RESUMO
Variation in nutritional components and amounts of secondary metabolites not only affects the growth, development and survival of insect herbivores but also indirectly influences expression of host plant resistance to insects. In this study, we examined the role of different biochemical and morphological factors in sorghum as host plant defense against the spotted stem borer, Chilo partellus (Swinhoe). The genotypes IS 2205 and IS 2123 suffered lower deadheart incidence, and exhibited deleterious effects on development and survival of C. partellus, followed by ICSV 700, ICSV 708 and ICSV 25066 than the susceptible check, Swarna. The anthocyanin pigmentation in sorghum seedlings and C. partellus deadhearts were found significantly and negatively correlated with the larval period (r = −0.60** to −0.88**), while positively correlated with the larval and pupal weights, and larval survival and adult emergence (r = 0.58* to 0.95**). Conversely, the numbers of C. partellus exit holes in the stalk, larvae recovered, number of tunnels and stem tunneling length were significantly and positively correlated (*, ** = P ≤0.05 and 0.01, respectively) with the larval period (r = 0.72** to 0.89**), but significantly and negatively correlated with larval and pupal weights, larval survival and adult emergence (r = −0.54* to −0.84**). Although there was significant variation in morphological traits and biochemical composition of the sorghum genotypes, there was no direct link to expression of resistance to this insect, but for a few cases. The significant and negative association of total carotenoid, p-coumaric acid, zinc and iron contents with growth, development and survival of C. partellus, and damage parameters (r = −0.48* to −0.72**), indicated their role in expression of resistance to C. partellus in sorghum. However, the interaction among different biochemical compounds and the morphological traits, rather than a particular biochemical constituent played a greater role in host plant defense against C. Partellus.
RESUMO
In order to better understand the biochemical interactions and to identify new biomarkers for plant resistance against insects, we proposed a suitable lipophilic profiling method for insects and their host plants. The critical components of GC-MS based analysis are: sample amount, extraction, derivatization, temperature gradient, run time, and identification of peaks. For lipophilic metabolite profiling of maize and sorghum, and their insect pest, spotted stem borer larvae, we recommend 100 mg sample weight for seeds and insect samples (whole insect body), and 200 mg for seedlings. Maize and sorghum seeds required less time for fat extraction in comparison to their seedlings and the pest fed on these seedlings. GC-MS was standardized for better separation and intensity of peaks using different temperature gradients in the range of 180-300 C. A total of 48 lipophilic compounds encompassing various classes based on their functional groups such as fatty acids, fatty alcohols, hydrocarbons, sterols and terpenoids, vitamin derivative, etc. were separated in the seedlings (30), seeds (14), and the pest (26) in the retention time range of 3.22 to 29.41 min. This method could be useful to study nutritional aspects of different field crops in relation to various stresses apart from the analysis of lipophilic compounds for better understanding of insect-plant interactions.