African Crop Science Journal, Vol. 8. No. 1, pp. 77-83, 2000
PEARL MILLET GRAIN SIZE AND HARDNESS IN RELATION TO RESISTANCE TO SITOPHILUS ORYZEA (L.) (COLEOPTERA: CURCULIONIDAE)
K. Leuschner, E. S. Monyo1, E. Chinhema, E. Tembo and D. Martin
Genetic Resources and Enhancement Program (GREP), International
(Received 1 September, 1998; accepted 12 November, 1999)
Code Number: CS00007
Fifty-seven varieties of pearl millet (Pennisetum glaucum (L.) R. Br.) were evaluated for resistance to the rice weevil Sitophilus oryzea (L.) by using artificial infestation. Kernels of each cultivar were separated into three grain sizes; small, medium and large, to remove the effect of grain size from that of variety. Grain hardness was measured using the sodium nitrate specific gravity floaters test. In general, larger and softer grains supported more weevils. However, there seems to be a good spread of variability for weevil progeny production within the large grain fraction, suggesting the possibility of selecting for resistance among large grains. Most of the soft grain types had floury endosperm, but no relationship was found between grain size and endosperm type. This suggests that floury and/or vitreous endosperm is not influenced by size, and that it may be possible to develop varieties with a combination of large grain, vitreous endosperm, and weevil resistance. Irrespective of grain size, SDMV 90016, Nandi Code 24, TSPM 91018, and SDMV 89001 were resistant compared to the farmers local variety.
Key Words: Grain size and hardness, Pennisetum glaucum, rice weevil
Cinquante-sept variétés de mil, (Pennisetum glaucum (L.) R. Br.) ont été évaluées pour leur résistance contre le charançon du riz Sitophilus oryzea (L.) à travers une infestation artificielle. Les graines de chaque cultivar ont été séparées en trois groupes en fonction de leurs grosseurs - petites, moyennes et grosses - pour différencier leffet de la grosseur de celle de la variété. La dureté des graines a été mesurée en utilisant le test de grains flottants de la gravité spécifique du nitrate de sodium. En général, les graines les plus grosses et les plus tendres supportaient mieux les charançons. Mais, il semblait y avoir une large gamme de variabilité pour la production de descendances de charançons dans la fraction des grosses graines, ce qui suggère la possibilité de sélectionner pour la résistance parmi les grosses graines. La plupart des types de graines tendres avaient une endosperme farineuse, mais il navait pas été trouvé de relations entre la taille de la graine et le type dendosperme. Ceci suggère que lendosperme farineuse et/ou vitreuse nest pas influencée par la taille et quil serait possible de développer des variétés ayant une combinaison de grosses graines, dendosperme vitreuse et de résistance aux charançons. Pour toutes les tailles de graines, SDMV 90016, Nandi Code 24, TSPM 91018 et SDMV 89001 se sont montrées résistantes par rapport à la variété locale paysanne.
Mots Clés: taille et dureté de la graine, Pennisetum glaucum, charançon du riz
Pearl millet (Pennisetum glaucum (L.) R. Br.) is a major food crop in the semi-arid areas of Africa. The crop is grown on 3.6 m ha in the Southern and Eastern Africa Region (UNFAO, 1995), mainly in Sudan, Namibia, Tanzania, Zimbabwe, and Eritrea.
After harvest, pearl millet grain is stored in traditional granaries, usually without insecticides application for protection against storage pests. The major storage pests are the angoumois grain moth Sitotroga cerealella (Oliver) and the rice weevil Sitophilus oryzae (L.) (Hill, 1987). No data on storage losses are available specifically for pearl millet. Only recently did farmers in Namibia complain about poor storability of Okashana-1, a newly released pearl millet cultivar with large grain (ICRISAT, 1995).
Kernel hardness and size are reported to be the main grain characteristics contributing to grain weevil resistance in sorghum and small grains like pearl millet (Doggett, 1957; Davey, 1965; Seifelnasr and Mills, 1985; McFarlane et al., 1995). Grain size in particular affects resistance to Sitophilus spp. Most information on these two resistance traits is related to research on sorghum. However, Seifelnasr and Mills (1985) found a high correlation (r=-0.648*) between kernel size and S. oryzea infestation in pearl millet; smaller kernels were attacked more often. Unlike S. cerealella, which can feed on several grains during the larval stage, each oryzea larva feeds and develops within a single grain (Russel, 1961). This may explain the relationship with grain size. Farmers perhaps did not complain about weevil damage because grains of traditional pearl millet landraces often have hard endosperm and are generally too small to sustain the development of weevil larva.
