Apical dominance — variability among cotton genotypes and its association with resistance to insect herbivory

Abstract

We investigated variability in apical dominance among cotton genotypes ( Gossypium hirsutum, G. barbadense) and the relationship between degree of apical dominance and resistance to insect herbivory. Two groups of genotypes were compared: a ‘normal’ group, including genotypes with naturally occurring concentrations of tannins and terpenoid aldehydes (characteristics of the tribe Gossypiae), and an ‘enhanced’ group, including experimental lines selected for improved defences. The degree of apical dominance ( AD, dimensionless) and the apparent time required for the activation of axillary buds after the removal of the apical bud ( t, days) were measured in a glasshouse experiment including 24 genotypes and two damage treatments, i.e damaged plants, in which the apical bud was removed 40 days after sowing, and undisturbed controls. To quantify AD we assumed that the stronger the apical dominance, the greater the distance ( d) between the plant apex and the nearest vegetative branch; we measured d and plant height ( h) in control plants, and calculated AD = d h . To calculate t, we counted periodically the leaves in the axillary bud closest to the apex of damaged plants, fitted linear regressions between leaf number and time, and took the x-intercept of the fitted functions as an estimate of t. Between 12 and 19 of the genotypes used in the glasshouse were grown in field experiments involving two treatments: control, protected with insecticides, and unprotected crops exposed to naturally occurring herbivorous insects, including Helicoverpa spp. (Lepidoptera: Noctuidae) and sucking mirid bugs (Hemiptera: Miridae). The ratio between the yield of unprotected and protected treatments for each genotype, R, was taken as an empirical measure of herbivory resistance. Degree of apical dominance ranged from 0.35 to 0.80, the height of undamaged plants from 0.68 to 1.0 m, and t from 1 to 7 days. Within the normal group, genotypes with strong apical dominance were taller, and normally had axillary buds that required more time for activation after release of apical dominance. A significant, non-linear relationship between R and AD was found for this group of genotypes, which indicates a broad optimum and a consistent decline in resistance with apical dominance increasing over a threshold (≈0.6). None of these relationships was found for genotypes with ‘enhanced’ defences, partially because this group had a narrower range of apical dominance. We conclude that a large variability in the degree of apical dominance among cotton genotypes exists that can influence resistance to herbivory, but interactions between degree of apical dominance and defences cannot be disregarded.