HU Ya-jie, WU Pei, ZHANG Hong-cheng, DAI Qi-gen, HUO Zhong-yang, XU Ke, GAO Hui, WEI Hai-yan,GUO Bao-wei, CUI Pei-yuan

Jiangsu Key Laboratory of Crop Genetics and Physiology/Jiangsu Co-Innovation Center for Modern Production Technology of Grain Crops, Yangzhou University, Yangzhou 225009, P.R.China

1. Introduction

Rice (Oryza sativaL.), the world’s important cereal crop,provides staple food for more than half of the world’s population (Nguyen and Ferrero 2006). Global rice production was expected to increase by more than an estimated 1% per year in the next 25 years to meet growing demand (Khush 2013). The world must learn how to increase rice production continuously. China serves as an important rice producing and consuming country. With recent economic development and urbanization, China would find it very difficult to expand the land area used for rice production, in part because of the scarcity of water for agriculture (Penget al. 2009; Veeck 2013; Cuiet al. 2014).Therefore, increasing yield per unit area may provide an effective approach to increasing rice production.

In the past several decades, the discovery of semidwarf genes (Zhang 2011), the application of hybrid rice(Penget al. 2008; Wu 2009), and improved crop cultivation technology (Reddyet al. 2003; Chenet al. 2011) have resulted in improved rice yield. In 1996, China’s super rice mega project research was established to obtain the targeted yield of 9.0–10.5 t ha?1by 2000, 12.0 t ha?1by 2005,and 13.5 t ha?1by 2015 (Chenget al. 2007). The plan to produce super rice effectively has promoted an increase in yield on farms in China. To date, 130 super rice varieties have been identified by the Ministry of Agriculture of China(statistical data from the Ministry of Agriculture of China,http://www.ricedata.cn/variety/superice.htm). To expand the potential to increase rice yield, the breeding of intersub-specific hybrid rice (IHR) has been widely conducted because IHR has strong inter-sub-specific heterosis and higher yield potential overindicaorjaponicahybrid rice,bothindicaandjaponicahybrid rice have a higher yield potential than inbred rice (Jinet al. 2013; Ma and Yuan 2015). However, the use of IHR had been restricted by the obstacle created by hybrid sterility (Miet al. 2016), where the F1generation produces a low percentage of filled grain during on-farm production. In recent years, breeders have made many efforts to overcome inter-sub-specific hybrid sterility (Shahidet al. 2013; Penget al. 2015; Linet al.2016). Recently, several varieties with super-high-yield potential have been released in China, such as Yongyou 12,Yongyou 538, Yongyou 2640, and Chunyou 927. Wanget al. (2014) reported Yongyou 12 yielded an average of 14.2 t ha?1over two years on a 6.7-ha farm. Our research group indicated that Yongyou 2640 yielded over 13.0 t ha?1at multiple sites under a rice-wheat crop rotation system in the lower reaches of Yangtze River, China (Huet al. 2014b).Many previous research studies have shown that IHR plants produced higher biomass and had better super-high-yield potential when compared with other typed cultivars (Xuet al. 2010; Huaet al. 2015). However, little information was available related to the comparison of agronomic performance between IHR and inbredjaponicarice (IJR)under mechanical transplanting methods.

Recently, mechanical transplanting has been progressively replacing the conventional method for cultivating rice, manual transplanting, because of a shortage of rural labor and a simultaneous increase in labor costs (Knightet al. 2011;Liuet al. 2015). Mechanical transplanting rice is currently widely used in Japan and Korea because of the ease of cultivation and stable yield. However, farmers in China have only mechanically planted 33.7% of all rice cultivation areas in recent years (Cuiet al. 2014). The main mechanical method is primarily combined with carpet seedlings for rice.CS have been popularized and applied slowly because some factors have restricted its application; for example, a high sowing rate as well as plant and root injury that occurs with machine transplanting caused the production of weak young seedlings (15–20 days) (Zhanget al. 2010). A new method involving the use of PS has been applied in rice production in recent years, and has exhibited favorable results. The PS method displays some advantages such as the use of older seedlings (28–35 days); this method produces stronger seedlings with very little injury to plant and root during mechanical transplanting when compared with that under CS (Zhanget al. 2013). Nevertheless, the research that compares agronomic performance between PS and CS for both IHR and IJR remains limited.

