CHAPTER 10
Advances in the
Venezuelan Recurrent
Selection Programme
for Population
Improvement in Rice

Carlos Eduardo Gamboa*
Eduardo José Graterol
Ramiro de la Cruz
Yorman Jayaro

* Editors’ note: Dr Carlos Eduardo Gamboa passed away in May 2004 before he was able to revise his chapter for the English-language version. We have lightly edited some passages in his chapter, but otherwise the text closely follows the Spanish version as published in October 2003.

Carlos Eduardo Gamboa

DANAC Foundation, Apdo. Postal 182, San Felipe, Estado Yaracuy, Venezuela.

Abstract

The DANAC Foundation, the Venezuelan National Institute of Agricultural Research (INIA) and the National Rice Foundation (FUNDARROZ) recently released several rice varieties for Venezuela. However, the varieties show limitations that traditional breeding techniques cannot fully resolve. This chapter aims to show the progress made by the DANAC Foundation’s rice breeding programme, focusing on how populations PFD-1 and PFD-2 were managed and their segregating lines obtained. Results from PFD-1 indicate that sufficient variability exists to develop lines with good potential for yield and blast resistance. However, efforts must be directed towards reducing chalkiness and white belly in the grain, and improving milling quality. PFD-2 is still in the initial stages of improvement, and little can be said of its potential. Based on evaluations of the S_0:2 families of PFD-1, a new and genetically narrow population [PFD-1 (BE)] was developed. After extracting lines, the DANAC Foundation began population improvement for PCT-16. Population PFD-1, in addition to the above-mentioned improvement needs, presented limited potential for the trait tiller thickness. To solve it, population PCTFD-20\0\0\0 was created by introducing six lines from the cross BG90-2/Oryza rufipogon. Simultaneously, lines were derived from it for varietal development. Line development was regarded as an opportunity to learn about the populations’ potential as sources of varieties and parental materials. These preliminary results suggest that population improvement methodology is a promising alternative for the DANAC Foundation’s strategy portfolio.

Resumen

La Fundación DANAC (FD), el Instituto Nacional de Investigación Agrícola (INIA) y la Fundación Nacional del Arroz (FUNDARROZ) liberaron variedades de arroz para Venezuela recientemente. A pesar de ello, aun siguen presentes algunas limitantes que las técnicas tradicionales de mejoramiento no han resuelto. El objetivo de este capítulo es mostrar los avances logrados por el programa de mejoramiento de la FD dando énfasis al manejo de las poblaciones PFD-1 y PFD-2 y al proceso de obtención de líneas segregantes de éstas. No obstante que los resultados de la PFD-1 indican que ésta presenta variabilidad para el desarrollo de líneas con buen potencial de rendimiento y resistencia a Piricularia, se podrían dedicar esfuerzos para disminuir yeso y centro blanco y mejorar su calidad molinera. La PFD-2 aún se encuentra en sus etapas iniciales del proceso de mejoramiento y poco se puede decir sobre su potencial. Con base en la evaluación realizada en las familias S_0:2 de la PFD-1 se creó una nueva población de base genética estrecha: la PFD-1 (BE). La FD después de extraer líneas de la PCT-16 decidió iniciar el proceso de mejoramiento poblacional de la misma. La PFD-1 presenta un limitado potencial para la característica grosor de los tallos y por ello se decidió crear la PCTFD-20\0\0\0. Se introdujeron en la PFD-1 seis líneas del cruce BG90-2/Oryza rufipogon. Al mismo tiempo que se realizaba el mejoramiento poblacional se derivaban líneas para el programa de desarrollo de variedades. Esta etapa se considera como una oportunidad para conocer el potencial de las poblaciones como fuente de variedades y progenitores potenciales. Con base en estos resultados iniciales el mejoramiento poblacional se considera como una alternativa promisoria para el portafolio de estrategia de la FD.

Introduction

The DANAC Foundation released the rice varieties D-Primera in 2001 and D-Sativa in 2002. At about the same time, other Venezuelan organizations such as the National Institute of Agricultural Research (INIA) and the National Rice Foundation (FUNDARROZ) also released new varieties. Thus, a 10-year dearth of new alternatives for Venezuelan farmers was broken. According to reports by the National Seed Service (SENASEM), experimental yield trials showed that the new materials yielded about 5% more than the older varieties. This means that the genetic improvement programmes are generating materials that produce yield gains of about 0.5% per year.

