Phenotypic and biochemical responses of cowpea (Vigna unguiculata l. Walp) to the cowpea aphid (Aphis craccivora koch)

Abstract

Cowpea (Vigna unguiculata L. Walp.) (Fabaceae) is a vital legume crop cultivated by many smallholder farmers. The production of cowpea is constrained by several abiotic and biotic factors such as insects, diseases, and parasitic weeds. Cowpea aphid (Aphis craccivora Koch) is one of the most important pests of cowpea and many other crop species worldwide that causes significant losses directly through feeding on plants, and indirectly as a viral vector, reducing the leaf area, grain yield, and quality. A number of aphid control measures developed have proved to be ineffective necessitating the development of host-plant resistance. In the development of insect-resistant varieties, a better understanding of the cowpea-aphid interaction processes and mechanisms of resistance is critical. Aphid attack leads to signal transductions that trigger plant defense responses such as oxidative burst, accumulation of secondary metabolites, and defensive proteins. In particular, changes in the composition and levels of secondary metabolites have been reported in a few crops and could be a possible defense mechanism of cowpea in response to aphid attack. Although resistant cowpea lines were identified in several locations, resistance-breakdown have been reported to occur because of the emergence of new biotypes in several plants–aphid systems and locations. This study sought to understand the mechanisms of cowpea resistance to the cowpea aphid, by identifying sources of resistance of cowpea to aphid in Uganda that can be used in the development of new resistant varieties. Fourty eight genotypes were screened against aphid resistance in a screen house experiment laid out in RCBD, replicated 3 times with each plot containing four plants. Five aphids were introduced to each plant at ten days after planting under free choice and data on aphid counts was collected at five, nine, thirteen, and seventeen days after infestation (DAI). The damage was scored at seventeen DAI on a scale of 1-5, where 1 was no damage and 5 was highly damaged by aphids. In addition to this, ten genotypes were selected from the first screening and screened further, in the same manner, using aphids collected from Ngetta, Kabanyolo, and Serere as the major cowpea growing areas. Furthermore, the changes of biochemical components in cowpea leaves of six selected genotypes in response to aphid infestation were determined for their relevance in the development of host resistant cowpea cultivars to aphids. Six cowpea genotypes, three resistant and three susceptibles were planted in the screen house, under two treatments of infested and non-infested and aphid damage scores and the number of aphids per plant were subjected to analysis of variance using Genstat 18th edition. Mean separations were done using Fisher’s protected least significant difference (LSD) at 5% of significance. Pearson correlation analysis was done. The flavonoid, alkaloids, proteins, carbohydrates, phenolics, and tannins leaf contents were determined and compared. A T-test analysis was conducted to compare the levels of the different secondary substrates on the genotypes’ between infested and uninfested treatments during and after infestations. Variations were observed among genotypes in their response to aphid infestation and damage. There were no significant differences in the response of genotypes to aphids collected from the three locations across Uganda. Sanzi and the wild relative TVNu 1158 showed a high level of resistance to aphids, however, the known to be resistant genotypes from IITA, Tvu-310 was found to be susceptible, while IT97K-556-6 was found to be moderately resistant to Ugandan biotypes and therefore cannot be used as sources of resistance in Uganda. Additionally, infestation increased biochemical constituents in cowpea leaves for most of the genotypes, and most metabolites had positive correlations amongst each other under infestation and negative correlations under non-infestation. The increase of metabolites in the cowpea leaves incited by infestation was an indication of their defensive role and complementation of metabolites in response to aphid attack. Alkaloids, tannins, and total phenols were found to be associated with a low aphid population and less damage to the plants. The aphid biotypes across the three locations in Uganda did not differ from each other but were different from the aphid biotypes in Nigeria where IT97K-556-6 was found to be resistant. Genotype TVNu 1158, Sanzi, and NE 51 are possible sources of aphid resistance and can be used in the breeding program to introgress resistance to aphids. There is a need for research on further identification of specific classes of metabolites studied within each group of metabolites, to determine for example the type of alkaloids that contributes to the resistance to aphids in cowpea.