Water availability affects tolerance and resistance to aphids but not the trade-off between the two

This study evaluates the effect of water availability on tolerance and resistance to the aphid Chaitophorus leucomelas by clones of the double hybrid [(Populus trichocarpa x P. maximowiczii Henry) x (P. trichocarpa x P. maximowiczii)] (TM x TM), a hybrid that was previously described as resistant to this aphid. In a 2 x 2 experimental design implemented in a nursery, we were able to assess growth (branch length, number of leaves and branch base diameter) in saplings reared in the natural presence of aphids (natural aphid damage) and aphid-controlled conditions (undamaged) under both well-watered and drought stress conditions. We found that resistance was reduced under drought stress conditions, while tolerance in branch length was increased. Cost of resistance was detected as clones displaying higher tolerance grew less in the absence of aphids, whereas no evidence of costs associated with tolerance was found in any of the growth traits measured, A genetic trade-off between tolerance and resistance was detected, but this trade-off was not affected by water availability. Considering the average response of both defence strategies, well-watered trees seem to allocate more resources to resistance than to tolerance, whereas drought-stressed trees allocate more to tolerance than to resistance. This suggests that tolerance would imply a lower cost than resistance, and the shift to either strategy could be modulated by resource availability.