Anthropogenic inputs of carbon dioxide to the atmosphere are likely to alter leaf chemistry, potentially affecting the growth of insect herbivores.  Exposure to elevated CO₂ typically increases the carbon-nitrogen ratio of foliage, leading to increased consumption by insect larvae to meet their nitrogen demand to support protein synthesis.   To understand the effects of these future changes, mutant Arabidopsis thaliana plants with altered starch metabolism were exposed to herbivory by Trichoplusia ni neonates.  Two starchless mutants deficient in enzymes involved in starch biosynthesis and three starch-accumulating mutants lacking enzymes that promote starch degradation were examined. Five independent experiments were conducted to compare T. ni growth on each mutant type versus wildtype after 2 or 12 days of feeding.  Larval growth and consumption were measured and compared to various aspects of leaf chemistry, such as C:N, soluble carbohydrates, starch and glucosinolates. 

Results/Conclusions

The two starchless mutants had opposite effects on larval growth.  After 12 days of feeding, pgm1-2 larvae had 2.5-fold greater growth than wildtype, while adg1-1 larvae only grew a third as much of those on wildtype.  The difference appeared to be related to the reduced glucosinolate content of pgm1-2.  Larvae exhibited compensatory feeding on two of the starch-accumulating mutants.  At the highest level of starch accumulation, larvae seemed to reach their maximum ability to compensate, because developmental rate was slower.  Our model system suggests that, in the future, elevated carbon dioxide will have variable effects on herbivore performance depending on how it alters the nutritional quality and defense capability of plants.