ammonium nitrogen. However, the difference in the percentage of establishment between high- and low-nitrogen seeds sown on submerged soil was much smaller. Therefore, the increase of seed nitrogen content may improve the percentage of establishment of seeds sown in submerged soil, presumably because the increase accelerates the spear growth and emergence. Consequently, for direct sowing in submerged soil, ammonium nitrogen concentration in soil solution should remain low because ammonium nitrogen exerts an adverse effect on seedling establishment, and vigorous seeds with a high nitrogen content should be sown because seed nitrogen exerts a beneficial effect on seedling establishment.

Effect of rhizobia inoculation on the seed (herbage) production of mungbean (Phaseolus aureusRoxb.) grown at Uzbekistan

In field trials conducted at Uzbekistan, inoculation of mungbean (Phaseolus aureusRoxb.) with commercial strain CIAM1901 ofBradyrhizobiumsp. (Phaseolus) increased (on average for two cultivars) the herbage mass by 46·6±6·0%, seed mass by 39·2 ± 3·6%, mass of 1000 seeds by 16·0 ± 0·8%, nitrogen content in seeds by 58·3 ± 8·9%, starch content in seeds by 30·0 ± 5·5% and number of nodules by 254%. Inoculation with this strain produced the same herbage (seed) mass as NH4NO3application (120kg ha−1of N), while the combined treatment of rhizobia and 60kg ha−1of N produced significantly higher yields than rhizobia inoculation alone or application of 120kg ha−1of N. The rhizobia strain M11 was isolated from Uzbekistan soils and significantly exceeded the commercial strain CIAM1901 in its influence on herbage mass by 3·9–10·6%. Two-factor analysis of variance demonstrated that the herbage mass and number of nodules are controlled mainly by the rhizobia strain genotypes, while seed yield, number of pods, mass of 1000 seeds and N and starch content in seeds are influenced by the plant cultivar genotypes.

Influence of Seed Phosphorus Content on Seedling Growth in Wheat: Implications for Organic and Conventional Farm Management in South East Australia

ABSTRACTFertilizer trials involving application of phosphorus (P) as both rock phosphate and superphosphate were conducted on adjacent organic and conventional farms in SE-Australia in 1991 and 1992. Seed P concentration and seed P content (P concentration multiplied by seed mass) were higher on the organic farm in 1991 when rainfall was below average, but were higher on the conventional farm in 1992, a wetter year. Addition of superphosphate increased seed P content and concentration on both farms in 1991 and on the organic farm in 1992. These results indicated that seed produced on a conventional farm with addition of fertilizers containing soluble P is likely to have a significantly greater seed P content than seed produced on an organic farm with application of rock phosphate or with no fertilizer addition.Seed harvested from these trials were sorted into small and large seeds to provide seed with a wide range of P contents: 29–169 μg. Heavier seeds had a higher P content. Seeds were germinated and grown for three weeks in field soil in a glasshouse. Heavier seeds were more likely to have germinated after 48 h. Seedling shoot dry mass after three weeks was significantly positively influenced by seed P concentration and seed mass, that is, shoot dry mass positively correlated with seed P content. Root dry mass after three weeks also correlated positively with seed P content. However, seedling relative growth rate was not correlated with seed P content. Seed nitrogen content and concentration did not significantly influence seedling growth. The origin of the seed, that is, whether it was produced on an organic or conventional farm, the year it was harvested, and the fertilizer treatment the parent crop had received all did not significantly affect seedling growth. Enhanced early growth of seedlings may be particularly important on organic farms where herbicides are not applied and seedlings may face increased weed competition. It is suggested that organic farmers may benefit from grading out heavier seeds for sowing.

Influence of Split Ammonium Sulfate on Nitrogen Availability from 15N-Labeled Matured Soybean as Green Manure for Buckwheat(Fagopyrum esculentum Moench).

A pot experiment was conducted at the Experimental Farm of Gifu University to study the effects of 15N-labeled matured soybean as green manure, with and without splitted ammonium sulfate, on the growth and nitrogen (N) nutrition of buckwheat. Crop yield ranged between 0.7 to 2.8g/pot. Applying green manure together with ammonium sulfate split in a ratio of 2:1:1 enhanced higher dry matter yield and seed nitrogen content. The crop derived 5.3% N from green manure when applied alone, and between 7.4 to 13.4% N when applied in combination with split ammonium sulfate and basal P2O5 and K2O. Nitrogen from green manure was most efficiently utilizcd by the crop when green manure was applied together with split ammonium sulphate at the ratio of 2:1:1. The "added nitrogen interaction" (ANI) was positive, indicating that the green manure did not furnish the crop with only N but also caused the effective utilization of soil resources as well.

