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CROP BREEDING & GENETICS

Grain Yield and Yield Attributes of New Plant Type and Hybrid Rice

^a Crop Physiology and Ecology Research Division, National Institute of Crop Science, Rural Development Administration, 209 Seodundong, Suwon 441-857, Republic of Korea
^b Crop and Environmental Sciences Division, IRRI, DAPO Box 7777, Metro Manila, Philippines
^c Institute of Biological Sciences, College of Arts and Sciences, Univ. of the Philippines, College, Los Baños, 4031 Laguna, Philippines

^* Corresponding author (s.peng{at}cgiar.org).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Development of new plant types (NPTs) and hybrids are two major approaches for improving the yield potential of irrigated rice (Oryza sativa L.). This study was conducted (i) to compare grain yield and yield attributes among three high-yielding groups of rice, namely indica inbred, indica/indica F₁ hybrid, and second-generation NPT, and (ii) to identify the morphophysiological traits responsible for the yield difference among the three groups. Fifteen genotypes, five from each of the three groups, were grown in the dry (DS) and wet seasons (WS) of 2003 and 2004 at the International Rice Research Institute, Philippines. On average, hybrids produced 11 to 14% greater grain yield than indica inbreds and NPTs in the DS. In the WS, the difference in grain yield was relatively small among the three groups. High grain yield of hybrids in the DS was the result of high number of spikelets per square meter due to a large number of spikelets per panicle and high harvest index rather than biomass production. The NPTs did not show yield advantage over the indica inbreds and demonstrated significantly lower yield than hybrids, mainly because of fewer spikelets per panicle and per square meter. Spikelet production efficiency per unit of N uptake and per unit of aboveground biomass at physiological maturity was generally higher in hybrids than indica inbreds or NPTs. Increasing harvest index and spikelet production efficiency by developing NPTs with more spikelets per panicle should be emphasized for improving the grain yield of NPTs.

Abbreviations: DS, dry season • NPT, new plant type • WS, wet season

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
IMPROVING RICE (Oryza sativa L.) grain yield per unit land area is the only way to achieve increased rice production because of the reduction in area devoted to rice production (Cassman, 1999). Irrigated rice contributes >75% of total rice production, although it accounts for about 55% of total rice area. Rice varieties with higher yield potential must be developed to enhance the average farm yields of irrigated rice to increase the world's total rice production.

In 1966 IRRI released IR8, the first high-yielding modern rice cultivar for the tropical irrigated lowlands. In the late 1960s and early 1970s, yields of 9 to 10 t ha^–1 were often reported for IR8 under favorable irrigated conditions (Chandler, 1969). Since then, IRRI has developed many high-yielding indica inbred cultivars such as IR36, IR64, and IR72 (Peng and Khush, 2003). Maximum grain yield of these indica inbred cultivars was also recorded as high as 10 t ha^–1 in the tropical irrigated lowlands (Peng et al., 2000). This suggests that a yield plateau has been reached in the indica inbred breeding approach under tropical irrigated conditions (Peng et al., 1999). To break the yield barrier founded in indica inbred varieties, development of hybrid rice was begun in the late 1970s and new plant type (NPT) rice in the late 1980s at IRRI (Virmani et al., 1982; Khush, 1995).

Hybrid rice has been grown in China since 1976 and today >50% of China's rice area is now planted to rice hybrids (Yuan, 2003). Hybrid rice had a yield advantage of about 15% over the best inbred cultivars in farmers' fields (Yuan, 1994). Recently, hybrid rice cultivars have been commercialized in India, Vietnam, and the Philippines (Virmani and Kumar, 2004). Peng et al. (1999) reported that indica/indica hybrid rice increased yield potential by about 9% compared with the best indica inbred cultivars in experimental fields under tropical irrigated conditions. The reasons for yield advantages of hybrid rice have been studied extensively. Hybrids have a higher growth rate during early vegetative stages as a result of rapid expansion of leaf area (Yamauchi, 1994; Laza et al., 2001). Cao et al. (1980) and Kabaki (1993) reported that hybrids have more efficient sink formation relative to the rate of dry matter accumulation at the flowering stage than inbreds. Hybrids maintain a relatively high grain-filling percentage despite the large number of spikelets because the hybrids translocate carbohydrate accumulated in the culm and sheath to developing grains more efficiently than inbred cultivars (Yan, 1988; Song et al., 1990). It is unknown, however, if the hybrids have a yield advantage over the NPT lines.

The development of NPT rice at IRRI was inspired by Donald's (1968) ideotype breeding approach. The goal was to develop NPT cultivars with a yield potential 20 to 25% higher than current existing semidwarf rice cultivars under a tropical environment during the dry season. The NPT was designed based on the results of simulation modeling and the new traits were mostly morphological since they are easier to select than physiological traits in breeding programs. The proposed NPT has a low tillering capacity (three to four tillers when direct seeded), few unproductive tillers, 200 to 250 grains per panicle, a plant height of 90 to 100 cm, thick and sturdy stems, dark green and erect leaves, a vigorous root system, 100- to 130-d growth duration, and increased harvest index (Peng et al., 1994).

