Abstract
Plant leaves of different ages differ in nutrients and toxic metabolites and thus exhibit various resistance levels against insect herbivores. However, little is known about the influence of leaf ontogeny on plant resistance to phloem-feeding insects. In this study, we found that the green peach aphid, Myzus persicae, preferred to settle on young cabbage leaves compared with mature or old leaves, although young leaves contained the highest concentration of glucosinolates. Furthermore, aphids feeding on young leaves had higher levels of glucosinolates in their body, but aphids performed better on young leaves in terms of body weight and population growth. Phloem sap of young leaves had higher amino acid:sugar molar ratio than mature leaves, and aphids feeding on young leaves showed two times longer phloem feeding time and five times more honeydew excretion than on other leaves. These results indicate that aphids acquired the highest amount of nutrients and defensive metabolites when feeding on young cabbage leaves that are strong natural plant sinks. Accordingly, we propose that aphids generally prefer to obtain more nutrition rather than avoiding host plant defense, and total amount of nutrition that aphids could obtain is significantly influenced by leaf ontogeny or source-sink status of feeding sites.
Highlights
The optimal-defense hypothesis suggests that valuable tissues of a plant should be better defended against insect herbivores because young leaves of plants usually have higher growth capacity and are more valuable than older ones [1,2]
More aphids settled on young leaves than on mature leaves after 3 h (t = 4.741, df = 9, P = 0.001) and 8 h (t = 5.063, df = 9, P = 0.001; Fig 1A), while more aphids preferred to settle on young leaves compared to old leaves since 1 h after aphid release (t = 3.475, df = 9, P = 0.007; Fig 1B)
M. persicae exhibited a clear preference for young leaves, which agrees with previous findings for other aphid species [10,14,15]
Summary
The optimal-defense hypothesis suggests that valuable tissues of a plant should be better defended against insect herbivores because young leaves of plants usually have higher growth capacity and are more valuable than older ones [1,2]. Young leaves generally contain more defensive metabolites than older leaves, and insect herbivores are expected to avoiding feeding young leaves to minimize the plant resistance conferred by defensive metabolites [2]. Many insects still prefer and grow better on young leaves, which generally contain more nutrients, suggesting that insects may have evolved strategies to minimize the negative impacts imposed by defensive metabolites [1,3]. The relationship between leaf age/ontogeny and plant defense against insects with chewing mouthparts have been examined by some studies, but the influence of leaf age on phloem-feeding insects remains largely unknown [1,2].
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