Influence of Seedling Age and Seeding Rate on Grain Yield in Double Cropping Rice with Machine Transplantation

ZHENG Hua-bin, HUAN Shun, LI Bo, TANG Qi-yuan

College of Agronomy, Hunan Agricultural University, Changsha 410128, PRC

Abstract In this study, Zhongzao 39, Zhuliangyou 189, Hyou 518 and Taiyou 390 four rice cultivars were used to explore the effects of increased seedling age on the seedling quality, tillering rate, panicle-bearing tiller rate, biomass, and seed yield attributes with a low seeding rate grown under machine-transplanted conditions. The results showed that compared to 20-day-old (15) rice seedlings with a conventional seeding rate, rice seedlings with a low seeding rate showed a significantly increased grain yield at higher seedling ages (5~10 d). In seedlings with a low seeding rate, the average number of spikelets per panicle was 18.9% higher in 25-day-old early rice and 15.9% higher in 20-day-old late rice compared to seedlings with a conventional seeding rate. There was no significant difference in panicles per square meter between the seedlings with a low seeding rate and the seedlings with a conventional seeding rate. In the low seeding rate plants, an increased seedling age (5~10 d) resulted in higher seedling quality and higher tillering and panicle-bearing tiller rates. Our results suggested that the prolonged aging of the seedlings with a low seeding rate compensated for the negative influence of delayed transplantation under machinetransplanted conditions.

Key words Seedling age; Seed rate; Yield

1. Introduction

To improve the operational efficiency and reduce the labor intensity of large-scale farming, machine-transplanted rice has largely replaced rice transplantation by hand in the cultivation of Chinese rice. The mechanical transplantation of young rice seedlings was first reported in Japan, with about 200 g/tray (60 cm×28 cm) of seeding rate and 2.1 tillers/plant of seedling. Subsequently, other ecological and production conditions were introduced, including mid-growth (3.1~3.5 tillers/plant of seedling and 60~100 g/tray of seeding rate) and late-growth (3.6~4.0 tillers/plant of seedling and 35~50 g/tray of seeding rate) transplantation. In Southern China, where the amount of sunlight and the temperature are suitable for year-round rice cultivation, about 20% of the planting area is taken up by early- and late-transplantation rice. The mid-growth transplantation of seedlings is usually due to a strained growing season. Generally, the seedling age with mid-growth transplantation is 20 d in the early rice and 15 d in the late rice. The seedling age with machine transplantation are about 5~10 d younger than that with rice transplantation by hand. When multiple cropping is utilized, mechanical transplantation shortens the seedling age and can affect rice yield. To overcome this problem, highyield, short-duration cultivars have been developed and older seedlings have been cultivated.

The aging or delayed transplantation of rice seedlings can be detrimental to plant growth and rice yieldsmainly due to low tiller numbers, a shortened vegetative period, decreased dry matter accumulation, and a decreased filling ratio. Therefore, how to compensate for the loss of grain yield caused by the use of older seedlings has become an important issue for researchers. LAMPAYAN R M

et al

.found that lowering the seeding rate could improve the grain yield of older rice seedlings. LIU Q H

et al

.reported that in addition to cultivation density, enhancing the amount of pre-anthesis dry matter and the remobilization efficiency could mitigate the loss of grain yield with 30-day-old and 35-day-old seedlings transplantation. YU L H

et al

.suggested that for mechanically transplanted rice, adopting dry seedling nursery technology could enhance the quality of the seedlings and prolong the age at which they are suitable for transplantation. These reports support the notion that various agronomic methods can be used to overcome the negative effects of older seedlings.It was hypothesized that a low seeding rate could increase the quality of older rice seedlings (

e.g

., improve tillering rate, panicle-bearing tiller rate, and increase grain yield). To investigate this hypothesis, field experiments were conducted to determine the effects of delayed transplantation of Zhongzao 39 (early rice), Zhuliangyou 189 (early rice), Hyou 518 (late rice) and Taiyou 390 (late rice) 4 rice cultivars on seedling quality, tillering rate, panicle-bearing tiller rate, biomass, and seed yield under machinetransplanted conditions.

2. Materials and Methods

2.1. Plant materials and experimental design

The field experiments were carried out in Yongan (28°14′N, 113°18′E), Liuyang County, Hunan Province, between March and October in 2016. The planting site is a typical double cropping rice district in China. The maximum and minimum temperatures between March 25and November 22were 26.4°C and 19.3°C, respectively. The early rice cultivars were Zhongzao 39 (inbred) and Zhuliangyou 189 (hybrid). The late rice cultivars were Hyou 518 (hybrid) and Taiyou 390 (hybrid), the cultivars most commonly planted locally.

