In this study, the yield and yield components were studied using a conventional variety Zhongshuang 11 (ZS 11) and a hybrid variety Zhongyouza 12 (ZYZ 12) at varying plant densities. The increase in plant density led to an initial increase in seed yield and pod numbers per unit area, followed by a decrease. The optimal plant density was 58.5 × 104 plants ha−1 in both ZS 11 and ZYZ 12. The further researches on physiological traits showed a rapid decrease in the green leaf area index (GLAI) and chlorophyll content and a remarkable increase in malondialdehyde content in high plant density (HPD) population than did the low plant density (LPD) population, which indicated the rapid leaf senescence. However, HPD had higher values in terms of pod area index (PAI), pod photosynthesis, and radiation use efficiency (RUE) after peak anthesis. A significantly higher level of dry matter accumulation and nitrogen utilization efficiency were observed, which resulted in higher yield. HPD resulted in a rapid decrease in root morphological parameters (root length, root tips, root surface area, and root volume). These results suggested that increasing the plant density within a certain range was a promising option for high seed yield in winter rapeseed in China.
Oilseed rape is one of the most important sources of edible oil in the human diet. In recent years, the seed yield has lagged behind the increasing demands driven by population growth. Therefore, the yields of rapeseed crops must be significantly increased [
Plant density is an important factor affecting seed yield and yield components of oilseed rape [
Previous studies have demonstrated that photosynthate supply plays an important role in pod and seed development [
In winter oilseed rape, the leaves are the main photosynthetic source before anthesis, whereas the lower part of the plant canopy becomes part of the source after anthesis, and during pod development, the photosynthetic rate from green pods during seed filling contributes to approximately 2/3 of the total seed weight [
The field trials were conducted from 2010 to 2014 at Yangluo Experimental Station of the Oil Crops Research Institute in Wuhan, Hubei, China (30°6′N, 114°1′E), which is located approximately in the center of the Yangtze River basin. This area is characterized by yellow-brown soil in the experimental field. The surface soil (0–30 cm) was sampled at the beginning of each growing season. The soil samples were air dried, ground, and analyzed for pH value, dissolved organic carbon (DOC), total nitrogen, alkaline digested N, available phosphorus, available potassium, and available boron contents (Table
Soil properties measured at the beginning of each growing season from 2010 to 2014.
Parameter | Unit | 2010-2011 | 2011-2012 | 2012-2013 | 2013-2014 |
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pH | 6.65 | 6.82 | 6.70 | 6.91 | |
Dissolved organic carbon | mg kg−1 | 85.3 | 95.1 | 104.1 | 100.6 |
Total N | g kg−1 | 1.51 | 1.69 | 1.79 | 1.56 |
Alkaline digested N | mg kg−1 | 74.2 | 78.2 | 79.3 | 77.8 |
Available phosphorus | mg kg−1 | 46.2 | 45.7 | 50.5 | 49.7 |
Available potassium | mg kg−1 | 63.1 | 60.3 | 68.4 | 65.4 |
Available boron | mg kg−1 | 0.45 | 0.51 | 0.66 | 0.63 |
The first experiment was conducted during the 2010-2011 and 2011-2012 growing seasons to evaluate the effects of plant density on seed yield and yield components. Conventional winter rapeseed variety Zhongshuang 11 (ZS 11) and the hybrid variety Zhongyouza 12 (ZYZ 12), two elite winter rapeseed varieties commonly grown in the Yangtze River basin, were used. The seeds were sown on 28 September in both 2010 and 2011. A split-plot design was used with three replicates. The main plots comprised five plant densities (27.0 × 104, 37.5 × 104, 48.0 × 104, 58.5 × 104, and 69.0 × 104 plants ha−1), and the subplots comprised two varieties. Each subplot was 2 × 10 m, with rows approximately 30–35 cm apart (three rows per meter). The plants were finalized by hand when the seedlings had fully developed 4-5 true leaves, and the spaces between seedlings ranged from 4 to 11 cm to achieve different planting densities. Each plot was fertilized at the average fertilizer level in the Yangtze River basin with urea (195 kg N ha−1), superphosphate (75 kg P2O5 ha−1), potassium chloride (105 kg K2O ha−1), and borax (9 kg boron ha−1). Approximately 60% of the nitrogen fertilizer was applied at sowing and the remaining 40% of the nitrogen fertilizer was applied at the seedling stage, whereas phosphorus, potassium, and borax were all applied at sowing.
The second experiment was conducted during the 2012-2013 and 2013-2014 seasons to study physiological traits of different populations. In the first experiment, low seed yield was obtained at a plant density of 27.0 × 104 plant ha−1, referred to as the low plant density (LPD) population. The highest yield was obtained at a plant density of 58.5 × 104 plant ha−1, referred to as the high plant density (HPD) population for both varieties. The second experiment was a randomized complete block design with three replicates. The seeds were sown on 28 September in both 2012 and 2013. The plot area was 10 m long × 2 m wide and comprised 30 rows. A 1 m border surrounded each plot. The application rates of N, P2O5, and K2O were the same as those used in the 2010-2011 and 2011-2012 growing seasons.
