INTRODUCTION Among the arthropod pests, chilli thrips, Scirtothrips dorsalis Hood (Thysanoptera: Thripidae) is known to pose serious threat to chilli cultivation and is primarily responsible for low productivity in India. Fruit yield loss due to this dreaded pest in India is estimated to be to the tune of 50-90 per cent (Kandasamy et al. 1990). Frequency of insecticide application have increased over the years in chilli ecosystem for managing S. dorsalis as result of which the cost of cultivation has increased enormously and making cultivation of chilli highly risky. In addition to this, pesticidal sprays became a threat to chilli ecosystem causing problems of resistance, resurgence of pests, pesticidal residues and menace to natural enemies fauna (David 1986; Awasthi et al., 2011). Host plant resistance having compatibility with all other methods of pest control without causing any adverse effect in chilli ecosystem has been considered as an important IPM component. Identification of resistant/tolerant chilli germplasm is the most vital option to manage this dreaded pest for which knowledge on morphological and biochemical bases of resistance is highly essential. Several biochemicals in crop plants are known to provide resistance to insect pests. In the present investigation an attempt has been made to study the biochemical components in the leaves of the some chilli germplasms and their relation with thrips incidence so as to identify the source of resistance for use in breeding programme. MATERIALS AND METHODS Twelve chilli germplasms viz., BC-25, BC-79-1, BC-27-2-2, Utkal Abha, BC-21, BC-406, BC-28, LCA-620, BC-78-1-2, BC-24-1 along with resistant check BC-7-2-1 and susceptible check Byadagi kaddi were evaluated under pot culture experiment in Department of Entomology, Odisha University of Agriculture and Technology, Bhubaneswar, Odisha during 2019-20 under insect free conditions. The experiment was laid out in randomized block design with three replications. The chilli germplasms were raised in pot tray and transplanted at six weeks after germination. Three plants per genotype were planted in 10 x 12 inches poly bag. Plants were spaced 60 cm between rows and 45 cm between plants in a row. Agronomic practices recommended for the pot culture crop were followed. Observations on population of nymphs and adults of S. dorsalis were recorded on three leaves of chilli at top, middle and bottom canopy from three plants at weekly interval from the appearance of the pest to last picking of the chilli fruits. The population was counted visually by using a magnifying lens in early morning hours (Bhede et al., 2008). For studies on biochemical bases of resistance in selected chilli germplasm to S. dorsalis, leaf samples from pot culture experiment at 60 DAT coinciding with the peak activity period of S. dorsalis were used. Standard procedures as suggested by various researchers followed for estimating the biochemicals viz., proline content (Bates et al., 1973), phenol content (Malick and Singh 1980), protein content (Lowry et al., 1951), total soluble sugar (Hedge and Hofreieter 1962), reducing sugar (Somogyi, 1952), non-reducing sugar content (Somogyi, 1952) and chlorophyll content (Arnon, 1949). Each sample was replicated thrice and the data on all these biochemical factors of leaf samples were subjected to statistical analysis. The chemical constituents of leaf samples of various test chilli germplasm were correlated with the population of S. dorsalis. RESULTS AND DISCUSSION Results of analysis of mean pool data of 14DAT, 21 DAT, 28 DAT, 35 DAT, 42 DAT, 56 DAT, 63 DAT, 70 DAT, 77 DAT, 84 DAT, 91 DAT and 98 DAT on population of S. dorsalis in different chilli germplasm revealed significantly lowest mean population of S. dorsalis in the resistant germplasm BC-7-2-1(resistant check) (0.70) which was at par with other resistant germplasm BC-25 (0.72). Lower population of S. dorsalis ranging from 1.18 to 1.31 per leaf was observed in the five moderately resistant germplasm viz., BC-27-2-2 (1.18), BC-21 (1.22), BC-79-1 (1.25), Utkal Abha (1.29) and BC-406 (1.31). The susceptible check Byadagi kaddi recorded the highest population of S. dorsalis (2.46/leaf) which was at par with the other susceptible germplasm BC-24-1 (2.38/leaf). The population of S. dorsalis ranged from 1.86/leaf (BC-28) to 1.93/leaf (BC-78-1-2) in rest of the susceptible germplasm (Table 1). The results of the study on biochemical compositions of leaves of twelve selected chilli germplasm revealed significant difference in proline, phenol, protein, total sugars, reducing sugars, non-reducing sugars and chlorophyll content amongst them (Table 2). Proline: The proline content in the leaves of twelve test chilli germplasm ranged between 3.76 µ moles/ gram and 8.92 µ moles/ gram (Table 2). The resistant check germplasm BC-7-2-1 had lowest proline content of 3.76 µ moles/ gram which was closely followed by the other resistant chilli germplasm BC-25 (3.89 µ moles/ gram). The moderately resistant germplasm viz., BC-27-2-2, BC-21, BC-79-1, Utkal Abha and BC-406 had comparatively low proline content of 4.47, 4.82, 5.33, 6.09 and 6.33 µ moles/ gram, respectively than the susceptible and highly susceptible germplasm where the proline content ranged between 7.3 and 8.92 µ moles/ gram. Highest proline content was recorded in the susceptible check Byadagi kaddi (8.92 µ moles/ gram) followed by the other susceptible germplasm BC-24-1 (8.55 µ moles/ gram). Proline, an amino acid has been shown to accumulate in many plants in response to abiotic and biotic stresses, where it plays a protective role including antioxidant function, protein protection and synthesis (as chaperone) and as a signalling molecule (Szabados and Savoure 2009). It is known to participate in a number of physiological functions in insects. Herbivory of plants generally stimulates accumulation of proline whereas total carbohydrate content decreases and content of phenolics remains unaffected (Khattab, 2007). The excessive synthesis of proline may have resulted in the cytosol at the expense of protein and resulted into the accumulation of excessive proline in the tissues as defensive arsenal in proportion to severity of herbivores. Diet selection based on the level of proline in an insect's host plant has been observed for a number of phytophagous insects. Information on relationship of proline content with the incidence of S. dorsalis in chilli is meagre in published literature. Khattab and Khattab (2005) reported that proline concentration in insect infested eucalyptus leaves was higher. Herbivory caused by grasshopper, Choreodocus illustris resulted more damaged leaves which reduced the plant growth due to loss of chlorophyll content and led to proline accumulation at the cost of protein (Rehman et al., 2016). According to Nasrin et al. (2021), the susceptible chilli variety BINA Morich 2 had highest proline activity in the leaves as compared to less proline content in the leaves of moderately resistant variety BARI Morich 2. Thus, the present observations confirmed the findings of these authors. Evidence in support of the significantly positive correlation between the incidence of S. dorsalis in chilli and the proline activity was also reported by Nasrin et al. (2021). Phenol: The phenol content of leaf samples of the twelve selected test germplasm varied from 1.46 mg g-1 to 4.12 mg g-1, the lowest being in susceptible check germplasm Byadagi kaddi and highest in the resistant check germplasm BC-7-2-1 (Table 2). The phenol content was higher (2.96 mg g-1 to 4.12 mg g-1) in the leaves of germplasm viz., BC-7-2-1 (resistant check) (4.12mg g-1), BC-25 (3.86 mg g-1), BC-27-2-2 (3.74 mg g-1), BC-21 (3.67 mg g-1), BC-79-1 (3.58 mg g-1), Utkal Abha (3.28 mg g-1) and BC-406 (2.96mg g-1) showing moderate resistance reaction to S. dorsalis as compared to the highly susceptible and susceptible germplasm viz., Byadagi kaddi (susceptible check), BC-24-1, BC-78-1-2, LCA-620 and BC-28 where it ranged from 1.46 to 2.38 mg g-1. Phenolics are the aromatic benzene ring compounds with one or more hydroxyl groups