The experiment used a randomized block design with three replications and a plot of 25 m. In the early season, there were two different dry seeding rates: a low seeding rate (30 g/tray for inbred cultivars and 15 g/tray for hybrid cultivars) and conventional seeding rate (100 g/tray for inbred cultivars and 70 g/tray for hybrid cultivars). Rice seeds were uniformly sown on the 25of March. The low seeding rate seedlings were grown for 20, 25, 30, and 35 d; 20-day-old conventional seeding rate seedlings were used as a control. In the late season, there were two different dry seed rates: a low seeding rate (15 g/tray for hybrid cultivars) and a conventional seeding rate (70 g/tray for hybrid cultivars). The low seeding rate seedlings were grown for 25, 30, and 35 d; conventional seeding rate with 20-day-old seedlings were used as a control. The rice seedlings were uniformly transplanted on July 26.

For the low seeding rate seedlings, a new seed sowing system for single seedling machinetransplanted rice production was adopted. For the conventional seeding rate seedlings, seeds were manually sown in 58 cm×28 cm seedling trays. The transplantation density was 28.5 tillers/min the early season and it was 36.4 tillers/min the late season. The application amount of pure nitrogen was 150 kg/hmin the early season and 165 kg/hmin the late season, with 70% of the total N applied at basal dressing and 30% applied at panicle initiation. The application amount of POwere 75.0 kg/hmin the early season and 82.5 kg/hmin the late season. The application amount of KO were 120.0 kg/hmin the early season and 132.0 kg/hmin the late season. Phosphorus was applied at the basal dressing and the potassium was split equally between the basal dressing and the panicle initiation. Insects, diseases, and weeds were controlled using chemical methods to avoid yield loss.

2.2. Sampling and determination

Three seedling samples (30 seedlings per sample) were randomly selected for each seedling age on the day of transplantation to examine the traits of the seedlings, including the leaf number per plant, plant height, stem thickness, and dry shoot weight. Excluding the three border plants, 20 hills were labeled in each plot to count the basic seedlings 3 d after transplanting and to count the maximum number of tillers at 3-d intervals from 30 d to 40 d after transplanting. 10 hills were sampled to determine the aboveground biomass at the flowering stage. At the mature stage, 12 hills were sampled to determine yield components, including the spikelets per panicle, filling ratio, and 1 000-grain weight. At the end of the growth period, a 5 marea was selected and sampled to determine the grain yield and 20 hills in the same area were used to determine the number of effective panicles per square meter. The tillering rate was calculated as the difference in maximum tillers and basic seedlings per square meter and the basic seedlings. The panicle-bearing tiller rate was calculated as the number of panicles per square meter and the maximum number of tillers per square meter.

2.3. Data analysis

Based on the above parameters, the biomass per plant height was calculated using shoot dry matter and plant height, the tillering rate was calculated as the difference between the maximum tillers and basic seedlings per square meter and basic seedlings, and the panicle-bearing tiller rate was calculated as the number of panicles per square meter and the maximum number of tillers per square meter. Statistical analysis was performed using one-way analysis of variance (ANOVA) with Statistix 8. The means of the treatments and years were compared using the least significant difference test; statistical significance was set at

P

<0.05.

3. Results

3.1. Effects of seeding rate and seedling age on Seedling traits

For 20-day-old early rice seedlings or 15-dayold late rice seedlings, the height of the seedlings with a low seeding rate was higher than that of the seedlings with a conventional seeding rate, and the biomass per plant height of the seedlings with a low seeding rate was equal to or slightly lower than that of the seedlings with a conventional seeding rate (Table 1). With increasing age, the traits of the seedlings with a low seeding rate were improved compared to the seedlings with a conventional seeding rate. In addition, the biomass per plant height in the early rice increased continually with increased seedling age, whereas in the late rice, an initial increase was followed by a decrease. The maximum biomass per plant height was observed in the 20~25-day-old seedlings.