Nitrogenase activity, nodulation and seed production inVicia fabaas affected by methabenzthiazuron

SUMMARYThe effects of recommended application rates of the herbicide methabenzthiazuron (MBT) (2·5 and 4 kg ha-1) on vegetative growth, nodulation and N2fixation in faba bean plants were studied in the field at Barcelona during 1990 and 1991. Nodulation, specific nitrogenase activity (SNA) of excised nodules, dry weight of plants and seed nitrogen content at harvest were compared in MBT-treated, hand-weeded (HW) and control plants. In 1990 the higher MBT application (4 kg ha-1) caused a decrease in nodulation and acetylene reduction activity (ARA) per plant during the mid-period of pod-fill, and also in vegetative growth, seed production and N content of seeds, whereas the lower rate (2·5 kg ha-1) did not have a significant effect on any of these factors. However, in 1991, growth and herbicide effects were different from those in the previous year. No inhibitory effect of MBT on nodulation, SNA or ARA per plant was observed and vegetative growth, seed production and N content of seeds were not significantly different from those of plants from HW plots. In 1991, both 4 kg ha-1of MBT and the HW treatment caused a stimulatory effect on seed N recovery and yield compared to control plants. Rates of ARA per plant at late pod-fill were positively related to N recovery by seeds. These differences could have been due to the low rainfall in 1990, which may have permitted MBT to remain in the soil for a longer period and to inhibit N fixation and plant growth in theVicia fabacrop.

Effects of Ascochyta blight caused by Mycosphaerella pinodes on the translocation of carbohydrates and nitrogenous compounds from the leaf and hull to the seed of dried‐pea

The effects of Ascochyta blight (caused by Mycosphaerella pinodes) on changes in dry weight and in water, carbohydrate, nitrogen and free amino acid contents were studied in the leaf, the hull and the seeds on the second fructifer node of pea (Pisum sativum). Pea plants were grown in a glasshouse and inoculated with various concentrations of conidia at the beginning of seed filling, with uninoculated plants as controls. Disease induced a premature water loss of hulls and leaves, accelerated seed desiccation and reduced seed weight. Biochemical analyses revealed a decline in the carbohydrate content and a lower nitrogen remobilization in diseased leaves and hulls. Thus, Ascochyta blight alters carbohydrate metabolism, protein remobilization and free amino acid translocation from these organs. Disease also reduced carbohydrate and nitrogen content in seeds and, in case of high disease severity, the carbohydrate/nitrogen ratio in the seeds was also affected (seed protein concentration increased and starch concentration decreased).

Effect of methabenzthiazuron on growth and nitrogenase activity in Vicia faba

The effects of the herbicide methabenzthiazuron (175 and 220 g ha-1) on vegetative and reproductive growth, nodulation and nitrogenase activity of Vicia faba were studied in the field under Mediterranean conditions. Nitrogenase activity of excised nodules was estimated using the acetylene reduction assay four times during the developmental period. Leaf area index, dry weight and nitrogen content of the different parts of the plants were measured. Methabenzthiazuron-treated plants showed an increase in nodulation, nitrogenase activity and vegetative growth at early pod fill. Methabenzthiazuron also caused an increase in leaf N content and fruits. These were transient effects found during early and mid pot fill. Nevertheless, plants treated with these sublethal doses of herbicide improved seed production and nitrogen content of seeds at harvest time. The stimulatory effect of methabenzthiazuron on N2 fixation and vegetative growth seems not be related with the transient stimulatory effect on photosynthetic capacity, also caused by the herbicide, since the stimulatory effect on N2 fixation was apparent during pod fill, when photosynthetic capacity declined and was not modified by methabenzthiazuron.

Nitrogen content of seeds of Zostera capricorni and Zostera marina

The nitrogen (N) content of mature seeds of Zostera capricorni Aschers, was found by Kjeldahl analysis to be 1.70% of dry matter. The larger seeds of Zostera marina L. were analysed individually by the more sensitive Heraeus procedure. The mean seed N content of these seeds was 1.37% of dry matter (range 1.20–1.56%, standard deviation (SD) 0.13). Seed water contents were similar for both species, 34.7% and 32.3% of seed mass, respectively. The ecological, physiological and evolutionary significance of these observations is discussed.

The Influence of nutrient nitrogen source on the growth and productivity of sunflower (Helianthus annuus var. Dwarf Sungold)

Helianthus annuus L. plants were germinated and grown under environmentally controlled conditions in Long Ashton nutrient medium using nitrogen supplied in the form of 2 mM nitrate, 2 mM ammonium or 1 mM nitrate + 1 mM ammonium and harvested after either 4 or 10 weeks of growth. Ammonium oxidation was prevented by the inclusion of nitrapyrin to the nutrient solutions. After 4 weeks of growth the mixed feed plants had 18% greater leaf area than the nitrate-only and 23% greater leaf area than the ammonium-only fed plants. Both the 4- and 10-week-old mixed-feed plants had greater fresh and dry mass, lower transpiration rates and greater water use efficiency than plants of either of the other two nutrient regimes. The fruit mass for the mixed feed plants was 25% greater than the nitrate-only and more than double that of the ammonium-only fed plants. The bound nitrogen content of both the nitrate-only and mixed-feed plants was over 30% greater than that of the ammonium-only fed plants, although individual seed nitrogen concentration was the same. Combined ammonium + nitrate is therefore considered to be a more efficient nitrogen source for sunflower production than either nitrate or ammonium fed singly.