The objectives of this study were to: (1) evaluate the advantages of high-yield and the characteristics of agronomic performance in PS, and (2) identify the different characteristics of creating high yield by comparing IHR and IJR.

2. Materials and methods

2.1. Plant materials and growth conditions

Field experiments were conducted in a paddy field in Diaoyu Town, Xinghua County, Jiangsu Province, China (33°05′N(xiāo),119°58′E) in 2013 and 2014. The cropping system in this experiment employed a wheat-rice rotation system. The loamy clay soil in the paddy field of this experiment had 25.9 g kg?1organic matter, 1.7 g kg?1total N, 12.5 mg kg?1Olsen-P, and 135.2 mg kg?1available K. The mean temperature, precipitation, and sunshine hours during the rice growing seasons of 2013 and 2014 were measured at a weather station close to the experimental site (Fig. 1).There were higher temperature, less precipitation, and more sunshine hours at the middle and late growth stages in 2013 than 2014.

An IHR cultivar, Yongyou 2640 (YY2640), and an IJR cultivar, Wuyunjing 24 (WYJ24), that are currently commonly planted in local production were used in the experiments.Ningbo Academy of Agricultural Science (Ningbo, Zhejiang Province) and Wujing Rice Research Institute (Wujing,Jiangsu Province) provided the seeds of YY2640 and WYJ24, respectively. The growth stages and periods of the two rice cultivars varied in the planting methods under the two years of the field trials (Table 1). The entire experiment was a part of a rice-wheat field rotation each year; that is,wheat was planted after the harvesting of rice and rice was transplanted after wheat was harvested. All experimental treatments were conducted using the same cultivation methods, such as fertilizer and irrigation water management.The total amount of nitrogen (N) applied was 300 kg ha?1.In addition, N (90 kg ha?1as urea), phosphorus (120 kg ha?1as single superphosphate), and potassium (120 kg ha?1as KCl) were applied and incorporated before transplanting.N was also applied as urea at the tillering (7 days after transplanting, 90 kg ha?1), panicle branch differentiation(60 kg ha?1), and panicle spikelet differentiation (60 kg ha?1) stages as top dressing. The water levels in the paddy fields were maintained at 1–2 cm deep for 7 days after transplanting. Then, water was held at 2–3 cm deep for tillering. When 80% of the tillers of the population produced productive panicles, the fields were drained and then remained dry until the soil water potential reached–15 KPa. Next, fields were irrigated to keep 3–5 cm water level until heading. After heading, the fields were irrigated and then drained before the next irrigation. This wet-dry cycle was repeated until 7 days before harvest. Weeds,insects, and diseases were controlled as required to avoid a reduction in yield.

Fig. 1 Sunshine hours, mean temperature, and actual precipitation during the rice growing seasons of 2013 and 2014.

Table 1 Growth stages of two rice cultivars under two mechanical transplanting methods: mechanically transplanting of both pot seedling (PS) and carpet seedlings (CS)

2.2. Treatments

The experiment employed a split-plot design with three replications. Two mechanical transplanting methods(treatments), mechanically transplanted PS and CS, were assigned as main plots, with the two rice cultivars analyzed here being allotted to sub-plots. The 5.8 m×10 m plots were separated by a 0.5-m wide alley covered with plastic film that was inserted into the soil to a depth of 0.30 m. Each plot was independently irrigated and drained.