The development of rice cultivars in Latin America, using conventional methods of breeding, has produced significant results, similar to or better than those observed in Venezuela. However, several recent studies have indicated that genetic gains are diminishing because the genetic base of the new cultivars has narrowed, and that, probably, a ceiling for yield has been reached (Cuevas-Pérez et al., 1992). Consequently, average productivity in the region has stagnated over several years despite many genetic improvement programmes having worked to increase yields in new materials.

A breeding programme needs to rely on a range of methods if it is to release cultivars that have not only high yield in the field, but also high milling quality, adequate culinary qualities and resistance to diseases and insect pests.

In Venezuela, as is apparently occurring in most rice-producing countries, the yield ceiling mentioned above is being reached, even in newly released cultivars. Two ways of counteracting these limitations are managing rice genetic resources through creating populations with broad genetic bases and using a methodology for improvement that continuously accumulates favourable alleles. Moreover, the implemented strategy permits a greater integration of activities executed by different breeding programmes existing in the country, which integration has been consolidated through the joint execution of projects cofinanced by the National Fund for Science, Technology and Innovation (FONACIT, its Spanish acronym).

Among the available breeding methods, population improvement through recurrent selection fulfils these two aspects. Starting with this premise, the DANAC Foundation initiated its activities in population improvement through recurrent selection by introducing six populations and one gene pool segregating for a recessive male-sterility gene (Graterol, 2000). This germplasm was introduced through collaborative projects carried out with CIAT, Cali, Colombia, and CIRAD-CA.

The management of this germplasm under irrigated conditions in Calabozo, State of Guárico, Venezuela, ensured the selection of populations PCT-6 and PCT-7 as genetically adapted bases and as sources of the genetic male sterility needed to develop populations PFD-1 and PFD-2. These two populations were created to improve them for rainy and dry cropping seasons, respectively. Moreover, as the recurrent cycles advanced, lines obtained from these populations were expected to be useful as parental materials with broad genetic bases that were different to those used by the DANAC Foundation’s programme. They were even expected to be useful as possible sources of commercial varieties.

By emphasizing the management of populations PFD-1 and PFD-2 under recurrent selection and the process of obtaining segregating lines from them, this chapter aims to show the advances already achieved in the search to break the yield ceiling observed in the country.

Population improvement

The DANAC Foundation’s rice project applies population improvement with the expectation of generating new and improved populations that will be used as sources of variability for line development. This activity, carried out in parallel with improvement activities, uses conventional methods such as pedigree, mass selection and others. We will first describe the general strategy used for managing populations and then discuss the procedures used to improve PFD-2 and later PFD-1.

The DANAC Foundation’s population improvement efforts are based on cycles of recurrent selection with evaluation and selection of S_0:2 families. The strategy requires conducting studies in two types of environments:

• Outside the rice-producing regions to advance the S_0:1 generation to the S_0:2 generation, and also to conduct recombinations between selection cycles. This environment is found at the DANAC Foundation’s headquarters in San Javier (State of Yaracuy)

• Within rice-producing regions, that is, at the sites for which the populations are being generated. Experimental areas have been selected in Calabozo (State of Guárico) and Acarigua (State of Portuguesa).

Generations S₀ and S_0:2 are managed within the rice-producing regions, and plants and lines, respectively, are selected from them.

To facilitate the distribution of activities and improve populations for the two planting seasons in the country, PFD-1 was generated with germplasm more adapted to ‘winter’ (i.e., cooler, drier season) and PFD-2 with germplasm adapted to ‘summer’ (i.e., hotter, wetter season). These populations, created for different conditions, require programmed evaluations. Thus, the generations in which evaluations and selections are made for PFD-1 (S₀ and S_0:2) are always planted in winter and, for PFD-2, in summer.

While the study is the DANAC Foundation’s responsibility, the evaluation and selection of S_0:2 lines are shared by the DANAC Foundation and INIA, with occasional participation of staff from universities such as the Universidad Central de Venezuela in Maracay. These activities helped increase and improve integration between the different national institutions that work in rice genetic improvement in Venezuela.

Table 1. Strategy for managing irrigated rice populations to be improved by the DANAC Foundation.

a. Winter (W) is a cooler, drier season; and summer (S), a hotter, wetter season.
b. Population under improvement.
c. Population for extracting lines.

