EFFECT OF INFESTATION BY APHIDS ON THE WATER STATUS OF BARLEY AND INSECT DEVELOPMENT

To compare the effects of aphid infestation with some effects of wounding and drought-stress, several physiological parameters and metabolite concentrations were measured in infested, mechanically wounded or water-stressed young barley plants (Hordeum vulgare cv Aramir). Barley plants infested with the greenbug (Schizaphis graminum) had lower water potentials and CO2 assimilation than non-infested plants. Abscisic acid content increased by 55% in leaves after 72 hr of infestation. Water potentials and stomatal resistance of barley plants changed only as a consequence of infestation by the greenbug or by drought-stress. Proline concentration increased in leaves subjected to infestation or drought by 11- and 14-fold, respectively. Leaves with artificial damage showed the same reduction in chlorophyll contents as leaves of drought-stressed plants. Greenbug infestation caused higher chlorosis than other treatments. Contents of soluble carbohydrates and proteins decreased 52 and 38% by infestation, 38 and 28% by drought, and 14 and 8% by artificial leaf damage, respectively. To study the influence of these treatments on the quality of barley plants as a food source for the aphids, developmental rates (1/D) and the mean relative growth rates (MRGR) of nymphs reared on these seedlings were compared. New aphids reared on previously infested seedlings had the lowest MRGR and 1/D (ca 82 and 68%, respectively) compared to aphids on control plants without previous infestation. Aphids reared on plants subjected to drought also had lower MRGR and 1/D (ca 89% and 77%). Greenbugs on wounded leaves had similar MRGR and 1/D rates to nymphs reared on control plants. These results show that greenbug infestation of barley produced changes similar to those observed in plants subjected to drought-stress and that aphids feeding on both groups of seedlings had lower developmental and mean relative growth rates. Water-stress caused in barley by aphid infestation or drought would probably affect greenbug development due to the effects of stress on the chemical composition of the plant.