Similarities between the effects of aphid infestation and cytokinin application on dark respiration and plant growth of legumes

Cowpea (Vigna unguiculata (L.) Walp. cv. Caloona), broad bean (Vicia faba L. cv. Aquadulce), and garden pea (Pisum sativum L. cv. Victory Freezer) seedlings were infested with cowpea aphids (Aphis craccivora Koch) or pea aphids (Acyrthosiphon pisum (Harris)), both Homoptera: Aphididae, for 10 days and then infested host plant tissue was examined for foreign substances injected by the aphids. No foreign compound was detected in any of the aphid-infested plant tissues. Both aphid species were also assayed for plant growth substances, utilizing the epinastic response of tomato (Lycopersicon esculentum Mill. cv. Roma Teardrop) seedlings, and both aphid species contained plant growth substances in concentrations higher than plant physiological concentrations. Broad bean and pea seedlings were also treated with foliar and root applications of 6-benzylaminopurine to determine if there were any similarities in plant growth or respiratory responses, following aphid infestation or 6-benzylaminopurine treatment. Root respiration in 6-benzylaminopurine treated plants decreased while shoot respiration increased in a response analogous to that observed for aphid-infested tissue. However, the alternative respiratory pathway was engaged for all 6-benzylaminopurine treatments, whereas in aphid-infested plant roots and shoots it was not. Both 6-benzylaminopurine treated and aphid-infested plants displayed a loss of apical dominance. These data suggest that part of the physiological response of the plant to aphid feeding is induced by changes in the cytokinin to auxin ratio.