Population PFD-2

This population, described by Graterol (2000), was generated for summer conditions through the introduction of four advanced lines from PCT-7. Population PCT-7 had been created to deal with the rice planthopper (sogata or Tagosodes orizicolus Muir), a pest that makes its presence strongly felt in summer. This pest also transmits the rice ‘hoja blanca’ virus (RHBV), causing white leaf disease. Population PCT-7 was also created to deal with the limited potential of yield of currently cultivated varieties.

Population PFD-2 was first evaluated in the 1999/00 summer (Table 1), when 6000 S₀ (cycle 0) plants were sown and transplanted in Calabozo. At physiological maturity, about 120 days after planting (DAP), 271 fertile plants were selected for their phenotypic traits. The criteria used were good tillering, medium grain length (as required by local industry), intermediate plant height, good panicle exsertion and a short-to- medium growth cycle. Their performance under natural RHBV pressure was observed, together with the mechanical damage caused by the planthopper.

To advance the S_0:1 generation to the S_0:2 generation in the 2000 winter, the 271 plants selected from the S₀ generation were planted as families. During this stage, more than 60% of the families had problems with plant height, being taller than the desired standards. This performance led to the decision of selecting shorter families for recombination. In the 2000/01 summer, the remnant S_0:1 seeds, forming a mixture of selected families, were planted in San Javier. In the stage before flowering, the tallest plants were eliminated and the others recombined.

Because improving population PFD-2 was seen as an alternative for generating materials for the summer cycle, and given that the following planting cycle was in winter, we made another recombination during the 2001 winter. This time, we planted 6000 S₀ plants and, at flowering, those plants that were taller than the population’s average height were eliminated, whereas the rest were recombined. In summary, we obtained a population (PFD-2) that was initially subjected to two selections for plant height (S_0:1 and S₀), then a recombination, and another selection cycle (S₀) and recombination (Table 1).

The first recurrent selection cycle based on S_0:2 families was begun in the 2001/02 summer. In Calabozo, about 6000 plants from the base population (S₀ of cycle 0) were established by transplanting, using the same methodology mentioned above. Visual selection of the best plants was carried out at 120 DAP, when 478 S₀ fertile plants were chosen. The criteria were also similar to those mentioned above, that is, good tillering, medium grain length, intermediate plant height, good panicle exsertion and a short-to-medium growth cycle. Performance under RHBV pressure and mechanical damage were also considered.

Following the methodology proposed by Graterol (2000), the 478 S₀ plants were advanced from generation S_0:1 to generation S_0:2 in the 2002 winter. Although this phase was not for selection, 28 S_0:1 families were eliminated because they presented serious problems with lodging, and susceptibility to blast, helminthosporiosis and sheath rot (caused by Sarocladium spp.). For the 2002/03 summer, 450 S_0:2 families were evaluated at the two sites (Acarigua and Calabozo) and the selected ones were recombined in the 2003 winter.

Population PFD-1

To develop materials better adapted to winter cropping, the DANAC Foundation decided to create population PFD-1, based on the introduction of five improved lines to PCT-6 (Graterol, 2000). The objective was to produce germplasm with resistance to the main diseases prevalent during this season, as well as maintaining the milling and culinary standards required by industry and those of productivity demanded by farmers.

In Calabozo, in the 1999 season, 6000 plants of the base material (S₀ of cycle 0) were transplanted. At maturity (at about 120 DAP), 306 fertile S₀ plants were selected. The criteria for selection were good tillering, medium grain length, intermediate plant height, strong stems with a low-to-intermediate inclination angle, good panicle exsertion and a short-to-medium growth cycle. Only those plants that were not susceptible to neck blast, helminthosporiosis, sheath rot and grain discoloration were selected.

During the 1999/00 summer in San Javier, 306 families were advanced from S_0:1 to S_0:2 by transplanting. Of the 306 S_0:1 families, 75 were rejected for their tall plants, long cycle, very open architecture and weak tillers. The other 231 S_0:2 families were kept and their seeds harvested for evaluation in the following cycle. At harvest, each S_0:2 family underwent mass selection, with six fertile plants selected according to their phenotype.

The evaluation of the 231 S_0:2 families was carried out at four sites, three for evaluating yield and milling quality, and one for determining performance under blast pressure. To evaluate yield, one trial was established at Acarigua and two at Calabozo. The trial at the first site was located at the INIA-Araure Experiment Station, and those of the second site were located at Plot 182 of the Rio Guárico Irrigation System and at the INIA-Calabozo Experiment Station. Evaluations of reactions to blast were carried out at the INIA-Barinas Experiment Station where pathogen pressure is high, compared with the other sites under evaluation.

