BIOCHEMICAL BASIS FOR THE RESISTANCE OF BARLEY TO APHIDS

Barley plants may be severely damaged by aphids, mainly because they may transmit viruses, remove essential nutrients, and disrupt tissues. This review discusses possible resistance factors of this plant, such as morphological defences and natural chemicals that have been shown or suggested to be involved in protection of barley against aphids. The available evidence for the role played by waxes, gramine, aconitic acid, phenolics and amino acids is presented. A discussion is included on other potentially protective molecules, such as protease inhibitors, that need to be studied. Environmental stress also affects plant-aphid interactions because the chemical composition of the plant changes. Water stress increases susceptibility, and NaCl and temperature increase resistance to aphids. The compatible solute glycine betaine, which accumulates under several types of stress, increases reproduction of aphids. Temperature and availability of nitrates increase gramine content of the leaves and, therefore, resistance to the aphids. A summary of conclusions and future perspectives focuses on the paramount importance of environmental stress in ant resistance and on the need to identify inducible resistance factors.