The first-generation NPT lines based on tropical japonicas were developed in 1993. As intended, the NPT lines had large panicles, few unproductive tillers, and lodging resistance. They did not yield well, however, because of limited biomass production and poor grain filling (Peng and Khush, 2003). It was speculated that the excessive reduction in tillering capacity resulted in low biomass production of the first-generation NPT lines. The first-generation NPT lines were susceptible to diseases and insects, and had poor grain quality, thus the lines could not be released as cultivars but were valuable genetic materials for rice breeding programs.

In 1995, development of the second-generation NPT lines was begun by crossing the first-generation tropical japonica NPT lines with elite indica parents. Indica parents have effectively increased tillering capacity and reduced panicle size (i.e., number of spikelets per panicle) in the second-generation NPT lines. Indica germplasm also helped improve other NPT attributes such as grain quality and disease and insect resistance. Some second-generation NPT lines (F₅ generation) were planted in a replicated observational trial for the first time in the 1998 wet season (Laza et al., 2003). Overall, the second-generation NPT lines showed yield advantage over the first-generation NPT lines. A rigorous comparison between the second-generation NPT lines and indica inbred varieties is needed, however, to determine the progress of improving rice grain yield by NPT breeding.

The objectives of this study were to: (i) compare yield performance of recently developed second-generation NPT, indica/indica hybrid, and indica inbred rice cultivars; and (ii) identify the morphophysiological traits responsible for the yield differences among the three high-yielding groups of rice cultivars.

	MATERIALS AND METHODS

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Field experiments were conducted in the same field in the dry seasons (DS) and the wet seasons (WS) of 2003 and 2004 at the research farm of IRRI, Los Baños, Laguna, Philippines (14°11'N, 121°15'E, 21-m elevation). The soil was an Aquandic Epiaquoll with pH 6.0, 16.2 g organic C kg^–1, 1.50 g total N kg^–1, and 32.9 cmol kg^–1 cation exchange capacity.

In each cropping season, five rice genotypes each from three high-yielding groups of rice cultivars, namely indica inbred, indica/indica F₁ hybrid, and the second-generation NPT from a cross between tropical japonica and indica, were used in the study. The genotypes were not exactly the same across the four seasons. For breeding lines, the selection was based on the improved grain yield in replicated yield trials done in a breeding nursery, thus a few newly developed lines replaced old ones in 2004. For hybrid cultivars, the replacement was due to the availability of F₁ hybrid seeds. In addition, some cultivars such as IR8 and SL-8H could not be grown in the WS due to disease susceptibility. All genotypes were developed by IRRI except SL-8H and SL-11H, which were developed by a private seed company in the Philippines.

Pregerminated seeds were sown on seedling trays to raise uniform seedlings. Fourteen-day-old seedlings were manually transplanted on 9 January for the DS and 17 June for the WS for both years at a hill spacing of 0.2 by 0.2 m with four seedlings per hill. All fertilizers were broadcast manually and incorporation was done only for basal application. Basal fertilizers were applied at a rate of 30 kg P ha^–1, 40 kg K ha^–1, and 5 kg Zn ha^–1 in the DS, and 15 kg P ha^–1, 20 kg K ha^–1, and 5 kg Zn ha^–1 were applied in the WS. Nitrogen in the form of urea was split applied: 60 kg ha^–1 at basal, 40 kg ha^–1 at mid-tillering, 60 kg ha^–1 at panicle initiation, and 40 kg ha^–1 at flowering in the DS. In the WS, 90 kg N ha^–1 was split applied equally at basal, mid-tillering, and panicle initiation.

Plots were laid out in a randomized complete block design with four replications. Plot size was 25 m² and plots were separated by levees. Crop management followed the standard cultural practices in all seasons. The experimental field was flooded 4 d after transplanting and 5- to 10-cm water depth was maintained until 7 d before physiological maturity for each genotype, at which time the field was drained. Herbicide was used to control weeds. Pests were intensively controlled using chemicals to avoid yield loss. Stem borers were controlled by applying carbofuran (dimethylarsinic acid) at 1 kg a.i. ha^–1 during the tillering stage. Benomyl (methyl [1-[(butylamino)carbonyl]-1H-benzimidazol-2-yl]carbamate) at 0.3 g a.i. ha^–1 was sprayed at booting and at 2 wk after booting to control sheath blight. Pretilachlor [2-chloro-N-(2,6-diethylphenyl)-N-(2-propoxyethyl)acetamide] at 1 L a.i. ha^–1 was applied immediately after transplanting to control weeds.