For the three Acarigua and Calabozo trials, the experimental design used was Federer’s augmented blocks (FAB), and the local checks were Cimarrón, Fonaiap 1, Fundarroz PN-1 and Palmar. To evaluate resistance to blast, the scale recommended by IRRI (IRRI, 1988) was used.

Once the checks were analysed and comparisons made for each block per site, 30% of the evaluated families were selected, that is, 70 families. Table 2, which gives the results of some of the selected families, also shows that several lines selected for yield presented values for chalkiness + white belly that were higher than the standards accepted by industry (maximum of 17 percentage points). Consequently, to begin the next recurrent selection cycle, we selected a large number of S₀ plants and evaluated their rates for white belly to augment the probability of selecting and advancing only plants that will perform well for this variable.

Although some high-yielding families scored poorly (4 to 6) for resistance to blast, others selected scored a type ‘1’ reaction to leaf blast. This indicates that genes for resistance exist in the population and that progress can be made when emphasizing this trait.

Table 2 shows that family no. 18 scored high for yield but also had unfavourably high values for chalkiness + white belly. To prevent the improved population being skewed towards only yield, we selected families such as no. 130, which carried the desired genes for chalkiness + white belly and family no. 85, which carried resistance to leaf blast. Use of a selection index (I. O. Geraldi, Chapter 3, this volume) would help deal with this problem.

Table 2. Results for yield, milling quality and resistance to blast of some irrigated rice families selected for recombination.

a. Desirable score is 17 percentage points or fewer.
b. On a scale of 1 to 9, where 1 is resistant and 9 is highly susceptible.

To complete the first recurrent selection cycle for population PFD-1, we had to recombine the selected families and create the improved population from cycle 1 (C₁). In the 2000/01 summer, the 70 S_0:2 families selected in the previous semester were recombined (Table 1). To do this, remnant seeds from generation S_0:1 were physically mixed at 3 g of seed from each of the 70 selected families. Transplanting was carried out at San Javier, where three planting dates were established to coincide flowering and thus promote crossing among early and late-maturing plants. The total number of plants planted was 8000, of which 841 (i.e. 14%) were male-sterile.

As observed for the S_0:2 evaluation, the population did not present suitable levels of chalkiness + white belly. Hence, we decided to change the methodology to better exploit the available variability in population PFD-1. We increased the number of plants selected from S₀ and decided to subject them to an early process of evaluation for white belly, as already mentioned.

To give continuity to the population improvement effort, 2000 and 6000 plants from C₁S₀ seeds were planted during the 2001 rainy season at two sites, Araure (State of Portuguesa) and Calabozo. We selected 245 and 425 fertile plants in Araure and Calabozo respectively. The selection criteria were those of the previous cycle. Once the selected plants were harvested, we evaluated their principal panicles for panicle length, measuring from the base of the rachis to its tip, and also weighed the panicle. As mentioned before, grain quality (i.e. white belly) was also evaluated.

Figure 1. Distribution of white belly in the irrigated rice population PFD-1, Calabozo and Araure, Venezuela.

Once the evaluations of population PFD-1 in Araure and Calabozo were done for white belly, a distribution was obtained (Figure 1). Analysis of the results showed that the values obtained in Araure and Calabozo had a normal Shapiro and Wilk distribution of 0.983 and 0.976, respectively (Shapiro and Wilk, 1965). This indicates that, within the population, we could find a wide range of values for white belly from 0.2 to more than 2.5. Likewise, we point out that performance was very similar in the two sites, which showed that the trait is stable in these two environments. Moreover, we found that 40% of the plants had values of less than 1 for white belly, which is desirable.

According to reports on India and USA by Chang and Somrith (1979) and Lin and Ban (1981), the trait white belly is recessive monogenic. As such, a high probability was assumed for finding good characteristics of this trait in the plants’ progeny.

To give continuity to the population improvement of the 670 S₀ plants (S_0:1 seeds) evaluated, we selected 288 that presented values of less than 1.2 for white belly. These materials were advanced to S_0:2, following the timetable as presented in Table 1.