The pearl millet breeding research programme of ICRISAT (International Crops Research Institute for the Semi-Arid Tropics) in Southern and Eastern Africa focuses on developing large-grained cultivars that mature early (85-100 days), produce high yields, and have resistance to storage insect pests. The objective of the study reported here was to screen breeding lines developed by ICRISAT for resistance to S. oryzea and to assess the relationship between grain size, hardness, and number of weevil progeny produced.
MATERIALS AND METHODS
Adult rice weevils were obtained from a laboratory culture maintained at the ICRISAT station in Matopos, Zimbabwe. The culture originated from poorly stored pearl millet harvested in 1994. Insects were reared for one generation at 27°C and 70% relative humidity (RH) on kernels of a susceptible sorghum cultivar "Red Swazi".
Kernels of 57 pearl millet varieties were evaluated for resistance to the rice weevil. Of these varieties, 26 are at the on-farm testing stage and 31 in advanced regional cooperative trials with national programmes. The screening was part of an effort to ensure that no highly susceptible varieties were released for cultivation. None of these varieties had previously been screened for kernel resistance to S. oryzea; most of them were developed for large grain size, a trait highly preferred by farmers in the region. Grain from each line was separated into 3 size classes, 1.40-1.99 mm, 2.00-2.35 mm and >2.36 mm using Endecott sieves of appropriate sizes (Endecotts Ltd.). Twenty grams of grain from each line and grain-size class (replicated 3 times) were placed in plastic jars (5 cm diameter x 5 cm tall) and infested with 30 unsexed 10-day-old weevil adults. Weevils were allowed to lay eggs for 7 days and then removed. No susceptible or resistant controls were used because none had been identified previously. The average grain moisture content for all lines was 13.5 ± 1%. Infested grain was maintained at 27°C and 70% RH. Weevil progeny (emerging adults) produced were counted beginning 27 days after infestation and continuing at 2-days intervals for 20 days.
Grain hardness was determined using the floater test developed by Hallgren and Murthy (1983) for sorghum. For each variety, 100 kernels of each size class (replicated 3 times) were placed in a solution of sodium nitrate with a specific gravity of 1.315 g mL-1. The percentage of kernels that floated to the surface was used as a measure of grain hardness ("floaters" have lower density and softer endosperm). Hardness was also determined visually by bisecting 10 kernels from each sample and recording the proportion of soft floury endosperm in each kernel. For each trial, data were recorded on number of weevil progeny, % floaters, and proportion of floury endosperm. The data were subjected to analysis of variance and any effects that were more than three times the standard error of the mean declared significant. The relationship between grain size classes and each of these three parameters was also statistically analysed.
Significant differences in the number of emerging weevils were found among the pearl millet varieties (Tables 1 and 2). No controls were used in these trials. Therefore, the resistance/susceptibility of a variety (or size class within a variety) was judged by comparison with the corresponding values for the farmers local and the trial mean. This resulted in a somewhat arbitrary classification of varietal resistance, but was the only possible method given the lack of data on weevil resistance in pearl millet.
TABLE 1. Mean numbers of grain weevil progenies produced, percent floaters and proportion floury endosperm, in kernels of pearl millet varieties under cooperative regional advanced variety trial in relation to three different grain sizes
*Weevil progeny test not done
TABLE 2. Mean numbers of grain weevil progeny produced, percent floaters and proportion flour endosperm, in kernels of pearl millet varieties under on-farm trials in relation to three different grain sizes
*Weevil progeny test not done
Tables 1 and 2 show the number of weevil progenies produced, percent floaters and proportion floury endosperm in each size class in each of the 57 varieties tested. The 31 varieties in regional trials are shown in Table 1, and the 26 varieties under on-farm trials are shown in Table 2. Within each variety, there were significant differences between size classes for each of the three parameters. The trial means for the three size fractions (small, medium and large) in the advanced regional trial was respectively 101.29, 153.02 and 184.54, whereas the farmers local had 110, 170 and 178 weevil progenies for the same fractions. Similarly for the varieties under on-farm trials, the trial mean for the three size fractions was 101.35, 176.99 and 224.59 whereas the farmers local had 95, 168 and 240 weevil progenies.