Seedlings grown using the two methods were raised in different types of seedling nursery trays in the field. Seeds under PS were sown mechanically in special plastic nursery trays for pot seedling by a sowing machine (LSPE-40AM,AMEC Corporation, Changzhou, Jiangsu, China). The 61.8 cm long×31.5 cm wide×2.5 cm high trays each had 448 holes, with a hole diameter of 1.6 cm. In consideration of the difference of high yield formation for two type cultivars,two seeds per hole were used for IHR and four seeds per hole for IJR. The sowing dates under PS were 21 May 2013 and 20 May 2014 for both rice cultivars. 30-d-old PS were transplanted with one pot seedling per hill using a hill spacing of 12 cm×33 cm (25.5×104hill per hectare) by a machine transplanter (RX-60AM, AMEC Corporation, Changzhou,Jiangsu, China). Seeds of 110 g under CS were sown manually on a 58 cm long×28 cm wide×2.8 cm high plastic carpet seedling tray. The sowing dates under CS were 2 Jun 2013 and 1 Jun 2014 for both rice cultivars. Carpet seedlings of 18-d-old were transplanted with 2 seedlings per hill for IHR and 4 seedlings per hill for IJR, and at a hill spacing of 13.2 cm×30 cm (25.5×104hill per hectare) by a rice carpet seedling machine transplanter (VP6, Yanmar Corporation, Japan). The two mechanical transplanting methods were used simultaneously for the two cultivars on 20 Jun 2013 and 19 Jun 2014, and the same plant density was used in both years.

2.3. Sampling and measures

Thirty plants of rice in each plot were tagged for counting the number of tillers during four stages: at the middle of the tillering stage (MT, 20 days after transplanting), at panicle initiation stage (PI), at heading stage (HD), and at maturity stage (MA). The values of each set of 30 plants were averaged.

Leaf area index (LAI) was measured with a portable leaf area meter (Li-3000A, LI-COR, Lincoln, NE, USA) at the MT,PI, HD, and MA stages. Plants from five hills were sampled from each plot for each measurement. Leaf area duration(LAD) was calculated using eq. (1):

Where,L1andL2are the first and second measurements of leaf area index (m2m?2), respectively, andt1andt2represent the first and second days (d) of the measurement,respectively.

For the measurement of aboveground biomass, sample plants from five hills were separated into leaves, stems,and panicles (at the full HD) at the PI, HD, and MA stages.Samples of each plant part were then dried separately and weighed to determine total aboveground biomass per unit area per each plot. Each component of rice plants was oven-dried separately at 105°C for 30 min and then at 80°C in bags to a constant weight. The crop growth rate (CGR)was calculated using eq. (2):

Where,W1andW2are the first and second measurements of dry matter weight, respectively, andt1andt2represent the first and second days (d) of measurement, respectively.

The dried tissue samples were weighed, ground to powder, and sieved. Plant and grain samples were digested with H2SO4-H2O2, filtered, and separated into samples of constant volume. N content was determined by micro Kjeldahl digestion, distillation, and titration to calculate aboveground N uptake (Yoshidaet al. 1976). The equation of Xueet al. (2013) was used to calculate nitrogen use efficiency, i.e., nitrogen partial factor productivity (PFPN, the ratio of grain yield to the amount of N applied).

Once the rice had matured, grain yield was determined from a 10.0-m2harvest area in the middle of each plot and adjusted to 14% moisture. Yield components, i.e.,the number of panicles per m2, number of spikelets per panicle, percentage of filled grains, and grain weight, were determined from plants of 1.0 m2sampled randomly from each pot. The percentage of filled grains was determined as the ratio of filled grains to the total spikelets.

2.4. Statistical analysis

Microsoft Excel 2013 and SPSS 17.0 (SPSS, Chicago,IL, USA) were used to analyze the data. The analysis of variance and mean comparison were based on the least significant difference (LSD) test at theP＜0.05 probability level for each year. Figures were prepared using Origin 8.5(OriginLab, Hampton, MA, USA).

3. Results

3.1. Growth stage and period

The seedling age varied between PS and CS, but the transplanting date was the same. The PI, HD, and MA of rice cultivars under PS occurred earlier by 2–4, 3–5, and 5–7 days when compared with those under CS (Table 1).The HD and MA were delayed 2–4 days in 2014 when compared with those of 2013, which attributing to low temperature and few sunshine after elongation in 2014.More days were required for plants to grow from sowing to maturity under PS than those under CS. The growth stage was different between IHR and IJR under PS or CS, but the days from sowing to maturity were comparable.