Population PFD-1 (BE)

The evaluation of the S_0:2 families from PFD-1 during the 2000 winter led to the development of a new population [PFD-1 (BE)] with a narrower genetic base than the original. The idea was to produce a new population from only the five best-performing families of the evaluations conducted in year 2000. This population would be exploited through recurrent selection but with the intention of obtaining improved lines from the segregating generations that could be immediately incorporated into the conventional breeding programme.

Table 3. Composition of irrigated rice population PCTFD-20.

Management of this population underwent certain changes during its development (Table 1) when the programme re-planned its work strategy with the populations it handled. In Calabozo, 3000 plants from the S₀ generation were planted in the 2001 winter, from which 210 selected S₀ plants were advanced to S_0:2, S_0:3, and so on, following the timetable as described in Table 1.

Population PCT-16

Since the 2000 winter, this population has been used to extract lines. Given the potential shown by the selected lines, the DANAC Foundation decided to begin population improvement of PCT-16. In the 2002 winter, 3000 S₀ plants were planted, of which some were selected, following the timetable as presented in Table 1.

As a result of this strategy of line development for this population, we are currently evaluating PCT-16> 104-M-5-M-1-1 in yield trials for elite lines. Results are very encouraging: the mean percentage of entire grains is 55.6%, values for chalkiness + white belly are about 10.7, amylose content is 25.9% and yield varies from 4326 kg ha^-1 in the 2001/02 summer in Araure to 7875 kg ha^-1 in the 2002 winter in Calabozo. Studies with these lines will continue to seek for a possible new cultivar.

Population PCTFD-20

Researchers who had managed PFD-1 say that the population has limited potential for the trait tiller thickness and that lines derived from this population suffer from lodging. To correct this problem, CIAT decided to create a new population based on PFD-1. They began by choosing six lines originating from crosses made between variety BG90-2 of the cultivated species O. sativa and accessions of the wild species O. rufipogon.

The composition of PCTFD-20 (Table 3) was obtained from crosses between the lines and a sample of population PFD-1. This strategy is similar to that followed by Borrero et al. (2000).

Table 4. Steps in creating the irrigated rice population PCTFD-20.

Crossing was begun in 2001 by first planting population PDF-1\0\0\2 and seven parental materials: the six as listed in Table 3, and including CT14938-36-1-M-1. The cross obtained with this last parent was eliminated when the F₂ generation was harvested. At flowering, sterile panicles were chosen (six per parent) and pollinated with pollen from paternal lines. From each cross, an average of 50 seeds were planted to obtain the F₂ generation. Each F₂ plant within each cross was individually harvested and, once obtained, the seeds of the six crosses were mixed in equal quantities, giving rise to population PCTFD-20\0\0\0. The stages of creating the new population are illustrated in Table 4.

Developing lines

Genetic improvement programmes for rice that manage populations as an additional and alternative strategy seek to obtain results over the medium term. They do not expect to effectively use the populations from their first stages of development. However, no rules exist to stop plant breeders from exploiting the variability available in the base population and the segregating generations that originate from the improvement process itself. Hence, below we report on our experiences in deriving and evaluating segregating lines extracted from PFD-1.

In the 2000 winter cycle, considering data from the phenotypic acceptance score at maturity (PAM), 64 S_2:3 families were selected from 231 S_0:2 derived from PFD-1, following the flow of lines described in Table 5.

In the 2000/01 summer, the families were planted at two sites (Araure and Calabozo), where, in line with the PAM results, 46 S_3:4 lines were selected, then evaluated in the 2001 winter. During this cycle, a selection was carried out between and within families, taking into account yield-for which the FAB design was used-and PAM. Nine families were selected, from which 9 to 10 plants per family were chosen, producing 93 plants, which were then used in a trial of S_4:5 lines from the 2001/02 summer cycle. As a result of this evaluation, two outstanding lines were selected and evaluated in preliminary yield trials in the 2002 winter cycle. This stage was considered as an opportunity for discovering the potential of the population as a source of potential varieties and parental materials.

Table 5. Flow of S_0:2 lines selected in the 2000 winter cycle for producing fixed lines, starting with rice population PFD-1.

Regarding this summary of the stages for the S_0:2 lines selected from PFD-1 in the 2000 winter, it should be pointed out that the process is continuous and that new lines were obtained in the following segregating generations that are not discussed here. In the 2002 winter, for example, 94 lines were chosen, following a procedure similar to that reported here.

As seen in Table 6 and without statistically analysing the data, the lines extracted from the original population do not yet present a potential for yield that can compete with that of the commercial checks. This could be expected because the population has yet to be subjected to recurrence for this trait. However, the mean of these lines is high, indicating the population’s potential for the improvement programme’s future.