3.2. Effects of seeding rate and seedling age on grain yield and its components

The average yield in the early rice with 20-dayold of seedling age was 7.03% higher than that in the late rice with 15-day-old of seedling age under a low seeding rate. Moreover, the average yield of rice with a low seeding rate was 3.0% higher than that rice with a conventional seeding rate, but the difference was not significant (

P

>0.05, Table 2). As the seedling age increased in the rice with a low seeding rate, the maximum grain yield was recorded with the 25-dayold of seedling age in the early rice, the 20-day-old of seedling age in cultivar Hyou 518, and 30-day-old of seedling age in cultivar Taiyou 390. A significant difference (

P

<0.05, Table 2) was observed between older seedlings with a low seeding rate (25-day-old early rice seedlings and 20-day-old late rice seedlings) and 20-day-old (15) seedlings with a conventionalseeding rate. In terms of their yield attributes, the average of spikelet number per panicle in the treatment with a low seeding rate was 18.9% higher (20 d in the early rice) and 15.9% higher (15 d in the late rice) than that in the treatment with a conventional seeding rate. No significant differences were observed in the number of panicles per square meter, filling rate, or 1 000-grain weight between treatment with a low seeding rate and the control. In comparison to the control, an increase in seedling age (> 5 d) in the treatment with a low seeding rate significantly increased the grain yield from mechanically transplanted rice.

Table 1 Influence of seedling age on plant height, leaf number per plant, stem thickness and biomass per plant height of seedlings with different seeding rate

3.3. Effects of seeding rate and seedling age on aboveground biomass and HI

In the early rice, the total biomass in the treatment with 20-day-old seedlings and a low seeding rate was slightly higher than that in the control, and in the late rice it was slightly lower than that in the control (Table 3). In the treatment with a low seeding rate, the total biomass initially increased and then decreased as seedling age increased. In the early rice, the peak value of total biomass was 1 266 g/mfor cultivar Zhongzao 39 and 1 230 g/mfor cultivar Zhuliangyou 189, and in the late rice it was 1 915 g/mfor cultivar Hyou 518 and 1 888 g/mfor cultivar Taiyou 390. There was a significant difference in total biomass with seedling age increased in the treatment with a low seeding rate compared to the control (

P

<0.05). Furthermore, there was a significant difference in biomass with seedling age increased based on reproductive stage between the treatment with a low seeding rate and the control (

P

<0.05).

Table 2 Influence of seedling age on grain yield and its component changes with different seeding rate in the early rice and late rice

Table 3 Influence of seedling age on aboveground biomass and HI variation with different seeding rate and seedling age in the early- and late season

3.4. Effects of seeding rate and seedling age on basic seedling, tillering, and panicle-bearing tiller rates

In the early rice, the basic seedling rate in the control was twice as more as the treatment with a low seeding rate, and in the late rice, it was an average 30% more than that in the treatment with a low seeding rate (Fig. 1a and 1b). In the treatment with a low seeding rate, the maximum basic seedling rate was 68 seedlings/mfor early rice and 74 seedlings/mfor late rice. The tillering rate in the treatment with a low seeding rate was higher than that of the control (Fig. 1c and 1d). In the early rice, the panicle-bearing tiller rate of aged seedlings with a low seeding rate (0~10-day-old) was 9.6%~19.1% higher than that in the control, and there was a significant difference in the panicle-bearing tiller rate between the treatment with 25-day-old seedlings and the control (

P

<0.05). Similarly, for late rice, the panicle-bearing tiller rate in aged (0~10-day-old) seedlings with a low seeding rate was an average of 6.2% (-2.3%~14.6%) higher than that of age-matched seedlings with a conventional seeding rate (Fig. 1e and 1f).

Fig. 1 Infl uence of seedling age on basic seedling (seedling/m2), tillering rate and panicle bearing tiller rate (%) variation with different seeding rate in the early and late rice

4. Discussion

Seedling age is an important element that affects the number of spikelets per panicle, panicle length, grain weight, and grain yield in rice. Previous studies have indicated that timely transplantation and appropriate seedling age are essential for producing a higher grain yield. LAMPAYAN R M

et al

.reported that 20 d was the ideal seedling age, and thatthe grain yield reduced when the seedling age were more than 20-day-old. LI J

et al.

