Shanlan upland rice is a local variety of upland rice unique to Hainan, with strong drought resistance and disease resistance. Its rice is white, sticky, fragrant and delicious, and its crude protein content is high; Shanlan wine brewed with it has a very sweet taste and is famous at home and abroad (Cai et al., 2019); Moreover, the price is relatively high, with the general purchase price reaching 7 yuan/500 g, which has played a very good role in boosting farmers' poverty alleviation and income increase and rural revitalization. For example, Qingsong Village in Baisha County received more than 2 million yuan from planting rice in the village in 2019, which was awarded "Demonstration Village (Town) Led by Science and Technology for Rural Revitalization" by the Ministry of Agriculture and Rural Affairs. However, the outstanding problem of Shanlan upland rice is that its plant height is higher, generally reaching more than 140 cm, and its growth period is also in the typhoon prone period, which is easy to lodging, causing yield decline and seriously endangering production. The most direct and effective method is to use plant growth regulators to reduce plant height, improve the lodging resistance of rice, and then increase the yield of Shanlan upland rice (Zhao et al., 2009).

Paclobutrazol is a plant growth retarding agent, which has physiological effects such as promoting plant tillering, rooting, inhibiting stem elongation and increasing yield. It is widely used in crop production (Sinniah, 2012; Wu, 2012; Xie et al, 2019), Its action mechanism is mainly to inhibit the synthesis of gibberellin in plants, increase the activity of IAA oxidase, regulate the content of IAA and ABA, inhibit the growth of crops and promote their branching tillering, so as to achieve high yield (Yan et al., 2016). The results showed that paclobutrazol had certain effects on plant height, tillering and yield components of rice. Spraying PP333 at seedling stage can significantly reduce the plant height of 'Zhendao 11' and 'Yongyou 2640', increase the number of tillers, and improve the quality and yield of seedlings (Cheng et al., 2020); PP333 significantly affected the yield and lodging resistance of hybrid rice under different density fertilizer populations. The plant lodging resistance was enhanced, but the yield decreased due to the decrease of grains per spike, which had different effects on the yield (Xu et al., 2020). Spraying paclobutrazol 5 days before jointing significantly reduced plant height and shortened spike length of rice (Gu et al., 2018). The above studies provided a good reference for exploring the dwarfing cultivation of Shanlan upland rice, while the study on the effect of paclobutrazol on the yield of Shanlan upland rice plant height has not been reported.

On the basis of applying different levels of fertilizer as base fertilizer, this study sprayed paclobutrazol of different concentrations at 35 days after sowing to explore the mechanism of interaction between fertilizer and paclobutrazol on important agronomic traits of Ledong Shanlan upland rice 'LD-1'. The best concentration combination was selected to provide theoretical basis and technical support for high and stable yield of dwarf rice and high efficient cultivation.

1 Results and Analysis

1.1 F Test for significance of main agronomic traits of Shanlan upland rice under different fertilization and paclobutrazol treatments

Analysis of variance was conducted on the important agronomic characters of 'LD-1' (plant height, spike length, effective panicles per plant, number of filled grains, 1000 grain weight and yield). The effect of different fertilization levels on the number of grains per panicle reached a very significant level, and on the yield reached a significant level. The effects of different concentrations of paclobutrazol on plant height, panicle length, effective panicles per plant and yield reached a very significant level, and on the number of grains per panicle reached a significant level. When fertilizer interacts with paclobutrazol, it has a very significant impact on the number of grains per panicle, yield and plant height, and has a significant impact on the number of effective panicles per plant and 1000 grain weight (Table 1).

1.2 Effect of paclobutrazol on plant height of 'LD-1' under different fertilization levels

The suitable plant height is very important to improve the yield of Shanlan upland rice ‘The change of plant height of LD-1' is mainly regulated by paclobutrazol treatment, reaching a very significant level (F value=17.81**) (Figure 1), while the effect of fertilizer level on plant height is not significant (F value=1.70), and the effect of interaction of compound fertilizer and paclobutrazol on plant height reaches a very significant level (F value=5.12**) (Table 1). With the increase of paclobutrazol concentration, the change trend of plant height was first decreased and then increased. At B3 level, plant height was lower than other levels. The plant height of A3B3 combination was the lowest, with a mean of 106.3 cm, which was 25.29% lower than that of the control (142.3 cm). It indicated that the interaction of fertilizer with PP333 at appropriate concentration could significantly reduce the plant height of Shanlan upland rice.

