Heterosis Expression of Hybrid Rice in Natural- and Short-Day Length Conditions

Rice Science, 2015, 22(2): 81 88 Copyright 2015, China National Rice Research Institute Hosting by Elsevier B.V. All rights reserved DOI: 10.1016/S1672-6308(14)60286-9 Heterosis Expression of Hybrid Rice in Natural- and Short-Day Length Conditions HUANG Zhi-yuan, ZHAO Bing-ran, LV Qi-ming, FU Xi-qin, XIN Ye-yun, YUAN Long-ping (State Key Laboratory of Hybrid Rice, Hunan Hybrid Rice Research Center, Changsha 410125, China) Abstract: Development of hybrid rice is an important strategy to increase yields of irrigated rice in China. Day length is a highly important environmental factor for hybrid rice adaptability, productivity and quality. Heterosis of hybrid rice has been utilized under natural- (long) day length conditions in the middle and lower reaches plain of Yangtze River Valley in China for a long time. However, whether the hybrid rice can show heterosis under short-day length conditions is unknown. In this study, we analyzed plant height, grain number per panicle, and other yield related traits of hybrid rice Liangyoupeijiu (LYP9) and its parents (Pei ai 64S and 93-11) under natural- and short-day length conditions and compared the respective heterosis. Results showed that earlier heading, reduced plant height, and reduced grain number per panicle were found in these three varieties under short-day length conditions relative to those under natural-day length conditions. In either natural- or short-day length conditions, heterosis of LYP9 was mainly displayed in grain number per panicle and plant height. This study may help in breeding and selecting suitable hybrid rice for different regions with natural- or short-day length. Key words: hybrid rice; heterosis; photoperiod; short-day length Research on hybrid rice in China started in 1964. A three-line breeding system composed of a cytoplasmic male sterile (CMS) line, a CMS maintainer, and a CMS restorer line was established in 1973 to generate hybrid rice, and commercial production of hybrid rice began in 1976 (Yuan, 1986). Wild-abortive CMS (CMS-WA) is the first and main type of male sterility used in hybrid rice breeding. The most popular WA-type hybrid, Shanyou 63, with Minghui 63 as the male sterile line, yielded 7.5 9.0 t/hm 2 ; Shanyou 63 was planted on a total acreage of 6.2 10 7 hm 2 between 1984 and 2003, occupying the largest acreage for a single hybrid from 1987 to 2001, and the highest annual acreage of over 6.7 10 6 hm 2 in 1990 because of its good general combining ability. However, some constraints and problems are encountered in such system, such as its genetic vulnerability and plateaued yields, because of insufficient genetic diversity (Cheng and Min, 2000). In 1995, a two-line hybrid rice technology with an environment-sensitive genic male sterile (EGMS) line and a restore line (R line) was successfully commercialized in China and widely used in 1998 (Li and Yuan, 2000; Yuan, 2004). This Received: 2 September 2014; Accepted: 9 December 2014 Corresponding author: HUANG Zhi-yuan (huangzhiyuan@hhrrc.ac.cn) method is designed and developed based on the two new kinds of rice genetic materials, namely photosensitive and thermo-sensitive genic male sterile lines. Male sterility is mainly controlled by one or two pairs of recessive nuclear genes. The advantages of the two-line system over the three-line system include a wider range of germplasm resources used as breeding parents, better grain quality and higher yields, and simpler procedures for breeding and hybrid seed production (Yang et al, 2007). However, the dependency of male sterility on temperature and day length requires more attention from breeders and seed producers. To date, hybrid rice planting area has occupied more than 50% of the total rice planting area in China due to about 10% 20% yield advantage over inbred varieties (Cheng et al, 2007). In recent years, two-line hybrid rice has become an important type in rice production in China, accounting for about 20% of the total planting area of hybrid rice (Si et al, 2011). Thus, the current annual acreage of hybrid rice has increased from 1.4 10 5 hm 2 in 1976 to about 1.7 10 7 hm 2 in China. Heterosis is an important and complex trait of hybrid rice that exhibits superior phenotypes relative to its parents, including enhanced biomass production, development rate, grain yield, and stress tolerance.

