Plants in nature interact with a wide range of organisms that are either beneficial (such as mycorrhizae) or detrimental (such as herbivores). Since plants must adjust their primary metabolism to support beneficial organisms and to resist attackers, a trade-off between the two could occur. For example, as plants supply carbon to mycorrhizae in the roots in exchange for important nutrients, it may be unavailable for defense, growth, and storage. Here, we investigated how mycorrhizae and herbivory, individually and in combination, alter resource dynamics in storage and actively growing tissues. We measured carbon (soluble sugars, starch and %C) and nitrogen (protein and %N) pools in stems and roots (storage) and apical leaves (growth) of tomato plants after inoculation with a 1:1 mixture of the mycorrhizal fungi Glomus etunculatum and Glomus mossae and simulated herbivory by the specialist herbivore Manduca sexta.
Results/Conclusions
Mycorrhizae and herbivory had very different effects on resource pools. Mycorrhizae had no effect on protein or soluble sugar content but had an effect on starch content, decreasing in the apex and increasing in the roots. Stem %N was also increased in the presence of mycorrhizae. In contrast, leaf herbivory altered protein, soluble sugars and starch but did not affect %N. Plants in the herbivory treatment had lower protein content in the apex but higher protein in the stem compared to undamaged plants. Soluble sugars decreased in both apex and stems, and starch also decreased in the apex. Interestingly, carbon and nitrogen pools remained unaffected in the roots by the herbivory treatment, suggesting that in tomato the stem may be a more important site of storage. Overall, there was no interaction effect of mycorrhizae and herbivory except for stem %N. Plants with both mycorrhizae and herbivory had a higher stem %N than control plants. This indicates that a mutualist and an antagonist can alter plant metabolism in a non-additive manner, potentially having profound effects on plant fitness and future regrowth capacity after an herbivory event.