1.3 Effect of paclobutrazol on panicle height of 'LD-1' under different fertilization levels

The interaction of compound fertilizer and paclobutrazol has no significant effect on panicle height (F value=1.30) (Figure 2), and the fertilizer level has no significant effect on panicle height (F value=2.08). The effect of paclobutrazol treatment on panicle height has reached a very significant level (F value=8.62**) (Table 1). With the increase of paclobutrazol concentration, the change trend of panicle height was first decreased and then increased. At B3 level, the panicle height was shorter than that at other levels. The results showed that PP333 of appropriate concentration could significantly increase the panicle height of Shanlan upland rice.

1.4 Effect of paclobutrazol on effective panicle number per plant of 'LD-1' under different fertilization levels

The effect of interaction of compound fertilizer and paclobutrazol on effective panicle number per plant reached a significant level (F value=2.59**) (Figure 3), while the effect of fertilizer level on effective panicles per plant was not significant (F value=0.63), and the change of effective panicles was mainly regulated by paclobutrazol treatment, reaching a very significant level (F value=23.75**) (Table 1). With the increase of paclobutrazol concentration, the change trend of effective panicles was first increased and then decreased. At B3 level, the number of effective panicles was more than that at other levels, among which, the number of effective panicles of A3B3 combination was the largest, with a mean of 15.67, an increase of 34.28% compared with the control (11.67). The interaction of fertilizer and paclobutrazol with appropriate concentration could significantly increase the number of effective panicles of mountain fenced rice.

1.5 Effect of paclobutrazol on grain number per panicle of 'LD-1' under different fertilization levels

The change in the grain number per panicle of 'LD-1' was regulated by fertilizer level and paclobutrazol treatment at the same time, reaching a very significant level (F value=28.96**) (Figure 4). The effect of fertilizer level on the number of grains reached a very significant level (F value=10.82**), and the effect of paclobutrazol treatment on the grain number per panicle reached a significant level (F value=6.43*) (Table 1; Figure 5). The grain number per panicle decreased first and then increased with the increase of fertilizer level and paclobutrazol concentration. The grain number per panicle decreased with the increase of fertilizer level and paclobutrazol concentration.

Figure 4 Effects of fertilization level and paclobutrazol treatment on grain number per panicle

1.6 Effect of paclobutrazol on 1000 grain weight of 'LD-1' under different fertilization levels

The change of 1000 grain weight of 'LD-1' was regulated by fertilizer level and paclobutrazol treatment at the same time, reaching a significant level (F value=2.76*). When fertilizer level and paclobutrazol act as a single factor, they have no significant effect on 1000 grain weight (F values are 1.46 and 3.72 respectively) (Table 1).

1.7 Effect of paclobutrazol on 'LD-1' yield under different fertilization levels

The change of 'LD-1' yield is regulated by fertilizer level and paclobutrazol treatment at the same time. The effect of fertilizer level on yield reaches a significant level (F=5.44*) (Figure 6), while the effect of paclobutrazol treatment on yield reaches a very significant level (F=9.83**) (Table 1). With the increase of fertilizer level and paclobutrazol concentration, the yield showed a trend of first increasing and then decreasing. At B3 level, the yield was higher than that at other levels. The yield of A3B3 combination was the highest, 281.67 kg, which increased by 33.64% compared with the control (210.77 kg).

2 Discussion

In order to solve the problem of high plant height and easy lodging of Shanlan upland rice, the effects of different concentrations of compound fertilizer and PP333 compound treatment on plant height, yield components and other agronomic traits of Shanlan upland rice were studied. The results showed that compound fertilizer and paclobutrazol had interaction effects on the main agronomic traits of Shanlan upland rice. Applying 20 kg of Hefei per mu and spraying 225 mg/kg paclobutrazol 35 days after sowing, the plant height decreased by 25.29% compared with the control, the effective panicles per plant increased by 34.28%, and the yield increased by 33.64%. This provided an important theoretical basis and technical support for the high-yield cultivation of Shanlan upland rice, and was of great significance for promoting the development of Shanlan upland rice industry in Hainan.