82 Rice Science, Vol. 22, No. 2, 2015 Thus, it has been effectively utilized to increase rice production from 1970s in China (Zhang et al, 1994; Xiao et al, 1995; Yu et al, 1997; Luo et al, 2001; Hua et al, 2003). Utilization of indica-indica heterosis was a fundamental principle for rice yield improvement during 1970s to 1980s. The heterosis between subspecific varieties is an effective strategy to increase rice yield further (Lu and Zou, 2005). The pioneer super hybrid rice Liangyoupeijiu (LYP9), with the target yield of 10.5 t/hm 2 in 2000 (Yuan, 2006), has been well demonstrated under high-yielding cultivation conditions. LYP9 was developed by a cross of the paternal 93-11, an indica variety widely grown in China (Yu et al, 2002), with the female parent Pei ai 64S (PA64S), a javanica EGMS line with its wide compatibility gene (S n 5 ) from Padi, a cultivar from Indonesia, and its EGMS character from Nongken 58S, a mutant from a japonica cultivar Nongken 58. LYP9 has been widely cultivated for commercial production with 20% to 30% more yield per hectare than its parent lines in the rice growing area between 12 N and 34 N, including southern regions of China and Southeast Asia (Lu et al, 2004). The hybrid has a growth period of 150 ± 24 d with 15 16 leaves, and an averaged grain yield of 8.4 ± 2.6 t/hm 2 as single seasonal rice. The plant height of LYP9 is 112 ± 19 cm, its tillering ability is strong and the effective tiller number per square meter is usually 260 ± 50, and the 1000-grain weight is 26.2 ± 2.4 g (Lu and Zou, 2005). During 2000 to 2009, the planting area of LYP9 was 5.6 10 6 hm 2, accounting for 26.7% of the total area of two-line hybrid rice. The most suitable region for LYP9 is between 28 N to 30 N. The daily average temperature of 26 C to 28 C at heading is thought to be more favorable for getting a higher seed-setting rate and grain yield (Lu et al, 2004). The growth period of LYP9 is sensitive to day length. The heading promotion rate of LYP9 is 21.0% in the short-day length and high-temperature conditions, and 13.6% in natural- (long) day length and high-temperature conditions. LYP9 is widely cultivated in 28 N, where the day length is longer than 13 h, ensuring a stable stage for its growth and development (Zou et al, 2008). For example, the opportune sowing date of LYP9 is before 21 June in Wenzhou (27 28 N), China, so that when it is sowed after 21 June, the grain number per panicle and seed-setting rate would decrease significantly because the day length is respectively more than 13 h from 21 April to 21 August and 12 h from 1 October to 1 April in 26 30 N (Bai et al, 2006). Day length, defined as the interval between sunrise and sunset, is known as photoperiod. Rice, according to its subtropical origin, is considered as a short-day plant and can be classified into photoperiod-sensitive and photoperiod-insensitive types, the latter showing a low response or a slight delay in flowering with an increase in photoperiod. Most of the wild species of Oryza and many of the primitive cultivated rice species are photoperiod-sensitive and may be classified as short-day plants. Most of the cultivated rice species may eventually become photoperiodinsensitive ones (Vergara and Chang, 1985). During the process of rice introduction to the regions at high latitudes, rice breeders have selected lines with weak sensitivity to photoperiod to ensure maturity during the period of optimal climatic conditions (Izawa, 2007; Xue et al, 2008; Takahashi et al, 2009; Wei et al, 2010; Ebana et al, 2011). Photoperiod insensitivity plays an important role in expanding the range of rice cultivation to high latitudes (Fujino and Sekiguchi, 2005; Izawa, 2007; Xue et al, 2008). In hybrid rice breeding, most of R lines are insensitive or weakly sensitive to photoperiod, which is suitable for hybrid rice cultivation and extension. Heterosis of hybrid rice has been long utilized under natural-day length conditions in the middle and lower reaches plain of Yangtze River Valley in China. However, whether this hybrid rice can show heterosis in the short-day length conditions is unknown. In this study, we analyzed the heterosis relative phenotypes of LYP9 under natural- and artificial short-day length conditions in Changsha City (28 N) of China to determine the effects of photoperiod on heterosis expression. PA64S, LYP9 and 93-11 were grown in winter in Sanya City (18 N) for comparison. Results showed that the short-day length treatment affected the development and yielding traits of LYP9 and its parents to some degrees. LYP9 displayed heterosis in plant height and grain number per panicle under the natural- and short-day length conditions, which would help to reveal the mechanisms underlying day length regulation of the rice heterosis. MATERIALS AND METHODS Rice materials and growth conditions The experiment base is in Changsha, which experiences subtropical climate. Super hybrid rice LYP9 and its parental lines (93-11 and PA64S) were grown in the