3.2. Grain yields and yield components

Grain yield showed significant differences among cultivars and transplanting methods. Across two years of studies,grain yield under PS was significantly greater, by 6.8–10.8%,for both rice cultivars when compared with that under CS(Table 2). The larger grain yield under PS was attributed to an increase in the total number of spikelets per panicle which increased the total number of spikelets per plant. When the two cultivars were compared, IHR exhibited significantly higher grain yield, by 4.9–9.9%, than IJR under both PS and CS (Table 2). The number of panicles, filled grain percentage, and grain weight under IHR were significantly lower than those under IJR for PS and CS, but the opposite was true for the number of spikelets per panicle.

3.3. LAI and LAD

LAI under PS was slightly higher than that under CS at MT(Table 3), but the result was reversed at PI. When compared with that under CS, PS had markedly greater LAI at HD and MA. With the same mechanical transplanting method,IHR had significantly higher LAI than that under IJR at MT,HD, and MA.

LAD under PS was higher than that under CS during the period from SO to PI and from HD to MA (Fig. 2-A and B).However, it was comparable in LAD during the period from PI to HD between PS and CS, which was attributed to the longer growth stage from PI to HD in CS. A comparison of the two rice cultivars showed no difference in LAD occurredduring the early and middle growth periods between IHR and IJR. However, IHR had significantly higher LAD during the period from HD to MA than that under IJR.

Table 2 Grain yield and yield components of two rice cultivars under two mechanical transplanting methods, pot seedling (PS)and carpet seedling (CS), in 2013 and 2014

Table 3 Leaf area index of rice cultivars under two mechanical transplanting methods, pot seedling (PS) and carpet seedling(CS), in 2013 and 20141)

3.4. Biomass production and CGR

Shoot biomass under PS for both rice cultivars was significantly higher than that under CS at both HD and MA(Fig. 3-A and B). However, no significant difference in shoot biomass was observed between PS and CS at PI. When comparing the two rice cultivars, IHR had significantly greater shoot biomass than IJR at HD and MA than IJR.The harvest index under PS was significantly higher than that under CS for both rice cultivars (Fig. 4-A). However,no significant difference in harvest index was observed between IHR and IJR.

Dry matter accumulation under PS was significantly greater from PI to MA when compared with that under CS(Fig. 3-A and B). However, no significant difference in dry matter accumulation was found from SO to PI between the two planting methods. Under PS or CS, IHR had significantly greater dry matter accumulation than IJR at the middle and late growth stages, suggesting that IHR had greater dry matter production after PI.

Similar to dry matter accumulation, CGR was significantly higher under PS than that under CS from PI to MA (Fig. 2-C and D). When compared with IJR, IHR had significantly greater CGR during the period after PI.

3.5. Nitrogen uptake and use

Similar to shoot biomass during the main growth stage,nitrogen uptake for rice cultivars under PS was significantly higher than that under CS at HD and MA (Fig. 3-C and D).A comparison of the two rice cultivars showed that IHR had significantly higher nitrogen uptake at HD and MA when compared with that of IJR, suggesting IHR exhibited greater N absorption capacity during those stages. However, no significant difference in nitrogen uptake was observed at PI between the two transplanting methods or the two rice cultivars.

Fig. 2 Leaf area duration (A and B) and crop growth rate (C and D) of rice cultivars under different mechanical transplanting methods in 2013 and 2014. YY2640 and WYJ24 are inter-sub-specific hybrid rice cultivar Yongyou 2640 and inbred japonica rice cultivar Wuyunjing 24, respectively. PS and CS represent mechanically transplanted pot seedling and carpet seedling, respectively.SO, PI, HD, and MA represent sowing, panicle initiation, heading, and maturity stages, respectively. The same letters are not significantly different according to least significant difference (P＜0.05). Bars mean SE.

When compared with that under CS, N accumulation under PS was significantly higher from PI to HD and from HD to MA (Fig. 3-C and D), in agreement with the changes in dry matter accumulation. With the same transplanting method, IHR had remarkably higher N accumulation than that under IJR at the middle and late growth periods.

