The role of hydroxamic acids in Gramineae in conferring resistance to the aphids Sitobion avenae and Rhopalosiphum padi

Hydroxamic acids (Hx) occurring in cereal extracts, in particular the compound 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), have been shown to be involved in the resistance of cereals to bacteria, fungi and several insects. This investigation screened a wide genetic range of cultivars and species of the genera Triticum and Aegilops for antibiotic resistance to the cereal aphids S. avenae and Rhopalosiphum padi in relation to concentrations of Hx ([Hx]) of seedlings. A significant negative correlation (r = - 0.59) was found between S. avenae intrinsic rate of increase (rm) and [Hx] of 20 lines of tetraploid and hexaploid Triticum. Variation in R. padi rm was very small. Subsequent work assessed [Hx] in whole plants of a variety of ages and individual assessments were made of leaves of different ages in two-, three-, four- and five-leaved plants. [Hx] was found to decline with age but was always highest in newly emerging leaves, including the flag leaves of mature wheat. The effects of varying environmental conditions such as nitrogen and water supply and of damaging plants on subsequent [Hx] and S. avenae mean relative growth rate (RGR) were also studied. Small and relatively short-lived increases in [Hx] were seen following damage or aphid infestation sometimes accompanied by a decrease in mean RGR of S. avenae. Manipulating nutrient availability also affected [Hx] and mean RGR. Finally, a small group of seedling taxa maintained under carefully controlled conditions were re-examined for [Hx] and for aphid antibiosis (mean RGR). The same group of taxa were also assessed as mature plants just coming into ear (growth stage 47) for [Hx] and aphid mean RGR in a field study. The correlation between S. avenae mean RGR and [Hx] in the oldest leaf of seedlings was very strong (r = -0.69; P < 0.05). S. avenae mean RGR and the [Hx] of flag-leaves of the same cultivars were also strongly correlated (r = - 0.80; P < 0.05). The results are discussed in relation to the potential use of hydroxamic acids analysis in plant breeding programs, both as a standard when selecting promising lines for resistance to cereal aphids and also in the isolation of resistant genes.