HUANG Zhi-yuan, et al. Heterosis Expression of Hybrid Rice in Different Day Length Conditions 83 rice fields. Approximately 600 kg/hm 2 compound fertilizer (N-P 2 O 5 -K 2 O:16-16-16) were applied before seedling transplantation. The 3 4 d germinated seedlings of these varieties were raised in seedling nursery bed for about 25 d in the natural conditions and were transplanted to the paddy fields (spacing of 16.5 cm 20.0 cm) in a randomized block design with three replicates. PA64S was sowed 8 d before LYP9 and 93-11 to attain simultaneous flowering. In winter, super hybrid rice LYP9 and its parental lines were grown in Sanya, which belongs to tropical climate with the same cultivation method as comparison. Short-day length treatment In Changsha, the short-day length conditions started from 9:00 AM to 6:00 PM at the initial development of the main panicle of plant that was, LYP9, PA64S and 93-11 began the short-day length 55, 42 and 51 d after sowing. The plants were covered by a box wrapped with shade cloth. Approximately 100% of light was blocked by shade cloth (Fig. 1). No temperature difference existed between inner and outer sides of the box. The light time was 9 h and the dark time was 15 h in the short-day length conditions. By comparison, the light time was about 13 h and the dark time was about 11 h in the natural-day conditions. Thus, the light time was reduced by about 4 h in the short-day length conditions compared with the natural day conditions. In Sanya, the day length increased from 11 h to 12.5 h gradually from 20 December to 20 April. Growth observation and yield component characters examination During rice growth and development, the different growth stages were recorded. Rice yield is dependent on grain number, seed-setting rate, effective panicle number, and 1000-grain weight. Before grain maturity, effective panicle number per plant was examined on the basis of five points per plot, and each point included ten hills. Grain number per panicle, seed-setting rate, and 1000-grain weight of the plants were examined (six hills per plot). Mid-parent heterosis (MPH) and high-parent heterosis (HPH) were calculated according to the following formula: MPH = (F 1 MP) / MP 100%, and HPH = (F 1 HP) / HP 100%, where F 1 is the performance of the hybrid, MP is the average performance of the two parents, and HP is the best performance of the two parents. Hypothesis testing was performed using t-test. Theoretical yield = Effective panicle number per hectare Grain number per panicle Seed-setting rate 1000-grain weight. RESULTS Effects of short-day length on rice plant development The rice plants were treated under short-day length at the stages from initial development to full heading of the main panicle. Particularly, LYP9, PA64S and 93-11 plants were treated under short-day length for 32, 34 and 37 d, respectively because each variety had different sensitive to day length. The days from sowing to heading of LYP9 in the natural- and short-day length conditions were 94 and 85 d, respectively (Tables 1 and 2). LYP9 headed 9 d earlier in the short-day length conditions than that in the natural-day conditions. The entire growth periods of LYP9 in the natural- and short-day length conditions were 133 and 116 d, respectively. The harvesting of LYP9 in the short-day length conditions was 17 d earlier than that in the natural-day conditions. The days from sowing to heading of the paternal line 93-11 in the natural- and short-day length conditions were 94 and 87 d, respectively, indicating that the A B C D Fig. 1. Effects of short-day treatment on plant development of hybrid rice Liangyoupeijiu (LYP9) and its parents Pei ai 64S (PA64S) and 93-11. A, Rice plants were covered by the box wrapped with shade cloth; B, PA64S; C, LYP9; D, 93-11. Arrows showed that the panicles in the short-day length conditions headed earlier than those in the natural-day length conditions.