PFPNfor rice cultivars under PS was markedly higher than that under CS in 2013 and 2014 (Fig. 4-B), which was attributed to greater grain yield under PS than that under CS. When comparing the two rice cultivars, IHR had higher PFPNwhen compared with that under IJR and this difference was significant except for under CS in 2014.

Fig. 3 Dry matter weight (A and B) and nitrogen accumulation (C and D) of rice cultivars under different mechanical transplanting methods in 2013 and 2014. YY2640 and WYJ24 are inter-sub-specific hybrid rice cultivar Yongyou 2640 and inbred japonica rice cultivar Wuyunjing 24, respectively. PS and CS represent mechanically transplanted pot seedling and carpet seedling, respectively.SO, PI, HD, and MA represent sowing, panicle initiation, heading and maturity stages, respectively. The same letters are not significantly different according to least significant difference (P＜0.05). Bars mean SE.

4. Discussion

4.1. Effect of planting method and variety on grain yield

The expansion of sink size (the total number of spikelets per unit area) is a critical aspect related to increasing rice grain yield. Sink size can be increased by increasing the number of panicles per unit area (Huanget al. 2011) or by enhancing the number of spikelets per panicle (Katsuraet al.2007; Linet al. 2007; Yanget al. 2008). This means that varied strategies of increasing grain yield can be adopted for different planting methods or varieties. Variations in rice planting methods have a significant effect on grain yield (Sunet al. 2013; Zhouet al. 2016). Most previous research has reported that manually transplanting rice (MTR) increased grain yield when compared with that under CS (Liet al.2011), which could be caused by high seedling quality and lower position tillers under MTR. Our results showed that grain yield under PS increased significantly when compared with that under CS for two rice cultivars in 2013 and 2014(Table 2). Similar to MTR, PS had also better seeding quality and earlier tillering after transplanting than CS (Huet al. 2014a). In the present paper, the increase of grain yield under PS was mainly attributed to the greater number of total spikelets that resulted from enlarged panicle (greater number of spikelets per panicle). The results suggest that PS gained an advantage in grain yield formation by expanding the total number of spikelets through growing stable panicles and enlarging the panicle size.

A comparison of the characteristics of grain yield formation between hybrid and inbred rice has been completed using MTR (Jianget al. 2015; Weiet al. 2016);however, little information was available related to the different characteristics of grain yield formation between hybrid and inbred rice in mechanical transplanting method.In our study, IHR had markedly higher grain yield than IJR under PS or CS. The number of spikelets per panicle under IHR increased by more than 75%, and was significantly greater than that under SJR; however, the reverse results were found in panicle number, grain filled percentage and grain weight. Therefore, increased yield under IHR was attributed to the larger number of spikelets per panicle.Similar observations have been made by Huaet al. (2015)and Weiet al. (2016). The percentage of grain filled and the grain weight under IHR were significantly lower when compared with that under IJR. The possible cause of this problem was that the number of spikelets per panicle was negatively correlated with the grain filled percentage and grain weight. Breeders and agronomists should consider new methods to further improve the filled grain percentage and grain weight for IHR.

The grain yield was the highest by using PS with IHR compared to other combinations between planting method and cultivar type, which attributing to the highest of total spikelets. PS had good quality of starting population (high quality of seedlings) and stronger photosynthetic matter production, conducing to increase the spikelets per panicle of IHR. Hence, the combination of PS and IHR might be better rice cultivation pattern in southern China.

Fig. 4 Harvest index (A) and nitrogen partial factor productivity (B) of rice cultivars under different mechanical transplanting methods.YY2640 and WYJ24 are inter-sub-specific hybrid rice cultivar Yongyou 2640 and inbred japonica rice cultivar Wuyunjing 24,respectively. PS and CS represent mechanically transplanted pot seedling and carpet seedling, respectively. The same letters are not significantly different according to the least significant difference (P＜0.05). Bars mean SE.