In 2010–2014 growing seasons, at maturity, plants per unit area (m2) were sampled, and the yield components (i.e., pods per unit area, seeds per pod, and 1000-seed weight) at each plot were determined. Seed yield was determined by harvesting the plants of 5 m2 area in each plot, and the seed yields per unit area (ha−1) were calculated, with 9% standard moisture content.
In 2012-2013 and 2013-2014 seasons, the chlorophyll and malondialdehyde contents in the leaves of HPD population and LPD population were determined. The frozen leaves (0.2 g) were first ground to a fine powder in liquid nitrogen, and chlorophyll was extracted after immersing the powder with cold acetone overnight at 4°C. The supernatant containing chlorophyll was generated after centrifugation at 10,000 ×g for 30 min. The residue was washed several times with cold acetone until it became colorless. The pooled supernatant was diluted to 10 mL with acetone until the final acetone concentration was 80%. The chlorophyll content per fresh weight of leaves was calculated as previously described [
In 2012-2013 and 2013-2014 seasons, the green leaf area was measured by passing the leaves through a LI-3100 leaf area meter (LiCor, Lincoln, NE, USA) at 7-day intervals after peak anthesis. The gas exchange analysis was conducted in the LPD and HPD populations of two varieties using a Portable Photosynthesis System (LI-6400; LiCor) on the leaves from 09:00 to 11:00. The net photosynthetic rates (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) were determined. The data were collected automatically every 2-3 min with 10 replications for every plot.
At the seed-filling stage, fifty pods on the main inflorescences and all of the branches were randomly sampled to measure the pod length and width, and the pod wall area was calculated according to Clarke (1978) [
Canopy radiation interception was measured at 7-day intervals from flowering stage to maturity using SunScan Canopy Analysis System (Delta-T Devices Ltd., UK). To measure the transmitted radiation, the 1 m probe was placed perpendicular to rows near soil surface for each plot. Another sensor (model BF5) was located outside the canopy for measurement of incident photosynthetically active radiation (PAR) [
Different aerial organs at maturity at different densities were ground into powder, and an appropriate amount of plant material was used to determine the total nitrogen content using a modified Kjeldahl digestion method [
Root digging was performed according to Majdi (1996) [
We performed multiway ANOVA with critical values of
In the first experiment, which was conducted during the 2010-2011 and 2011-2012 growing seasons, the pod numbers per unit area were initially positively and then negatively affected after increasing plant density. Compared with a density of 27.0 × 104 plant ha−1, the pod numbers per unit area of ZS 11 and ZYZ 12 varieties increased significantly at 48.0 × 104 plant ha−1 and 58.5 × 104 plant ha−1, respectively, and the maximum pod numbers per unit area were obtained at a plant density of 58.5 × 104 plant ha−1 for both varieties (Table
Yield components of ZS 11 and ZYZ 12 populations at varying plant densities during the 2010-2011 and 2011-2012 growing seasons.
Year | Variety | Plant density (×104 plants ha−1) | Pod numbers per unit area (×103 m−2) | Seeds per pod | 1000-seed weight (g) |
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2010-2011 | ZS 11 | 27.0 |
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ZYZ 12 | 27.0 |
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2011-2012 | ZS 11 | 27.0 |
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ZYZ 12 | 27.0 |
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Experimental treatments had pronounced effects on seeds per pod. The seeds per pod of the two varieties were significantly decreased with increasing plant densities, but the 1000-seed weight showed no significant differences at the examined plant densities. The ANOVA results showed that the pod numbers and seeds per pod were obviously affected not only by the year, variety, and plant density but also by plant interactions, whereas the 1000-seed weight was not significantly affected.
The seed yields per unit area were also initially positively and then negatively affected with increasing plant density during the 2010-2011 and 2011-2012 growing seasons (Figure
Seed yield per unit area (kg ha−1) at various plant densities during the 2010-2011 and 2011-2012 growing seasons. (a) Seed yield per unit area (kg ha−1) at various plant densities during the 2010-2011. (b) Seed yield per unit area (kg ha−1) at various plant densities during the 2011-2012. Different letters indicate significant differences at
The highest and lowest values of the seed yields per plot were obtained at 58.5 × 104 and 27.0 × 104 plant ha−1 during the 2010-2011 and 2011-2012 growing seasons, respectively. Compared with 27.0 × 104 plant ha−1, the seed yields per unit area at 58.5 × 104 plant ha−1 for ZS 11 and ZYZ 12 significantly increased 23.3% and 18.5%, respectively, during the 2010-2011 season and 27.6% and 26.7%, respectively, during the 2011-2012 season. In addition, the pod numbers per unit area displayed a strong correlation with seed yield (
Regression of seed yield (kg ha−1) over pod numbers per unit area in the two varieties across the 2010-2011 and 2011-2012 growing seasons.
