Pigeonpea grains in storage suffer losses due to bruchid infestation. Therefore, farmers typically sell it as soon as possible after harvest. Storing the grain and selling it at a time when the prices have risen due to scarcity of dal in the market could provide an economic incentive for the farmers to store; however, their need for cash after harvest time, the lack of bruchid tolerant varieties, paucity of effective storage systems deter famers to sell pigeonpea at lesser prices during glut in the market. Because of the wide range of maturities, pigeonpea seed needs to be stored for variable periods of time (up to nine months). Bruchids, major storage pests of pigeonpea, causes substantial losses (
Grolleaud, 2002). Keeping this in view the present study was conducted to identify possible tolerant donors for use in pigeonpea breeding programme.
Screening of pigeonpea genotypes against C. maculatus
None of the genotypes were immune to
C. maculatus. However, laboratory screening in terms of GI revealed significant variations in their reaction to
C. Maculatus. GI ranging 1.10 to 0.45 with an average of 0.85 (Table 1). On the basis of GI, genotypes were grouped into four categories
viz. (1) resistant (≤ 0.50), (2) moderately resistant (0.51- 0.70), (3) moderately susceptible (0.71-0.90) and (4) susceptible (≥ 0.91) (Table 2).
Out of 52 genotypes, three genotypes
viz., ICP 89049, IPA 37 and Dholi dwarf DB were found to be resistant, with GI less than 0.50. Four genotypes with GI between 0.51 and 0.70 were classified as moderately resistant, while 41 were found to be moderately susceptible. Remaining four genotypes having GI above 0.91 were susceptible (Table 2). Earlier,
Chandel and Bhadauria (2015) reported substantial variation in terms of degree of damage by
C. maculatus in 12 pigeonpea varieties.
Dasbak et al., (2009a) reported varied influence of pigeonpea genotypes on bruchids infestation and identified ICPL 87 and ICPL 161 to be less susceptible to bruchids.
Physical seed parameters of pigeonpea genotypes
Seed hardness, seed diameter, proportion of seed coat, 100-seed weight and seed colour revealed significant variability among 52 genotypes (Table 1). Seed hardness among 52 genotypes ranged from 279.66 (IPA 7-5) to 153.37 (BDN 1) Newton with an average of 206.71 Newton. Average hardness of resistant and moderately resistant genotypes to
C. maculatus vis-a-vis moderately susceptible and susceptible genotypes (categorized on the basis of GI) was observed to 221.01, 225.02, 205.16 and 193.52 Newton, respectively. Seed diameter (mm) of different genotypes ranged from 19.27 (Ranchi local) to 12.93 (Maruti) with an average of 15.61 mm. Average seed diameters of resistant, moderately resistant, moderately susceptible and susceptible genotypes were 14.98, 15.58, 15.62 and 16.03 mm, respectively (Table 1). The proportion of seed coat ranged from 16.67 (ICP 89049) to 10.31% (Bahar) with mean of 13.45%. Average seed coat proportion of resistant, moderately resistant, moderately susceptible and susceptible genotypes was 14.56, 13.32, 13.45 and 12.69%, respectively (Table 1). Hundred-seed weight ranged from 24.72 (IPAV 16-1) to 8.51 g (ICPL 67B) with an average of 12.10 g. Average values for 100-seed weight of resistant, moderately resistant, moderately susceptible and susceptible genotypes were observed to be 10.88, 11.02, 12.28 and 12.22 g, respectively (Table 1).
Anamika and Jayalaxmi (2016) reported that the rate of increase in insect population is adversely affected by seed traits of resistant variety by hampering oviposition. Hard seed coat and more proportion of seed coat deter access of bruchids into the grains and thereby make it unsuitable for oviposition.
Resistance of C. maculatus vis-a-vis seed parameters
Seed hardness (-0.197) and proportion of seed coat (-0.162) had negative relation with GI of
C. Maculatus (Table 3). Seed coat acts as a physical defensive trait of seed that protects the embryo from external calamities like diseases, pests and insects.
Dasbak et al., (2009a) proposed testa thickness as a direct measure for estimation of proportion of seed coat content, and a comparison of it with susceptibility index or number of eggs laid suggested that an increase in testa thickness results into increased tolerance to bruchid. This inverse relationship could be attributed to the difficulty in larval penetration owing to the presence of thick grain coat barriers. The 100-seed weight had a significant positive relation with number of eggs laid (0.277) and GI (0.128) of
C. maculatus, respectively (Table 3), that suggested preference of bruchids on large-sized seeds for feeding and development.
Lamichaney et al., (2017) reported that large seeds have low proportion of seed coat. Therefore, preference of large sized seeds by bruchid may be because of low content of seed coat as they showed significantnegative relationship (-0.426). In other crops like rice, grain size was positively correlated with mean GI of
S. oryzae (Tripathi
et al., 2017).
Soumia et al., (2017) also established a positive correlation in greengram between susceptibility and grain size. Seed coat colour varied from brown to grey to dark brown regardless of their (resistant or susceptible) response to bruchids. Similarly, pattern of seed coat colour (uniform and mottled), and seed shape (elongate and globular) did not show any clear-cut relationship with GI (data not presented). It has been evident that grain hardness serves as a significant barrier for penetration by storage pests such as
Sitophilus zeamais in maize
(Zakka et al., 2013) and
Sitophilus oryzae (L.) in rice
(Tripathi et al., 2017).
Principal component analysis (PCA)
The PCA conducted on 52 pigeonpea genotypes using seed and bruchid quantitative characters revealed that the first three components (PCs) accounted for about 35, 24 and 17% of variances, with 75.79% as cumulative variance (Table 4). The scatter plot of ordinance using two most informative principals is illustrated in Fig 1. The genotypes ICP 89049, IPA 37 and Dholi dwarf DB formed a distinct cluster of resistant types.
