Do phloem feeders affect gas exchange? A case study of Aphis gossypii (Glover) on cotton

Abstract

AbstractCotton aphid (Aphis gossypii Glover) is a worldwide pest of cotton (Gossypium hirsutum L.). High aphid densities visibly damage cotton leaves, reducing leaf photosynthesis and yield. In contrast, lower aphid densities usually do not visibly damage leaves but their effect on photosynthesis is unknown, complicating the decisions on the need for their control. The effect of cotton aphid at lower densities on gas exchange of cotton leaves was investigated under field conditions in the Namoi Valley, Australia. Photosynthesis, leaf area, stomatal opening, and aphid density were assessed on the third mainstem leaf below the terminal of aphid infested and uninfested control plants. Photosynthesis was reduced by a maximum of 27% at a density of 27.6 aphids cm−2 of leaf tissue. Negative linear relationships were found between aphid density and relative photosynthesis, relative conductance and relative transpiration. The temperature of leaves infested with >5 aphids cm−2 was higher than control leaves. Stomatal conductance was reduced faster than photosynthesis, and 87% of stomata on aphid infested leaves were closed, compared to only 34% on control leaves. Closure of stomata was implicated as a cause of reduced photosynthesis in aphid infested cotton leaves. Stomatal closure may be a direct response to cell damage from aphid feeding, as a plant defense or as a response to aphid induced stress. The results suggest that low levels of aphids (e.g., <7 aphids cm−2) on leaves in the upper canopy of cotton crops may be tolerated with low risk to photosynthesis.