produced in the plant for protection against biotic stresses (Showket et al., 2017). These compounds are associated with the resistant traits of several crop plants (Link and Walker, 1933). The enhancement in the phenol contents in response to insect infestation is considered to be a general phenomenon as it reduces the growth and development of herbivores (Ramiro et al., 2006 and War et al., 2012). Phenolic compounds induced in the plants are either directly toxic to insects or mediate the signaling of various transduction pathways, which in turn produce toxic secondary metabolites and activate various defensive enzymes (Helmi and Mohamed 2016). Higher level of phenols in several resistant/moderately resistant crops leads to low incidence of thrips possibly due to unpalatability of the food materials. Low incidence of S. dorsalis in chilli due to higher phenol content in the leaves was earlier reported by Mondal et al. (2013); Rameash et al. (2015); Rameash et al. (2017); Latha and Hunumanthraya (2018); Chaudhary and Pandya (2019). The findings on significantly negative correlation between phenol content in the chilli leaves and the incidence of S. dorsalis are in conformity with the results reported by Roopa (2013); Megharaj et al. (2016); Latha and Hunumanthraya (2018). Protein: The protein content in the leaves of tested chilli germplasm ranged between 13.56 mg g-1 (BC-7-2-1) (resistant check) and 18.54 mg g-1 (Byadagi kaddi) (susceptible check) (Table 2). The protein content was comparatively low (13.56 mg g-1to 15.96 mg g-1) in the leaves of resistant and moderately resistant germplasm viz.,BC-7-2-1 (13.56 mg g-1), BC-25 (13.96 mg g-1), BC-27-2-2 (14.22 mg g-1), BC-21 (14.39 mg g-1), BC-79-1 (14.94 mg g-1), Utkal Abha (15.36 mg g-1) and BC-406 (15.96 mg g-1). The germplasm viz., BC-28, LCA-620, BC-78-1-2, BC-24-1 and Byadagi kaddi (susceptible check) having higher infestation of S. dorsalis were with higher protein content of (16.32 mg g-1to18.54 mg g-1). Proteins are the central biomolecules that are responsible for all cellular functions in the living organism. Modifications in plant protein profiles are the first response of plants to insect herbivory. Such qualitative and quantitative changes in proteins play an important role in signal transduction and oxidative defense of plants (Green and Ryan 1972; Rafi et al., 1996; Ni et al., 2001). Information on change in total protein content in the leaves of chilli due to attack of S. dorsalis is scarce in published literature. The present results are comparable with Chen et al. (2009) who stated that an increase in protein content is a general phenomenon in plants in response to insect damage as defence mechanism. Chilli cultivars with higher quantity of total proteins enhanced susceptibility to thrips damage (Alabi et al., 2005). Roopa (2013) reported higher content of total proteins in the susceptible chilli genotypes to S. dorsalis as compared to the moderately resistant genotypes. According to Chaudhary and Pandya (2019), the chilli variety GVC-111 harbouring a moderately population of S. dorsalis had minimum protein content in the leaves as compared to maximum protein content in the leaves of a susceptible chilli genotypes GCH-3. All these findings of previous workers are in full agreement with the results of the present investigation. The positive relationship as found in the present study between the total protein content of leaves and infestation of S. dorsalis corroborates with the findings of Roopa (2013); Chaudhary and Pandya (2019). Total sugar: The total sugar content in the leaves of twelve test chilli germplasm varied from 3.59 per cent to 4.59 per cent (Table 2). The resistant check germplasm BC-7-2-1 had lowest total sugar content of 3.59 per cent which was closely followed by the other resistant chilli germplasm BC-25 (3.62 per cent). The moderately resistant germplasm viz., BC-27-2-2, BC-21, BC-79-1, Utkal Abha and BC-406 had comparatively low total sugar content of 3.85, 3.90, 3.92, 3.98 and 3.99 per cent respectively, than the