84 Rice Science, Vol. 22, No. 2, 2015 Table 1. Treatments in the natural- and short-day length conditions in hybrid rice Liangyoupeijiu (LYP9) and its parents Pei ai 64S (PA64S) and 93-11 in 2012, Changsha, China. Variety Treatment Days from Sowing to heading (d) Days from sowing to harvest (d) Treatment time (Month-Day) Short day treatment (d) PA64S Natural-day 77 NA a Short-day 70 NA 07-21 to 08-24 34 LYP9 Natural-day 94 133 Short-day 85 116 07-26 to 08-27 32 93-11 Natural-day 94 133 Short-day 87 116 07-22 to 08-28 37 a PA64S had no seeds set because of male sterile. heading day of 93-11 is 7 d earlier in the short-day length conditions than in the natural-day conditions. The entire growth duration of 93-11 in the naturaland short-day length conditions were 133 and 116 d, respectively, leading to harvesting 17 d earlier in the short-day length conditions than in the natural-day conditions. The days from sowing to heading of PA64S in the natural- and short-day length conditions were 77 and 70 d, respectively. The PA64S was completely male sterile in both conditions and headed 7 d earlier in the short-day length conditions than in the natural-day conditions. These results showed that the short-day length conditions can promote the rice flowering and reduce the entire growth period. The difference between LYP9 and 93-11 in the short-day length conditions was small, but 93-11 needed more short-day length conditions time to head fully, thereby shortening the entire growth period. In Sanya, PA64S and LYP9 headed 13 15 d earlier than 93-11 (Table 2). The entire growth duration of LYP9 and 93-11 were 124 and 136 d, respectively. The difference of entire growth period between LYP9 and 93-11 was considerable. In Sanya, the harvesting of LYP9 was 9 d earlier than in the natural-day conditions in Changsha, but the harvesting day of 93-11 was only 3 d later than in the natural-day conditions in Changsha, with unremarkable difference. The day length of Sanya is shorter than that of Changsha in the natural-day conditions. The results showed that LYP9 was more sensitive to short-day length than 93-11. length conditions (Table 3 and Fig. 3-A). Furthermore, LYP9 was the tallest, whereas PA64S was the shortest among the three varieties. These results suggested that the effect of short-day length treatment on the plant height is the largest in LYP9 and relatively smallest in PA64S. The MPH and HPH of plant heights of LYP9 were 22.0% and 3.3% in the natural-day conditions, respectively, and were 20.3% and 1.5% in the short-day length conditions, respectively (Table 3), indicating that MPH of plant height in LYP9 was stronger than HPH. The plant heights of PA64S, LYP9 and 93-11 were 65.8, 96.4 and 90.8 cm, respectively, in Sanya, which were decreased 18.0, 28.3 and 29.9 cm compared with those in the natural-day conditions in Changsha. The MPH and HPH of plant height of LYP9 were 23.2% and 6.2%, respectively (Table 4). These results suggested that LYP9 displayed stronger heterosis in plant height both under the natural- and short-day length conditions. Effects of short-day length treatment on grain number per panicle Grain number per panicle is a key trait of rice yield. For LYP9, The grain number per panicle was 190.3 in the natural-day conditions and 179.0 in the short-day Effects of short-day length treatment on plant height Plant heights of LYP9 in the natural- and short-day length conditions in Changsha were 124.7 and 118.3 cm, respectively, whereas the corresponding plant heights of 93-11 were 120.7 and 116.6 cm. Similarly, plant heights of PA64S were 83.8 cm in the natural-day conditions and 80.2 cm in the short-day Fig. 2. Effects of short-day treatment on development of hybrid rice Liangyoupeijiu (LYP9) and its parents Pei ai 64S (PA64S) and 93-11. ND, Natural-day length conditions; SD, Short-day length conditions.

