Mediation of Impacts of Elevated CO 2 and Light Environment on Arabidopsis thaliana (L.) Chemical Defense against Insect Herbivory Via Photosynthesis.

Elevated CO ₂ alters C3 plant tolerance to insect herbivory, as well as the induction kinetics of defense hormones salicylic acid (SA) and jasmonic acid (JA), but the underlying physiological mechanism causing this response is not well understood. In principle, SA could be induced under elevated CO ₂ by reactive oxygen signals generated in photosynthesis, ultimately influencing chemical defense. To test whether the effects of elevated CO ₂ on C3 plant chemical defense against herbivorous insects are modulated by photosynthesis, Arabidopsis thaliana var. Col-0 plants were grown in two 2 × 2 × 2 nested factorial combinations of ambient (400 ppm) and elevated (800 ppm) CO ₂ , and two dimensions of light regimes comprising intensity ('mild' 150 μmol E m ^-2  s ^-1 vs. 'low' light, 75 μmol E m ^-2  s ^-1 ) and periodicity ('continuous', 150 μmol E m ^-2  s ^-1 vs. 'dynamic', in which lights were turned off, then on, for 15 min every 2 h). Plants were challenged with herbivore damage from third instar Trichoplusia ni (cabbage looper). Consistent with experimental predictions, elevated CO ₂ interacted with light as well as herbivory to induce foliar concentration of SA, while JA was suppressed. Under dynamic light, foliar content of total glucosinolates was reduced. Under combination of elevated CO ₂ and dynamic light, T. ni removed significantly more leaf tissue relative to control plants. The observations that CO ₂ and light interactively modulate defense against T. ni in A. thaliana provide an empirical argument for a role of photosynthesis in C3 plant chemical defense.