susceptible and highly susceptible germplasm where the total sugar content ranged between 4.32 and 4.59 per cent. Highest total sugar content was recorded in the susceptible check Byadagi kaddi (4.59 per cent) which was closely followed by the highly susceptible germplasm BC-24-1 (4.57 per cent). Sugars, the dominant soluble leaf carbohydrates of plants are the substrates in respiratory reactions or intermediate metabolites in many other biochemical processes. They have protective role against stress factors, for example, as osmoprotectants, donors of carbon skeletons or signaling molecules (Ciereszko 2009; Morkunas and Ratajczak 2014). These compounds are also involved in the synthesis of phenolic compounds, lectins, etc. as defence mechanism. The role of plant sugar contents in the resistance of various crops to sucking pest was earlier reported by Mittler (1967), Corcuera (1993) and Nawalgatti et al. (1993). The observations in respect of lower total sugar content in the leaves of resistant and moderately resistant chilli germplasm are in full conformity with the findings of Roopa (2013); Subhash et al. (2013); Chaudhary and Pandya (2019). Several earlier researchers also observed significant positive correlation between the incidence of S. dorsalis and the total sugar content in the leaves of chilli germplasm (Roopa, 2013; Subhash et al., 2013). Reducing sugar: The reducing sugar content in the leaves of twelve test chilli germplasm ranged between 1.01 per cent and 2.28 per cent (Table 2). Lowest reducing sugar content of 1.01 per cent was observed in the resistant check germplasm BC-7-2-1 which was closely followed by the other resistant chilli germplasm BC-25 (1.06 per cent). A lower range of reducing sugar content (1.33 to 1.54 per cent) was recorded in the moderately resistant germplasm viz., BC-27-2-2, BC-21, BC-79-1, Utkal Abha and BC-406 as compared to the susceptible and highly susceptible germplasm where the reducing sugar content ranged between 2.03 and 2.28 per cent. Highest reducing sugar content was recorded in the susceptible check Byadagi kaddi (2.28 per cent) which was closely followed by the highly susceptible germplasm BC-24-1 (2.27 per cent). Reducing sugars are the disaccharides, oligosaccharides, polysaccharides and all monosaccharides in the host plant which influence positively the feeding of insect pests. These are the essential component in insect nutrition. The more reducing sugars resulted in more sweetness of leaves which act as a feeding stimulant for sucking pests. Genotypes containing higher total sugars with high reducing sugars exhibited susceptibility towards thrips incidence. Lower amount of reducing sugar in the leaves of chilli genotypes resistant/moderately resistant to S. dorsalis was observed by several earlier researchers (Varadharajan and Veeravel 1996; Megharaj et al., 2016; Chaudhary and Pandya 2019). The present observations are in close akin with the findings of the above workers and that of Subhash et al. (2013) who have also reported positive relationship between reducing sugar content in the leaves and the incidence of S. dorsalis. Non- reducing sugar: The non- reducing sugar content of leaf samples of the selected test germplasm varied from 2.30 per cent to 2.58 per cent, the lowest being in highly susceptible germplasm BC-24-1 and highest in the resistant check germplasm BC-7-2-1 (Table 2). The leaves of germplasm viz., BC-7-2-1, BC-25, BC-27-2-2, BC-21, BC-79-1, Utkal Abha and BC-406 showing resistance reaction to S. dorsalis had comparatively higher non- reducing sugar content (2.45 to 2.58 per cent) as compared to the susceptible and highly susceptible germplasm viz., BC-24-1, Byadagi kaddi (SC), BC-78-1-2, LCA-620 and BC-28 where it ranged from 2.30 to 2.39 per cent. Non reducing sugars are the polysaccharides in plant and the most common example of non-reducing sugar is sucrose. Sucrose is the major product of photosynthesis and contributes to various regulatory mechanisms in plants including growth and development, differential gene expression