Among varieties in the advanced regional trial in the small grain size class (1.40-1.99mm), 5 varieties had £ 71.4 emerging weevil progenies, a number significantly fewer than the trial mean and the farmers local. Two varieties in the medium grain size class and three in the large grain size class were significantly superior to both the trial mean and the farmers local (<118.3 and 135.5 emerging weevil progenies, respectively). Only one variety SDMV 90016 showed weevil resistance across all three size classes (average across size fractions weevil progenies was 87), whereas two varieties; SDMV 87001 and SDMV 93002 exhibited resistance in the small and large size fractions (average across size fractions weevil progenies 100 and 92.5, respectively).
Among the varieties under on-farm trials in the small grain size class (1.40-1.99 mm), 5 varieties had significantly fewer emerging weevil progenies (£ 68) than the trial mean. Six varieties in the medium grain size class and four varieties in the large grain size class (£ 143 for the medium and £ 180.74 weevils for the large fraction) were significantly superior to the trial mean. Overall, three varieties, namely, Nandi code 24, TSPM 91018 and SDMV 89001, showed weevil resistance across all three size classes and were superior to both the trial mean and the farmers local check.
Fewer total weevils emerged from the smallest sized grain fraction (1.40-1.99 mm) than from the medium (2.00-2.35 mm) and large grain size fractions (³ 2.36 mm) (Tables 1 and 2). Larger grains generally supported more weevils. This indicates a relationship between kernel size and resistance/susceptibility with sufficient variability for weevil progeny production among the large grain fraction as determined by the standard error of the mean.
With regard to grain hardness as measured by the floater test, significant differences for hardness were observed for all size fractions (Tables 1 and 2). This was also true in the case of distribution of floury and non floury endosperm fractions. Larger numbers of weevil progenies were distributed among genotypes that had a higher proportion of floaters (or soft endosperm). In general hybrids tended to have a higher proportion of floaters than open-pollinated varieties and also tended to support more weevil progenies. This is reflected in the higher proportion of weevil progenies among the medium and large size fractions in Table 2. There were no hybrids entered in the collaborative advanced trials (Table 1).
No relationship was found between grain size and endosperm type or grain size and hardness (Table 3). Significant positive relationships were found between weevil progeny number and grain size (r=0.743**), grain hardness (r=0.360**), and proportion of floury endosperm (r=0.297**). It was also observed that both large and soft grains supported more weevils, and grain hardness was influenced by endosperm type as shown by the positive relationship (r=0.497**) between grain hardness (% floaters) and endosperm type (% floury endosperm).
TABLE 3. Correlation matrix for weevil progeny, percent floaters, proportion floury endosperm and grain size distribution in the pearl millet varieties under study
** Significant at <0.01 probability level
Storage pest problems in pearl millet in the region became more important with the release of improved varieties with higher yields and larger grains (ICRISAT, 1995). The increase in yields implied a larger harvest, and thus more time in storage than was earlier the case with local landrace varieties. The larger grain size also provided a more favourable environment for grain weevil larvae to feed and develop (Russel, 1961). The fact that some varieties were resistant/susceptible across all three size fractions implies that scope exists for improving grain size without necessarily increasing susceptibility to the grain weevil. Most of the genotypes with a low percentage of floaters produced significantly fewer weevil progeny. This implies that grain hardness is a possible component of resistance, as previously suggested by Seifernasr and Mills (1995) for pearl millet and Doggett (1957) for sorghum.
Most of the soft grain types had floury endosperm which in turn supported more weevils. However, the fact that grain size was not related to endosperm type implies that one can combine large kernels with vitreous endosperm. Vitreous endosperm, which is positively associated with grain hardness, is a trait that can easily be selected for in a breeding programme. We also noted that some varieties (SDMV 90016, Nandi Code 24, and PMV-1) produced relatively few weevil progenies and could thus be classified as resistant, even though they had a large percentage of floaters and a high proportion of floury endosperm. This suggests that other mechanisms of resistance are also in operation.
Pearl millet is grown largely by resource poor farmers and almost the entire crop is processed manually for food. Because small-seeded varieties take a long time to process into food, farmers in the region prefer large-seeded varieties, and in fact demand that this trait be incorporated into new improved varieties (Chintu et al., 1996; Ipinge et al., 1996; Letayo et al., 1996). Soft endosperm grains are also similarly difficult to process because of milling yield losses. The findings that grain size is not related to endosperm type, and irrespective of grain size, some varieties are resistant to weevil infestation, imply that it is possible to breed varieties with the combination of large grain size, hard grains, and weevil resistance. The existence of a good spread of variability for weevil progeny production among the large-grain fraction of the varieties further reinforces this potential.
©2000, African Crop Science Society