Put a Wal-Mart guy and a Whole Foods guy on the same stage to talk sustainability at a conference in Boulder, and what happens? Interesting things.

Fresh from their morning yoga and organic luncheon, a not particularly friendly audience of execs and marketers heard Wal-Mart senior director of corporate responsibility Rand Waddoups say: “Wal-Mart is not a green company.”

Countered Michael Besancon, southwest regional president for Whole Foods Market: “If Wal-Mart is not a green company, then Whole Foods is not a green company. We do a lot of green things, and we have green intentions, but we don’t believe that we are, and we try not to say that we are.”

Later in the discussion, asked by eco-journalist Simran Sethi whether Wal-Mart sells products containing genetically modified organisms, Waddoups answered, “Everybody is.”

Sethi started to pursue the point but Besancon interrupted. “We are too,” he said. “We sell GM foods. We can’t source corn and soy in every product… we can’t control everything manufacturers do.”

Because of this, consumers as well as retailers must push for transparency of the supply chain, Waddoups said. Added Besancon: “The more questions you ask, the more answers you get that you don’t want to hear.”

Sustainability, Besancon added, creates a great deal of tension between the three legs of the “triple bottom line”: People, planet, and profits. Replacing plastic bags, which cost a penny each, with paper bags, which cost as much as 17 cents each, is not a zero-sum move.

Waddoups was the salty snack buyer at Wal-Mart, sharing a cubicle with the water buyer, when the company responded to Hurricane Katrina more quickly and effectively than the government did — starting with 18-wheelers full of bottled water. That showed him — and the rest of management — that the world’s largest company could be a potent power for good. “We’re trying to be as good as we were during Hurricane Katrina all the time,” he said. “What’s happening now with the climate is like Hurricane Katrina in slow motion.”

Besancon, who went to work at a southern California health food store 38 years ago, noted that Waddoups started corporate and adopted a sustainable viewpoint, whereas “I started out as a hippie and became a hard-assed businessman.”