Plant height was measured from the plant base to the tip of the highest leaf (or panicle, whichever was longer) for all 12 hills in each plot at flowering. At physiological maturity when 95% of the grains turned yellow, 12 hills (0.48 m²) were sampled diagonally from a 5-m² harvest area from each plot to determine aboveground biomass production, harvest index, and yield components. Panicle number of each hill was counted to determine the panicle number per square meter. Plants were separated into straw and panicles. Straw dry weight was determined after oven drying at 70°C to constant weight. Panicles of all 12 hills were hand threshed and filled spikelets were separated from unfilled spikelets by submerging them in tap water. Three subsamples of 30 g of filled spikelets and 5 g of unfilled spikelets were taken to count the number of spikelets. The dry weight of the rachis and filled and unfilled spikelets were determined after oven drying at 70°C to constant weight. Aboveground biomass was the total dry matter of straw, rachis, and filled and unfilled spikelets. Spikelets per panicle (total spikelet number/panicle number of all 12 hills), grain-filling percentage (100 x filled spikelet number/total spikelet number), and harvest index were calculated. Grain yield was determined from the 5-m² area in each plot and adjusted to the standard moisture content of 0.14 kg H₂O kg^–1. Nitrogen concentrations of straw, rachis, and filled and unfilled spikelets were determined by micro-Kjeldahl digestion, distillation, and titration (Bremner and Mulvaney, 1982) to calculate aboveground total N uptake.

Spikelet production efficiency was calculated as the ratio of total spikelet number to aboveground total biomass and total N uptake at physiological maturity. Panicle size per unit plant height at flowering and the product of harvest index and plant height at physiological maturity were calculated for identifying genotypes with large panicles without an increase in plant height and with high harvest index without reduction in plant height, respectively. In the 2003 WS and the 2004 DS, tiller numbers of 10 hills were counted weekly from 2 wk after transplanting to physiological maturity to determine maximum tiller number. The panicle number of the 10 hills was measured at physiological maturity. The productive tiller percentage was the panicle percentage at physiological maturity over the maximum tiller number.

Climatic data were collected from the weather station located in the research farm at IRRI for both years. Data were analyzed following analysis of variance (SAS Institute, 1982) and means of genotypes and the groups of rice cultivars were compared based on the least significant difference test at the 0.05 probability level for each season and year.

	RESULTS

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Daily mean temperature in the WS was higher from transplanting to 70 d after transplanting but was lower 90 d after transplanting than in the DS in both years (Fig. 1A ). The difference in daily mean temperature between 2003 and 2004 was small and inconsistent in both DS and WS. There was no significant difference in daily mean solar radiation between the DS and WS from transplanting to 50 d after transplanting (Fig. 1B). From 60 d after transplanting to physiological maturity, the DS had consistently higher solar radiation than the WS. In both DS and WS, 2003 had slightly higher seasonal mean solar radiation than 2004. In general, the DS showed higher radiation during reproductive and ripening phases and lower temperature during vegetative and reproductive stages than the WS.

View larger version (23K): [in this window] [in a new window]   Figure 1. Daily mean (A) temperature and (B) solar radiation during the growth period of rice (Oryza sativa L.) in the dry and wet seasons of 2003 and 2004.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Grain yield in the DS was greater than in the WS because there was higher solar radiation during the reproductive and ripening phases and lower temperature during vegetative and reproductive stages in the DS than in the WS (Tables 1 and 2, Fig. 1). The greater yield in the 2003 DS than the 2004 DS and in the 2004 WS than the 2003 WS was probably due to the difference in solar radiation during the ripening phase. Evans and De Datta (1979) reported that high radiation during reproductive and ripening phases was closely associated with high grain yield. Akita (1989) indicated that high temperature during the vegetative stage reduced sink size due to reduced spikelet production efficiency. Lower temperature during the vegetative stage and higher solar radiation during the ripening phase contributed to greater sink formation and better grain filling in the DS than the WS (Fig. 1, Table 3).

On the average, hybrids produced 11 and 14% higher grain yield than indica inbreds and NPTs, respectively, in the DS (Table 1). Higher yielding ability of hybrids in the DS was mainly due to the higher harvest index rather than aboveground total biomass at physiological maturity compared with indica inbreds and NPTs. In the WS, hybrids had a higher grain yield than the NPTs but there was no difference in grain yield between hybrids and indica inbreds (Table 2). Higher yield of hybrids in the WS than NPTs was also due to the difference in harvest index rather than total biomass. The NPTs did not demonstrate a yield advantage over indica inbreds in any season. There was no consistent difference between the NPTs and indica inbreds in total biomass or harvest index.