and stress-related responses (Wind et al., 2010). Involvement of non reducing sugars having more insects feeding on plants has been reported in different plant insect interaction studies (Athar et al., 2011). Information on relationship of non reducing content in the leaves with the incidence of S. dorsalis in chilli is rather scarce in published literature except the report of Megharaj et al. (2016); Chaudhary and Pandya (2019) who recorded lower reducing sugar content in chilli germplasm harbouring moderate population of S. dorsalis. The negative relationship as found in the present study between the non reducing sugar content of leaves and infestation of S. dorsalis is in accordance with the findings of Megharaj et al. (2016); Chaudhary and Pandya (2019). Total chlorophyll content: The total chlorophyll content (sum of chlorophyll-a and chlorophyll-b) in the leaves of twelve test chilli germplasm varied from 9.96 mg g -1 to 16.44 mg g -1 (Table 2). The resistant check germplasm BC-7-2-1 had highest total chlorophyll content of 16.44 mg g-1 which was closely followed by the other resistant germplasm BC-25 (15.79 mg g-1). The moderately resistant germplasm viz., BC-27-2-2, BC-21, BC-79-1, Utkal Abha and BC-406 had comparatively higher total chlorophyll content of 14.31, 14.29, 14.19, 14.15 and 13.68 mg g-1 respectively, than susceptible and highly susceptible germplasm where total chlorophyll content ranged between 9.96 mg g-1 and 12.47 mg g-1. Lowest total chlorophyll content was recorded in the susceptible check Byadagi kaddi (9.96 mg g-1) which was closely followed by the highly susceptible germplasm BC-24-1 (10.63 mg g-1). Photosynthetic pigment viz., leaf chlorophyll content and carotenoids in plant tissue are the key parameters in the photosynthetic productivity which gets altered during defensive responses against the attacking insect pest (Gomez et al., 2004; Mao et al., 2007). Helmi and Rashwann (2015) reported that photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoids) showed negative relationship with sap sucking insects. The present observations on higher amount of chlorophyll content in the resistant and moderately resistant chilli germplasm corroborates with the report of Megharaj et al. (2016); Latha and Hanumantharay (2018); Chaudhary and Pandya (2019); Nasrin et al. (2021). The decrease in the photosynthetic pigment in the susceptible chilli germplasm might be ascribed to the inhibition of pigment biosynthesis which results from the alteration in mineral nutrition or lack of assimilates (Stacey and Keen 1996). Significantly negative relationship between total chlorophyll content of chilli leaves and the incidence of S. dorsalis was also reported earlier by Rameash et al. (2015); Megharaj et al. (2016); Latha and Hanumantharay (2018); Nasrin et al. (2021). The reason for negative association in the present study might be attributed to the reduction of leaf size and leaf curl owing to thrips infestation and consequent decline in the photosynthetic activity and chlorophyll content of leaves. Results on correlation studies between population of S. dorsalis and various biochemical parameters of chilli germplasm revealed that the population of S. dorsalis showed significant negative correlation with phenol (-0.975**), non-reducing sugars (-0.985**), chlorophyll a (-0.980**), chlorophyll b (-0.987**) and total chlorophyll (-0.990**) (Table 3). However, the relationship between proline (0.961**), protein (0.961**), total sugars (0.989**) and reducing sugars (0.989**) content of chilli leaves and the population of S. dorsalis was significantly positive. The multiple linear regression analysis indicated that various biochemical parameters of chilli germplasm viz., proline (X1=0.0247), phenol (X2=0.1165), protein (X3=0.0323), total sugars (X4=0.0367), reducing sugars (X5=0.0646), non-reducing sugars (X6=2.1829), chlorophyll a (X7=0.0435), chlorophyll b (X8=0.0654) and total chlorophyll (X9=0.0308) together influenced the population of S. dorsalis to an extent of 97.32 per cent (Table 4).