HUANG Zhi-yuan, et al. Heterosis Expression of Hybrid Rice in Different Day Length Conditions 85 Table 2. Growth stages of hybrid rice Liangyoupeijiu (LYP9) and its parents Pei ai 64S (PA64S) and 93-11 in 2012 2013, Sanya, China. Variety Sowing date (Year-Month-Day) Initial heading date (Year-Month-Day) Full heading date (Year-Month-Day) Days from sowing to full heading (d) Days from sowing to harvest (d) PA64S 2012-12-20 2013-03-25 87 LYP9 2012-12-10 2013-03-15 2013-04-13 89 124 93-11 2012-12-10 2013-03-25 2013-04-25 102 136 Table 3. Yield component characters of hybrid rice Liangyoupeijiu (LYP9) and its parents Pei ai 64S (PA64S) and 93-11 in Changsha, China. Variety and Plant height Effective panicle number Seed-setting rate 1000-grain weight Treatment Grain number per panicle heterosis (cm) per plant (%) (g) PA64S Natural-day 83.8 ± 3.3 Bb 10.6 ± 2.0 Bb 177.1 ± 18.1 Aa PA64S Short-day 80.2 ± 3.5 Aa 10.6 ± 3.9 Bb 159.4 ± 24.9 ABab LYP9 Natural-day 124.7 ± 4.7 Fe 8.2 ± 1.7 Aa 190.3 ± 23.2 ABb 65.0 Aa 26.42 ± 0.72 Aa LYP9 Short-day 118.3 ± 3.8 CDc 7.3 ± 1.6 Aa 179.0 ± 21.5 Bb 84.0 Bb 27.25 ± 0.75 Ab 93-11 Natural-day 120.7 ± 4.5 Dd 7.3 ± 1.5 Aa 178.2 ± 41.1 Bb 80.0 BCc 31.33 ± 1.32 Bc 93-11 Short-day 116.6 ± 4.9 Cc 7.3 ± 1.6 Aa 143.4 ± 41.0 Bb 88.0 Cc 31.44 ± 0.72 Bc MPH (%) Natural-day 22.0 7.1 HPH (%) Natural-day 3.3 6.8 MPH (%) Short-day 20.3 18.2 HPH (%) Short-day 1.5 12.3 MPH, Mid-parent heterosis; HPH, High-parent heterosis. Values followed by the same lowercase letter and uppercase letters in the same column are not significant at the 0.05 and 0.01 levels, respectively. Table 4. Economic characters of hybrid rice Liangyoupeijiu (LYP9) and its parents Pei ai 64S (PA64S) and 93-11 in 2012 2013, Sanya, China. Variety and heterosis Plant height (cm) Effective panicle number per plant Average grain number per panicle Seed-setting rate (%) 1000-grain weight (g) PA64S 65.8 ± 3. 7 Aa 8.3 ± 3.2 ab 153.3 ± 21.6 Aa LYP9 96.4 ± 4.0 Cc 10.4 ± 3.7 b 183.0 ± 21.1 Bb 72.0 26.59 93-11 90.8 ± 4.9 Bb 7.1 ± 1.8 a 156.2 ± 4.7 ABa 58.0 28.71 MPH (%) 23.2 35.5 18.2 HPH (%) 6.2 2.6 17.2 MPH, Mid-parent heterosis; HPH, High-parent heterosis. Values followed by the same lowercase letter and uppercase letters in the same column are not significant at the 0.05 and 0.01 levels, respectively. length conditions, whereas for 93-11 and PA64S, their grain numbers per panicle were 178.2 and 177.1 under natural-day conditions and 143.4 and 159.4 under short-day length conditions, respectively (Table 3 and Fig. 3-B). These results suggested that the effect of the short-day length treatment on grain number was the largest in 93-11 and the smallest in LYP9. The MPH and HPH of grain number per panicle of LYP9 were 7.1% and 6.8%, respectively, under natural-day conditions, and 18.2% and 12.3%, respectively, under short-day length conditions (Table 3). The grain numbers per panicle of PA64S, LYP9 and 93-11 were 153.3, 183.0 and 156.2, respectively in Sanya, which were decreased 23.8, 7.3 and 22.0 cm compared with those under natural-day conditions in Changsha. The MPH and HPH of plant heights for LYP9 were 18.2% and 17.2% (Table 4). These results suggested that LYP9 exhibited stronger heterosis in grain number per plant under the natural- and short-day length conditions. Effects of short-day length treatment on seed-setting rate and 1000-grain weight The seed-setting rate of LYP9 under natural- and short-day length conditions were 65% and 84%, respectively. The seed-setting rate of 93-11 under natural- and short-day length conditions were 80% and 88%, respectively. The difference of seed-setting rate of LYP9 between under natural- and short-day length conditions was significant (Table 3 and Fig. 3-C). The results showed that short-day length conditions can improve seed-setting rate. The 1000-grain weights of LYP9 under natural- and short-day length conditions were 26.42 and 27.25 g, respectively. The 1000-grain

86 Rice Science, Vol. 22, No. 2, 2015 Fig. 3. Effects of short-day treatment on hybrid rice Liangyoupeijiu (LYP9) and its parents Pei ai 64S (PA64S) and 93-11. A, Plant height; B, Average grain number of per panicle; C, Seed-setting rate. ND, Natural-day length conditions; SD, Short-day length conditions. weights of 93-11 under natural- and short-day length conditions were 31.33 and 31.44 g, respectively. Although the differences between LYP9 and 93-11 were small, the short-day length conditions were beneficial for 1000-grain weight in both varieties. PA64S was completely male sterile under natural- and short-day length conditions. The seed-setting rates of LYP9 and 93-11 were 72% and 58%, respectively, in Sanya. And the seed-setting rate of LYP9 was higher than that of 93-11. The 1000-grain weights of LYP9 and 93-11 were 26.59 and 28.71 g, respectively (Table 4). The 1000-grain weight