The chlorophyll content in the leaves decreased more rapidly in HPD than in LPD, and HPD showed a lower chlorophyll content (Figures
Leaf chlorophyll and malondialdehyde contents in the low plant density (LPD) population and high plant density (HPD) population in ZS 11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2012-2013 and 2013-2014 growing seasons. (a, b) Leaf chlorophyll in LPD and HPD in ZS 11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2012-2013 and 2013-2014 growing seasons. (c, d) Malondialdehyde contents in LPD and HPD in ZS 11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2012-2013 and 2013-2014 growing seasons. The bars indicate the SD.
Figure
Green leaf area index (GLAI) and pod area index (PAI) in the low plan density (LPD) population and high plant density (HPD) population in ZS 11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2012-2013 and 2013-2014 growing seasons. (a, b) GLAI in LPD and HPD in ZS 11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2012-2013 and 2013-2014 growing seasons. (c, d) PAI in LPD and HPD in ZS 11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2012-2013 and 2013-2014 growing seasons. The bars indicate the SD.
The GLAI in both HPD and LPD rapidly decreased after peak anthesis, and this value was lower in HPD than in LPD after 14 DAPA. During the 2012-2013 season, the GLAI from 0 to 28 days after peak anthesis in HPD decreased 85.5% and 84.7% in ZS 11 and ZYZ 12, respectively, whereas the GLAI in LPD decreased 64.0% and 69.3% in ZS 11 and in ZYZ 12, respectively (Figure
In both seasons, the photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) of leaves decreased rapidly after flowering, whereas the values declined rapidly in the high-yield population 21 days after peak anthesis (Figures
The leaf photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), transpiration rate (Tr), and pod photosynthetic rate in the low plan density (LPD) population and high plant density (HPD) population in ZS11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2012-2013 growing seasons. (a) Pn, (b) Gs, (c) Ci, (d) Tr, and (e) pod photosynthetic rate in LPD and HPD populations in ZS11 and ZYZ12 at 7-day intervals after peak anthesis during the 2012-2013 growing seasons. The bars indicate the SD.
The leaf photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), transpiration rate (Tr), and pod photosynthetic rate in the low plan density (LPD) population and high plant density (HPD) population in ZS11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2013-2014 growing seasons. (a) Pn, (b), Gs (c), Ci (d), Tr, and (e) pod photosynthetic rate in LPD and HPD populations in ZS11 and ZYZ12 at 7-day intervals after peak anthesis during the 2013-2014 growing seasons. The bars indicate the SD.
High plant density population had slightly lower accumulated incident radiation than the low plant density population owing to their shorter growth duration in two growing seasons in two varieties (Table
Radiation use efficiency and its related parameters for low plant density (LPD) population and high plant density (HPD) population in ZS 11 and ZYZ 12 from flowering to maturity during 2012-2013 and 2013-2014 growing seasons.
Sowing date (month/day) | Plant density (×104 plants ha−1) | Incident radiation (MJ m−2) | Intercepted radiation (MJ m−2) | Intercepted percent (%) | Total dry weight (g m−2) | Radiation use efficiency (g MJ−1) |
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However, the intercepted radiation was significantly higher in HPD population corresponding to their LPD populations. It is interesting that the higher canopy radiation interception in HPD population was due to the higher PAI. Compared with LPD, the dry matter weight in HPD increased 42.3% and 47.4% in ZS 11 during the 2012-2013 and 2013-2014 seasons, respectively, and the dry matter weight in ZYZ 12 increased 46.7% and 55.1%, respectively. In 2012-2013 and 2013-2014 seasons, HPD population had 18.7% and 24.3% higher RUE than LPD population in ZS 11, respectively. HPD population had 20.5% and 38.1% higher RUE than LPD population in ZYZ 12, respectively. The ANOVA results showed that the incident radiation, intercepted radiation, intercepted percent, total dry weight, and radiation use efficiency were all significantly or extremely significantly affected not only by the year, variety, and plant density but also by plant interactions.
