Effect of photosynthetic efficiency and water availability on tolerance of leaf removal in Amaranthus hybridus

1 Although substantial genetic variation in photosynthetic rate exists within natural populations, it is unclear how such variation might affect the capacity of plants to tolerate herbivore damage. 2 I tested if decreased efficiency of electron transport, caused by a mutation that confers resistance to the herbicide triazine, affected the ability of Amaranthus hybridus to tolerate leaf removal, and if this genotypic effect interacted with water availability. 3 Tolerance levels were compared with the root-to-shoot ratio and with compensatory photosynthesis, to investigate the mechanisms of tolerance. 4 Triazine-resistant A. hybridus was found to have lower tolerance to herbivory suggesting that herbivores should drive selection for higher photosynthetic capacity. 5 Although interactive effects between water availability and photosynthetic genotype were not detected, reduced water availability did cause an overall increase in the ability of plants to tolerate damage. 6 Greater tolerance of plants under low water availability and of the TS genotype was associated positively with root-to-shoot ratios, but there was a negative association between greater tolerance under low water availability and compensatory photosynthesis. 7 These results suggest that photosynthetic variation is more likely to alter tolerance indirectly though changes in resource allocation, and that direct associations between physiological variables and tolerance may be uncommon in natural systems.