Nitrogen Distribution and Dry Matter Accumulation in Oilseed Rape (Brassica napus L.) as Influenced by N Nutrition

AbstractA greenhouse experiment was conducted to study N distribution and dry matter accumulation in oilseed rape (Brassica napus L. cv. Callypso) in relation to N supply. Three levels of N supply (30, 100 or 170 ppm N) were tested as treatments. Stem and leaf dry weights increased at higher N fertility up to 170 ppm N but root dry weight did not respond to N. Dry matter yield during the vegetative phase was seriously depressed by N deficiency. Most of the plant dry matter was accumulated in the lower segments of the stem and roots. Dry weights of stem and axillary branches increased significantly as N supply increased up to 100 ppm N. Although hull dry weight increased with N supply up to 100 ppm N, seed dry weight did not respond to N. High root N concentration was maintained at 100 or 170 ppm N; but declined as plants advanced in age. N content of leaf and stem also declined with time. Leaf growth was particularly responsive to N fertility and N was mobilised from the older to the younger leaves over time. Nitrogen content of hulls and seeds increased significantly with N supply but N was translocated from the vegetative into the generative organs or from older into younger tissues during pod development. There is need for proper N fertility management in order to achieve successful rape production.

Effect of seed weight, and seed phosphorus and nitrogen concentrations on the early growth of wheat seedlings

Wheat seeds of uniform weight selected from 3 batches of seed with phosphorus (P) concentrations of 0.14, 0.17 and 0.19% were sown in soil with a range of applied P treatments. Seedling emergence was more rapid the higher the seed P. By 25 days after sowing all plants grown at the highest concentration of applied P were similar in size, but with lower applied P, plants from seed with higher P concentrations had an advantage. A range of weight classes was selected from 2 batches of seed raised at low or high concentrations of P. There were 11 groups ranging in mean seed weight from 28 to 58 mg and in seed P from 0.13 to 0.37%. Seedlings from the high P batch had larger first leaves, a higher dry weight, and longer roots than those from the low P batch when grown in a highly P-deficient sand culture system. Within each batch, heavier seeds produced larger leaves, heavier plants and longer roots. The yield differences between batches were largely accounted for if the seed P content (�g P/seed) was considered rather than either percentage P or seed weight alone. Differences in seed nitrogen concentration or content did not consistently account for differences in seedling growth.

Post-flowering photoperiodic effect on reproductive efficiency and seed growth in soybean

Photoperiod not only controls soybean ( Glycine max (L.) Merr.) flower induction, but also affects later stages of reproductive growth. Its effect on different yield-determining processes, however, is not well understood. The objective of this work was to determine the post-flowering photoperiodic effect on yield components, reproductive efficiency, and assimilate partitioning to developing seeds in the soybean cultivar Hood 75. Plants were induced to flower in short days. After flowering, three photoperiodic treatments were applied: (1) short days ( SD); (2) short days with interrupted night ( NI); and (3) a sequential combination of both ( NI-SD). Flower differentation, pod lenthening, and seed-filling periods were shortest in SD, intermediate in NI-SD, and longest in NI. The number of differentiated flowers, pods, and seeds were doubled by NI as compared to SD, but the percentages of setting and retained pods were not changed. The percentage of seeds that completed development was, however, increased by SD. More assimilates were partitioned into the seeds in SD, while NI increased the proportion partitioned into shoots. The nitrogen content of seeds was similar in NI and SD, while NI-SD seeds showed a slight increment in their N content. The N content of carpels, however, was high in NI, medium in NI-SD, and low in SD. Compared to NI, duration of the seed-filling period was decreased but the seed-growth rate and final seed size were increased by SD. Final yield was similar for plants in NI or SD, because seed number and seed growth rate compensated each other. Plants in NI-SD has a significantly higher yield, however, as a result of sequential increase in seed number caused by NI, followed by an increase in the rate of seed-filling caused by SD. The results support the existence of two sequential photoperiodic effects on soybean reproductive development: (1), the number of growing seeds is enhanced by NI and decreased by SD; and (2), the assimilate partitioning to seeds is enhanced by SD and decreased by NI. A non-destructive method for seed growth-rate calculation is also described.