of 93-11 was lower under natural-day conditions (Table 3). Effects of short-day length treatment on effective panicle number per plant The effective panicle number per plant of PA64S under natural- and short-day length conditions were both 10.6, and have the highest effective panicle numbers per plant. The effective panicle number per plant of 93-11 under natural- and short-day length conditions were both 7.3, whereas those of LYP9 were 8.2 and 7.3, respectively. The effective panicle number per plant of LYP9 was between those of PA64S and 93-11, and was higher under natural-day conditions, but the difference was not significant (Table 3). The effective panicle number per plant of PA64S, LYP9 and 93-11 were 8.3, 10.4 and 7.1, respectively, in Sanya (Table 4). The effective panicle number per plant of LYP9 was larger than those of PA64S and 93-11. DISCUSSION Hybrid rice has been used in commercial production for four decades, and it is a good choice for resolving food security problem. Currently, approximately 17 million hectares are allotted for hybrid rice production in China and another 4 million hectares in other tropical countries, mainly in Bangladesh, India, Indonesia, Myanmar, the Philippines and Vietnam (Xie et al, 2013). However, the hybrid rice extension area in these countries is still very small compared with that in China. Many Chinese hybrid rice perform poorly in tropical countries because of the environment changes, including day length and temperature. Our study revealed that the heterosis of LYP9 mainly displayed in grain number per panicle and plant height in both natural- and short-day length conditions. Results also showed that the short-day length treatment could not only shorten the flowering time but also reduce the plant height, as described by Ganashan and Whittington (1976) and Craufurd et al (2003). In China, CMS and EGMS lines usually flower earlier than R lines and hybrid rice. PA64S is a typical short-day plant with a short growth period. PA64S and 93-11 have some similarities in terms of plant development and height under short-day length conditions compared with those under natural-day conditions. They both flowered 7 d earlier, and their plant heights were reduced by 3 4 cm under short-day length conditions compared with those under naturalday conditions. Panicle primordial of rice plants are initialized in response to a short photoperiod and are delayed or inhibited under long photoperiods (Yoshida, 1981). This result also confirmed that rice is sensitive to short-day length when it initiated panicle primordial even in the rice hybrid. Notably, the grain number per panicle of PA64S and 93-11 were reduced by 17.7 and 34.9 under short-day length conditions compared with those under natural-day conditions, but the grain number per panicle of 93-11 was more affected under short-day length conditions. The development of LYP9 was tended to 93-11 under natural- and short-day length conditions. They both flowered 7 9 d earlier and matured 17 d earlier under short-day length conditions. However, 93-11 flowered and matured later than

HUANG Zhi-yuan, et al. Heterosis Expression of Hybrid Rice in Different Day Length Conditions 87 LYP9 when they grew in Sanya, owing to reduced day length and temperature change. The grain number per panicle of LYP9 was higher under natural-day conditions than under short-day length conditions, but the seed-setting rate of LYP9 was lower under natural-day conditions than under short-day length conditions. The effective panicle number per plant of LYP9 was PA64S and 93-11 in both conditions in Changsha, but in Sanya, the seed-setting rate of LYP9 was higher than that of 93-11, and the effective panicle number per plant of LYP9 was higher than those of PA64S and 93-11. The seed-setting rate and 1000-grain weight of 93-11 under short-day length conditions were relative higher under natural-day conditions. This result confirmed that the growth duration was shortened, plant height and total grain number per panicle were decreased, and effective panicle number per plant was increased when hybrid rice were grown under short-day length conditions (Mao et al, 2005). Based on the results, we can conclude that the plant height and grain number per panicle are strongly affected by day length. Although the grain numbers per panicle of PA64S, LYP9 and 93-11 were all decreased under short-day length conditions, LYP9 still showed heterosis with regard to grain number per panicle and plant height. The yield of hybrid rice in short-day length conditions was lower than that in natural-day length conditions because of decreased grain number per panicle. The heterosis expression of LYP9 under natural-day conditions is better than that under short-day length conditions. 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