The nitrogen utilization efficiency increased 25.8% and 24.2% in HPD of ZS 11, respectively, and 14.0% and 43.7% in HPD of ZYZ 12 during the 2012-2013 and 2013-2014 seasons, respectively (Figure
Nitrogen utilization efficiency and nitrogen harvest index (NHI) in the low plant density (LPD) population and high plant density (HPD) population in ZS 11 and ZYZ 12 during the 2012-2013 and 2013-2014 growing seasons. (a) Nitrogen utilization efficiency in LPD and HPD in ZS 11 and ZYZ 12 during the 2012-2013 and 2013-2014 growing seasons. (b) NHI in LPD and HPD in ZS 11 and ZYZ 12 during the 2012-2013 and 2013-2014 growing seasons. Different letters indicate significant differences at
The dry matter weight, radiation use efficiency, nitrogen utilization efficiency, and nitrogen harvest index were significantly correlated with seed yield (
Regression of seed yield (kg ha−1) over dry matter weight, radiation use efficiency, nitrogen utilization efficiency, and nitrogen harvest index in the two varieties across the 2012-2013 and 2013-2014 growing seasons. (a) Regression of seed yield (kg ha−1) over dry matter weight in the two varieties across the 2012-2013 and 2013-2014 growing seasons. (b) Regression of seed yield (kg ha−1) over radiation use efficiency in the two varieties across the 2012-2013 and 2013-2014 growing seasons. (c) Regression of seed yield (kg ha−1) over nitrogen utilization efficiency in the two varieties across the 2012-2013 and 2013-2014 growing seasons. (d) Regression of seed yield (kg ha−1) over nitrogen harvest index in the two varieties across the 2012-2013 and 2013-2014 growing seasons.
For both HPD and LPD populations in the two varieties, the root length, root tips, root surface area, and root volume per unit area declined after peak anthesis (Table
Root length, number of root tips, root surface area, and root volume per unit area (0.25 m3) for low plant density population (LPD) and high plant density population (HPD) in ZS 11 and ZYZ 12 at 7-day intervals after peak anthesis during the 2012-2013 and 2013-2014 growing seasons.
Year | Variety | Day after peak anthesis (DAPA) | Root length (m) | Root tips (×103) | Root surface area (m2) | Root volume (dm3) | ||||
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LPD | HPD | LPD | HPD | LPD | HPD | LPD | HPD | |||
2012-2013 | ZS 11 | 0 |
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2013-2014 | ZS11 | 0 |
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During the 2012-2013 season, these values decreased by 78.3%, 43.9%, 64.5%, and 45.3%, respectively, at 28 DAPA compared with 0 DAPA in HPD of ZS 11, whereas a decrease of 68.9%, 41.9%, 61.5%, and 37.8%, respectively, was observed in LPD. The root parameters showed a decrease of 64.6%, 54.3%, 67.6%, and 49.5%, respectively, in HPD of ZYZ 12, whereas a decrease of 62.5%, 41.3%, 53.6%, and 39.0%, respectively, was observed in LPD. Similarly, during the 2013-2014 season, these root values at 28 DAPA decreased by 77.8%, 46.9%, 54.8%, and 43.8%, respectively, in HPD of ZS 11, but a lower reduction of 67.0%, 37.8%, 50.0%, and 42.4%, respectively, was observed in LPD. In HPD of ZYZ 12, these values decreased by 64.5%, 49.0%, 54.5%, and 52.1%, respectively, and a lower reduction of 55.7%, 40.2%, 48.1%, and 51.0%, respectively, was observed in LPD. The ANOVA results showed that the year, variety, and DAPA significantly affected the root length, root tips, root surface area, and root volume. Additionally, the interactions among these factors significantly affect root morphology.
In the present study, the seed yield of winter oilseed rape can be effectively increased by increasing plant density from 27.0 × 104 plant ha−1 to 58.5 × 104 plant ha−1 and decreases at the plant density of 69.0 × 104 plant ha−1. It is consistent that an increasing the number of plants per unit area is associated with a better use of arable land and better light interception, but this does not always result in higher yielding capacity [
Previous studies have also indicated that high yield is associated with dry matter production [
The effects of five different plant densities were examined to optimize the population under modern cultivation systems and clarify the mechanism of high seed yield. The results indicated that a higher seed yield and optimal plant density were obtained after increasing the plant density to a certain range. In high plant density population, it showed a rapid decrease in GLAI and chlorophyll content as well as the rapid increase of MDA content after peak anthesis. The high yield highlighted the rapid increase of PAI and pod photosynthesis concomitant with accelerated leaf senescence after peak anthesis. The higher reduction in root morphological parameters, namely, root length, root tips, root surface area, and root volume, the higher accumulation in dry biomass, and higher N utilization efficiency in higher plant density treatment at peak anthesis suggested that that higher nutrient concentration could be available for shoot growth and seed filling.
The authors declare that there are no conflicts of interest regarding the publication of this paper. They declare that they do not have any commercial or associative interest that represents conflicts of interest in connection with the work submitted.
This research was financially supported through grants from the National Natural Science Foundation of China (nos. 31271671 and 31571619) and the Special Fund for Agro-Scientific Research in the Public Interest of China from the Ministry of Agriculture (no. 201503122).