reported that when rice seedlings were transplanted by machine at 20-day-old, the grain yield decreased by 4.66% compared to that by hand. LIU Q H

et al

.reported that the grain yield with 42-day-old seedlings that were mechanically transplanted was 31.3% lower than that of hand-transplanted seedlings. The results of this study showed that in comparison to the control, the treatment with 20-day-old older seedlings and a low seeding rate significantly increased the grain yield under mechanical transplantation. However, when the age of the seedlings with a low seeding rate was more than 10 d, the rice grain yield decreased. This result was inconsistent with previous studies. This contradiction may be ascribed to the intense competition among individual seedlings under the limited growth conditions of the seed-nursery bed.A higher seeding rate also affects seedling quality and grain yields. LIU Q H

et al

.found that the sowing density was 74 g/mfor hand-transplanted rice and 412 g/mfor mechanically transplanted rice. LI J

et al

.found that the seeding rate was 110 g/tray. A lower seed rate (25 g/m) could improve the grain yield with 30-day-old seedlings during the dry season. However, lowering the seeding density from 25 g/mto 12.5 g/mand increasing nitrogen topdressing has not shown to enhance grain yields. A possible reason for this is that the uniformity and order of seedings can affect seedling quality. It can be difficult to ensure a uniform and well-organized distribution of seedlings using hand transplantation, whereas uniform and well-organized distribution can be achieved using a mechanical seed-sowing system (as in the present study). Therefore, the use of older seedlings with a low seeding rate could compensate for the negative infl uence of older seedlings on grain yield.

In seedlings with a conventional seeding rate, young seedlings between 15 and 20 d old were characterized by a plant height ranging from 12~20 cm, and individual plants with a sufficient nutrient supply from the seedling-nursery tray are suitable for mechanical transplantation. Compared with the seedlings with a conventional seeding rate, a 5~10 d increase in age in the seedlings with a low seeding rate could improve seedling quality while not impacting the operation of the rice transplanter. Because of the higher seedling height, the average plant height in the early rice and the late rice was 15.8 and 22.6 cm, respectively. Our results also showed that there was an increase in leaf number per plant, an increase in stem thickness, and an increase in biomass per plant height of seedlings with a low seeding rate. The advantages of the older seedlings with a low seeding rate were: (1) a shorter period for seedling establishment, earlier tillering, and higher tillering rate (Fig. 1), improved spikelet per panicle, filling rate, and grain weight (Table 2) due to stronger rice seedlings; (2) increased seedling age could prolong the growth period and alleviate brought on by mechanical transplantation; and (3) the suitable transplantion period was increased by 5~10 d (15 [20] to 25 [30] d in seedlings with a conventional seeding rate), which is more suitable for large-scale mechanized rice production.

Our results showed that seedlings with a low seeding rate generally had fewer panicles per square meter seedlings with a conventional seeding rate, and the greatest panicle number per square meter was observed in 5~10 day-old seedlings with a low seeding rate. Panicle number per square meter is a function of the maximum tiller number per square meter and panicle-bearing tiller rate. In this study, the higher panicle number per square meter observed in older seedlings with a low seeding rate was mainly attributed to the high tillering rate in the late rice and the higher panicle-bearing tiller rate in the early rice compared with seedlings with a conventional seeding rate, whereas there was no significant difference between prolonged seedling age in seedlings with a low seeding rate and seedlings with a conventional seeding rate. This was in agreement with a report by HUANG M

et al

.who observed that there was a positive relationship between panicle number per square meter and the maximum tiller number per square meter. In contrast to panicle number per square meter, the spikelet number per panicle was greater in older seedlings with a low seeding rate than in older seedlings with a conventional seeding rate. In this regard, it was suggested that a strong compensatory mechanism exists between the panicle number per unit land area and spikelet number per panicle. Therefore, it was possible that a yield increase resulted from the spikelet number per panicle in older (5~10 d delay) seedlings with a low seeding rate, rather than the panicle number per unit land area. Further study is required to confirm this speculation.

Grain yield is determined by the aboveground biomass and harvest index. In this study, the harvest index was comparable between treatments; therefore, further improvements in rice yield may be more dependent on the ability to increase biomass production than on an increase in the harvest index. The highest total biomass was observed in 5 d delayed seedlings with a low seeding rate compared to agematched seedlings with a conventional seeding rate, whereas there was no significant difference between prolonged seedling age in low seeding rate versus conventional seeding rate seedlings. Further analysis found a significant difference in biomass in the reproductive stage, whereas not in the vegetative stage.

5. Conclusions

An increased seedling age in cultivars with a low seeding rate can result in improved seedling quality, an improved tillering rate, and an improved paniclebearing tiller rate, which in turn can improve biomass accumulation and grain yields. Compared with 15~20-day-old seedlings with a conventional seeding rate, a 5~10 d delay in seedling age in seedlings with a low seeding rate resulted in an increase in rice growth and development and a higher and more stable grain yield. Finally, the uniform and orderly transplantation of seedlings with a low seeding rate is important for mechanically transplanted seedlings.