Yield increase can be achieved either by increasing the biomass production or harvest index or both (Yoshida, 1981). The importance of biomass production and harvest index in determining grain yield has been a controversial issue. In this study, the high yield of hybrids was attributed to the high harvest index rather than biomass production compared with indica inbreds and NPTs. Laza et al. (2003) studied hybrids, indica inbreds, and NPTs and found that harvest index had a much closer relationship with grain yield than with biomass production. Peng et al. (1999), however, attributed the 9% advantage in yield potential of hybrids to greater biomass production rather than harvest index. Song et al. (1990) and Yamauchi (1994) also reported that hybrids had a higher yield than indica inbreds mainly due to an increase in total biomass production. The contribution of biomass production and harvest index to genetic gains in grain yield varied among different studies, probably due to different genotypes used and different growing conditions in these studies (Laza et al., 2003).

Harvest index is the ratio of grain weight to total biomass. There is a negative relationship between harvest index and plant height (Yoshida, 1981) and a positive relationship between biomass production and plant height (Kuroda et al., 1989). The product of harvest index and plant height at physiological maturity was significantly higher in hybrids than indica inbreds and NPTs (Table 5). This suggests that the hybrids produced significantly higher yield than indica inbreds and NPTs in the DS because hybrids did not follow the negative relationship between harvest index and plant height. In the WS, the high harvest index of hybrids did not result in yield advantage due to their low biomass production caused by short growth duration. The product of harvest index and plant height at physiological maturity could be a useful parameter for identifying genotypes with high harvest index without reduction in plant height, and consequently with high yield potential in future studies.

The yield advantage of hybrids in the DS was associated with large sink size compared with indica inbreds and NPTs (Tables 1 and 3). Sink size is a function of panicle number per unit area and spikelet number per panicle. These two parameters are negatively correlated (Yoshida, 1981). Therefore, their balance is important for attaining maximum sink size. Despite the lower panicle number, hybrids had greater sink size than indica inbreds and NPTs due to large panicles (Table 3). The NPT was designed to achieve higher grain yield from large panicles (Peng et al., 1994); however, the NPTs produced smaller panicles than the hybrids in both the DS and WS. The NPTs had a grain-filling percentage similar to hybrids but significantly lower than indica inbreds. An interesting observation is that high harvest index in hybrids was not associated with the high grain-filling percentage (Tables 1, 2, and 3). The grain-filling percentage of the hybrids was significantly lower than that of the indica inbreds, which was probably due to the difference in panicle size between the two groups. Large sink size could be responsible for the high harvest index of hybrids.

Panicle size is positively related to plant height (Yoshida, 1981). If the large panicle size is caused by tall plants, lodging could become a problem to limit rice yield. A new parameter, panicle size per unit plant height at flowering, was proposed to identify genotypes with large panicle size without an increase in plant height. Panicle size per unit plant height at flowering was higher in hybrids than in indica inbreds and NPTs in all four seasons (Table 5). This suggests that the large panicle size of hybrids was not due to taller plants than indica inbreds and NPTs. Spikelet production efficiency has a large influence on sink size (Akita, 1989). Spikelet production efficiency per unit biomass at physiological maturity was generally higher in hybrids than indica inbreds and NPTs (Table 5). Spikelet production efficiency per unit N uptake at physiological maturity was also higher in hybrids than in indica inbreds and NPTs, but the difference was much smaller than spikelet production efficiency per unit biomass at physiological maturity. Spikelet production efficiency per unit N uptake is associated with N use efficiency for grain production if the difference in grain-filling percentage is relatively small.

This is the first study to rigorously compare the second-generation NPT lines with indica inbred and hybrid cultivars. Overall, NPT has not shown yield superiority over hybrids and indica inbreds for the following reasons: (i) NPTs did not show higher biomass production or harvest index than hybrids and indica inbreds; (ii) the panicle size of NPT was not greater than that of hybrids; and (iii) the grain-filling percentage of NPT has not reached the same level as that of indica inbreds. To improve the grain yield of NPT, large panicle size is necessary to assure the increase in sink size. We may look for genes of large panicle size from donor parents other than tropical japonicas. Spikelet production efficiency and grain-filling percentage should be increased in NPTs. These traits plus lodging resistance are also important to further increase the grain yield of indica inbred and hybrid cultivars.

	ACKNOWLEDGMENTS

 
This project was partially supported by the U.S. Agency for International Development (USAID), the Japanese government, and the World Bank. Financial support was also partially provided by the National Natural Science Foundation of China (Projects no. 30528005 and 30671219).

	NOTES

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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication July 12, 2006.

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TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Yang, W. Articles by Dionisio-Sese, M. L. Search for Related Content PubMed Articles by Yang, W. Articles by Dionisio-Sese, M. L. Agricola Articles by Yang, W. Articles by Dionisio-Sese, M. L. Related Collections Rice Crop Growth and Development Germplasm Enhancement