Effect of environment on gramine content in barley leaves and susceptibility to the aphid Schizaphis graminum

Barley seedlings grown under different enviromnental conditions were infested with aphids. The population growth rate of the greenbug Schizaphis graminum decreased in seedlings maintained at temperatures above 25°. Plants grown with longer photoperiods were more resistant to aphids. Gramine concentration increased only in the second (youngest) leaf with high temperatures and photoperiod of 16 hr. The content in the first (oldest) leaf remained constant in all conditions. Water stress did not affect gramine content and aphid population growth rate in a cultivar with high alkaloid content. Changes in the levels of CO 2 exchange and water potential were similar in both barley cultivars grown at higher temperatures. It is suggested that barley seedlings grown under high temperatures and long photoperiod are more resistant to aphids, because these factors caused an increase in gramine concentration in the youngest leaves.