Morpho-anatomical characterization of interspecific derivatives of Gossypium hirsutum L. x G. armourianum Kearney cross for whitefly tolerance

The wide spread cultivation of transgenic Bt-cotton in India has altered the pest profile from bollworms to sap-sucking insect-pests especially whitefly [Bemisia tabaci (Gennadius)]. A whitefly epidemic in 2015 destroyed cotton crop on nearly 1.5 million hectares in North Indian cotton growing states. Management of whitefly is not easy due to its wider host adaptability, potential for insecticide resistance development and rapid evolution. Host plant resistance is the most viable approach for combating whitefly menace. A wild cotton species Gossypium armourianum Kearney Accession PAU 1 tolerant to various biotic stresses was used for the introgression of whitefly tolerance in Upland cotton (Gossypium hirsutum L.). Various morpho-anatomical characteristics (viz. stomatal frequency, gossypol glands, trichome density, leaf thickness) were studied from leaf discs in experimental material (i.e. parents G. armourianum Acc. PAU 1, G. hirsutum cv. F 1861 and LH 1556, F-1, BC1F1, BC2F1 derivatives). The BC2F1 individuals differed significantly in stomatal frequency, trichome density, number of gossypol glands and lamina thickness; and hence their response to whitefly tolerance. However, with age progression, a significant decline in stomatal frequency, gossypol glands and trichome density were observed in mature leaves. Correlation studies revealed a positive association of whitefly incidence with trichome density (r = 0.319*) and stomatal frequency (r = 0.372*) in young leaves. A positive correlation of stomatal frequency with gossypol gland (r(y) = 0.251* and r(m) = 0.296*) and trichome density (r(y) = 0.360* and r(m) = 0.305*) was observed in both young and mature leaves. G. armourianum possessed 4-5 -folds higher tannin contents than G. hirsutum parents before infestation. Surprisingly in G. armourianum, increase in tannin levels were observed much earlier (after 24 h of infestation) as compared to G. hirsutum.