Paclobutrazol, a triazole plant growth regulator, can inhibit endogenous gibberellin synthesis and control plant growth (Yang et al., 2018). In the research on the effect of paclobutrazol on rice, spraying 300 mg/kg and 100 mg/kg paclobutrazol at booting and heading stages of rice can inhibit the elongation of rice stems and increase yield and income (Wu et al., 2010). Spraying paclobutrazol can significantly reduce the plant height of machine transplanted rice, improve the bending resistance of the stem, and increase the 1000 grain weight and seed setting rate, but the number of grains is significantly reduced, so the purpose of increasing production is not achieved (Tang et al., 2017).

In the study on the effect of paclobutrazol on plant height and yield of indica japonica rice 'Puyou 201' in the field, plant height, spike length, grain number, number of stem nodes, internode length and stem dry matter weight were negatively correlated with paclobutrazol concentration; 1000 grain weight was positively correlated with paclobutrazol concentration, while spike number was not; At the same time, the concentration also has different effects on the growth period. Considering multiple factors such as plant height and yield composition, it is most appropriate to spray 900 mg/L paclobutrazol before jointing (Gu et al., 2018).

This study shows that different levels of fertilization mainly affect the yield by affecting the number of grains per ear, while paclobutrazol treatment significantly reduces plant height and increases the number of effective panicles per plant. The extremely significant increase in the number of effective panicles per plant compensates for the effect of shorter ear length and lower number of grains per plant on yield, so the yield is significantly improved. When the fertilization level and paclobutrazol treatment interact, it mainly reduces plant height, The yield of Ledong Shanlan upland rice 'LD-1' was increased by increasing the effective panicles per plant and 1 000 grain weight. The results of this study are similar to those reported above, but there are differences in the mechanism of increasing yield.

Paclobutrazol can reduce plant height, and the use of paclobutrazol alone may reduce plant biomass. In this study, compound fertilizer and paclobutrazol are used to compensate for the reduction effect of paclobutrazol, which not only reduces plant height, but also increases effective panicles and yield, filling the gap of dwarf cultivation technology of Shanlan upland rice, and has important application value.

3 Materials and Methods

3.1 Test materials

The experimental material was selected from Ledong Shanlan upland rice resources and named 'LD-1'. The tested fertilizer is compound fertilizer (N15: P15: K15); The applied growth regulator is Paclobutrazol 15% wettable powder (produced by Shanghai Yuelian Chemical Co., Ltd.).

3.2 Test design

Two factor split area design is adopted, with fertilization level as the main area and dwarfing treatment as the sub area (Table 2). Set 4 fertilization levels, which are A1 (0 kg), A2 (10 kg), A3 (20 kg) and A4 (30 kg) respectively, and apply them all as base fertilizer. Four paclobutrazol dwarfing treatments were sprayed 35 days after sowing, with the concentrations of B1 (0 mg/kg), B2 (150 mg/kg), B3 (225 mg/kg), and B4 (300 mg/kg) respectively. Each combination was repeated three times, and then sprayed. The row spacing is 30 cm, the plant spacing is 23 cm, and the plot area is 30 m².

Table 2  Design of crack zone for two factors on fertilization application and dwarfing treatment Note: The numbers in parentheses in the table represent 16 processing numbers

3.3 Measurement and statistics of important agronomic traits

At the maturity stage, 10 plants were randomly selected from each plot, and the plant height, effective panicles per plant, panicle length, grains per panicle and 1000 grain weight were determined, as well as the plot yield.

3.4 Data processing

Microsoft Excel was used for data processing and mapping, DPS 7.05 was used for variance analysis of data, and Duncan's new complex range method was used for multiple comparisons.

Authors’ Contributions

WXH and TLQ were the experimental design and executors of this study; WXH, TLQ and LYH completed the data analysis and the writing of the first draft of the paper; HX, WXN, XJ and ZHL participated in the experimental design and analyzed the experimental results; TLQ, the designer and person in charge of the project, guided the experimental design, data analysis, paper writing and revision. All authors read and approved the final manuscript.

Acknowledgments

This research was jointly supported by the Key Projects "Seven Major Crop Breeding" of the National Key Research and Development Plan of China (2016YFD0101102, 2017YFD0100100), the Scientific Research Project of Hainan Academician Innovation Platform in 2020 (Screening of High Yield and High Efficiency Regenerative Rice Varieties and Application Demonstration of Production Technology), and Special Project for the Construction of Modern Agricultural Industrial Technology System (CARS-01-89).

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