Observation of some of the selected lines (Table 7) shows that potential also exists for improvement of other traits. For milling yield, for example, several lines were found with more than 54% of entire grains and other materials had the combination chalkiness + white belly with values of less than the minimum 17 percentage points.

Although these lines present limitations for release as varieties for farmers, they can be used as parental materials in the crossing programme. Moreover, as well as carrying certain specific traits, they originate from a genetic base that is broader than the varieties currently grown in the country.

Table 8 summarizes the contribution of lines originating from the populations in the DANAC Foundation’s crossing programme. In the 2001/02 summer, they contributed more than 50% to the crosses and, in the 2002 winter, they contributed 80%. The contribution of these lines to the process of generating variability is therefore significant.

Table 6. Yield of S_2:4 lines derived and selected from the rice population PFD-1, Calabozo, State of Guárico, Venezuela, 2001 winter.

Table 7. S_4:5 lines derived from the rice population PFD-1 and selected during planting in the 2001/02 summer, Calabozo, State of Guárico, Venezuela.

Table 8. Percentage of contribution of lines derived from rice population PFD-1 to the crosses carried out by the DANAC Foundation in the 2001/02 summer and the 2002 winter.

Final comments

The DANAC Foundation’s experience with managing populations has helped open up new perspectives for genetic improvement programmes for rice in Venezuela for various reasons such as:

• Creating an additional alternative for plant breeders
• The populations providing new and continuous sources of genetic variability for plant breeders
• Generating increased integration among the different actors in genetic improvement of rice in the country
• The methodology providing a medium- term strategy for developing varieties.

According to results from different stages of improving populations PFD-1 and PFD-2, particularly the first, one can say that the variability found will permit the development of lines with good potential for yield and resistance to blast. However, more effort is needed to reduce the values for chalkiness and white belly, and to improve milling quality. If these objectives are reached, then the likelihood of developing lines for release as varieties is greater. Population PFD-2 is still in the initial stages of improvement and little can be said of its potential. At present, efforts are being made to correct the excessive height of its plants.

Working with the populations has increased and diversified the number of parental materials used in the crossing programme-one of the objectives of using this methodology in Venezuela. Another aspect that should be highlighted is the gain in experience in managing segregating populations with broad genetic bases under Venezuelan agroecological conditions. Such experience was lacking before this strategy was incorporated into the improvement programme. Overall, the generation of variability had concentrated on executing simple, triple or maximum double crosses, which had limited the use of the genetic resources available in the country.

Interacting with other institutions such as the Universidad Central de Venezuela also increased the efficiency of the DANAC Foundation’s rice genetic improvement programme.

References

Borrero, J.; Châtel, M. & Triana, M. 2000. Mejoramiento poblacional del arroz irrigado con énfasis en el virus de la hoja blanca. In E.P. Guimarães, ed. Avances en el mejoramiento poblacional en arroz, pp. 105-118. Santo Antônio de Goiás, Brazil, Embrapa Arroz e Feijão.

Chang, T.T. & Somrith, B. 1979. Genetic studies on the grain quality of rice. In Proc. workshop on Chemical Aspects of Rice Grain Quality, pp. 49-58. Los Baños, Philippines, International Rice Research Institute (IRRI).

Cuevas-Pérez, F.E.; Guimarães, E.P.; Berrío, L.E. & González, D.I. 1992. Genetic base of irrigated rice in Latin America and the Caribbean, 1971 to 1989. Crop Sci., 32(4): 1054-1059.

Graterol, M.E.J. 2000. Caracterización de poblaciones e introducción de variabilidad genética para iniciar un programa de mejoramiento poblacional del arroz en Venezuela. In E.P. Guimarães, ed. Avances en el mejoramiento poblacional en arroz, pp. 87-103. Santo Antônio de Goiás, Brazil, Embrapa Arroz e Feijão.

IRRI (International Rice Research Institute). 1988. Standard evaluation system for rice. 3rd ed. Los Baños, Philippines. 54 pp.

Lin, X.S. & Ban, R.D. 1981. The inheritance of the glutinous character in rice and its application in breeding. Fujian Agric. Sci. Technol., 4: 5-6.

Shapiro, S.S. & Wilk, M.B. 1965. An analysis of variance test for normality. Biometrika, 52: 591-611.