Photosynthetic Response of Cotton to Spider Mite Damage: Interaction with Light and Compensatory Mechanisms

We investigated the photosynthetic responses of cotton ('Gossypium hirsutum' L.) leaves to two-spotted spider mite ('Tetranychus urticae' K.) damage. Light-response curves of mite-infested (+M) and uninfested (-M) leaves diverged as mite populations increased. At 17 adult female mites per leaf, photosynthetic rate of +M leaves at photosynthetic photon flux density of about 1600 μmol m⁻²s⁻¹ was halved from 31 μmol CO₂ m⁻²s⁻¹ in -M to 16 μmol CO₂ m⁻²s⁻¹ in +M but there was no effect on either respiration or apparent maximum quantum yield. This has important implications when comparing the response to mites of individual leaves versus canopies. In the field (i) photosynthesis declined with crop age, but the rate of decline was faster in mite-infested leaves, and (ii) mite damage progressed downward in the canopy and from basal to distal leaf positions. We found no evidence of within-leaf (i.e., basal vs. distal section) or within-plant (top vs. mid or bottom leaf) increases in photosynthesis in compensation for mite damage, except for a minor enhancement of photosynthesis in bottom leaves of mite-infested crops due to greater light penetration in canopies severely defoliated by mite damage.