4.2. Effect of planting method and variety on biomass production

The accumulation and distribution of shoot biomass creates the basis for the formation of rice production. In previous studies, increasing CGR and/or LAD resulted in enhanced production of aboveground biomass in rice (Zhanget al.2009; Jianget al. 2015). In the present study, PS had greater total aboveground biomass than CS for both varieties(Fig. 3-A and B), which was attributed to the greater LAD during the period from HD to MA (Fig. 2-A and B) and the higher CGR during the periods from PI to HD and from HD to MA (Fig. 2-C and D). Grain yield depends on biomass production and the value of the harvest index. Achieving higher rice yield was mainly achieved by increasing aboveground biomass production (Penget al. 1999). We found that the harvest index under PS was significantly higher than that under CS (Fig. 4-A), meaning that PS synergistically improved total aboveground biomass and the harvest index, contributing to higher grain yield. Jianget al. (2015) reported that higher biomass production was responsible for higher grain yield under hybrid rice than under inbred rice. Ibrahimet al. (2013) stated that high biomass production in hybrid rice was attributed to greater CGR during the vegetative period. In our study, IHR had the advantage in producing greater aboveground biomass over that of IJR after PI (Fig. 3-A and B) because of its higher LAI at HD and MA (Table 3), higher LAD from HD to MA(Fig. 2-A and B), and greater CGR from PI to HD and from HD to MA (Fig. 2-C and D). The total aboveground biomass under IHR was significantly increased when compared with that under IJR. However, no significant difference in harvest index was observed between IHR and IJR under PS or CS(Fig. 4-A). Hence, high yield of IHR was mainly attributed to greater aboveground biomass production.

4.3. Effect of planting method and variety on N uptake and use

High N accumulation and use efficiency were important characteristics during the formation of high yield for rice.Yinget al. (1998) indicated that N accumulation during the vegetative stage could be as important as N accumulation during the reproductive stage in determining rice sink size.In our study, when compared with that under CS, from both PI to HD and from HD to MA N accumulation rates under PS were significantly higher than that under CS (Fig. 3-C and D), which contributed to the large sink size. N uptake,accumulation, and use efficiency were different among plant methods or genetic types (Huoet al. 2012). In the present paper, N uptake at HD and MA as well as PFPNwere significantly higher under PS than that under CS(Fig. 3-C and D and Fig. 4-B), meaning that PS had greater N uptake capacity that contributed to high N use efficiency.Therefore, high N accumulation and use efficiency provided the vital physiological base needed for high yield for PS.Compared with inbred rice, hybrid rice proved superior in post-anthesis dry matter production, N uptake at anthesis,and post-anthesis N accumulation (Mahajan and Chauhan 2016). In the present paper, N uptake at HD and MA were significantly higher under IHR than that under IJR (Fig. 3-C and D), because IHR resulted in higher aboveground biomass at both HD and MA. N accumulation from PI to HD and from HD to MA were significantly higher under IHR than that under IJR (Fig. 3-C and D), contributing to higher PFPNunder IHR (Fig. 4-B). These results suggest IHR exhibited superior productivity and N use efficiency when compared with IJR in N uptake and accumulation during the middle and late growth periods.

5. Conclusion

Grain yield was significantly higher under PS than that under CS for IHR and IJR, which was attributed to the larger sink size that resulted from the increased number of spikelets per panicle in PS. This was associated with higher N accumulation during the period from PI to maturity.Compared to that under CS, PS had markedly higher aboveground biomass production because of their greater LAI, LAD and higher CGR. When comparing the two types of cultivars, IHR had remarkably higher grain yield than IJR under both PS and CS. The higher grain yield in IHR was caused by the higher number of spikelets per panicle and greater aboveground biomass production. The total N uptake and accumulation in IHR was higher than those in IJR, which was attributed to the higher N use efficiency of IHR. By using IHR, higher grain yield could be obtained in PS, which exploiting both advantage and potential to increase the productivity.

Acknowledgements

We are grateful for grants from the National Key Research Program of China (2016YFD0300503), the Special Fund for Agro-scientific Research in the Public Interest, China(201303102), the Key Research Program of Jiangsu Province, China (BE2016344), the Major Independent Innovation Project in Jiangsu Province, China (CX(15)1002)and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions,China.

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