Defense and tolerance in Asclepias syriaca L. (Asclepiadaceae).

摘要

This study explored the evolution of resistance for Asclepias syriaca L. (Asclepiadaceae). Vertebrate herbivores caused much more damage than insect herbivores. Natural levels of damage ranged from 2% to 23% among 12 genets, suggesting that herbivores may decrease plant growth and that genetic variation in defense potentially exists.; In a common garden experiment, significant genetic variation in defense against Uromyces asclepiadis and almost significant genetic variation in defense against Danaus plexippus were detected. Selection pressure by these plant enemies could lead to increased levels of defense. In addition, this experiment detected genetic variation in tolerance to damage, as some genotypes were more able to tolerate foliar damage than other genotypes.; To further investigate the potential for and constraints on the evolution of plant tolerance to herbivores, a greenhouse experiment was performed in which soil nutrient concentration was manipulated. Families differed significantly in tolerance when experiencing high nutrient conditions, and families were almost significantly different in tolerance when grown under low nutrient conditions. The interaction between family and nutrient treatment for tolerance would not constrain the rate at which tolerance could evolve under each nutrient condition. Under low nutrient conditions, significant genetic variation was detected for allocation patterns and for compensatory ability. Furthermore, in the low nutrient treatment, resource allocation to storage was positively, genetically correlated both with compensatory ability and tolerance, and compensatory ability was positively, genetically correlated with tolerance. These results demonstrate that allocation of resources to storage provided a mechanism for tolerance. A negative genetic correlation between compensatory ability and plant growth when undamaged suggests that this tolerance mechanism entailed an allocation cost. Under high nutrient conditions, genetic variation in tolerance also existed, but allocation patterns did not explain tolerance. The importance of distinguishing between tolerance and compensatory ability is evaluated.