Source-sink status of plant leaves influence host plant resistance to aphids.

<p>Young leaves have higher growth capacity than older ones and contain more defensive metabolites. However, aphids can obtain more phloem sap when feeding on young leaves that are natural plant sinks where phloem sap flows to. In contrast, aphids feeding on mature or old leaves compete for phloem sap with natural plant sinks and may thus ingest less phloem sap. To maximize their fitness, aphids generally prefer to obtain more nutrients by feeding on plant sinks rather than avoiding ingesting more plant defense metabolites in these tissues. <i>Red arrows indicate phloem flow direction</i>.