The effect of hydroxamic acid concentration at late growth stages of wheat on the performance of the aphid Sitobion avenae

From a previous worldwide screen of wheat cultivars as seedlings, five were selected to encompass a wide concentration range of the hydroxamic acid DIMBOA (0.99-8.07 mmol kg(-1) fr. wt). Compared with those in seedlings, the concentrations of DIMBOA in flag leaves of the same cultivars were relatively low (0.06-0.87 mmol kg(-1) fr. wt). Concentrations of hydroxamic acids in whole ears at anthesis and at the early milk stage were even lower, ranging from undetectable to 0.05 mmol kg(-1) fr. wt. The three-day mean relative growth rate (MRGR) was determined for nymphs of Sitobion avenae on the flag leaf (GS 39) and on the ear (GS 60-62) of the five cultivars. No significant differences in MRGR were found between cultivars at the same growth stage, although MRGR was higher on ears than on flag leaves. The results are discussed in relation to the potential of hydroxamic acids in programmes aimed at breeding mature-plant aphid resistance into wheat cultivars.