A Maryland legislative panel today soundly rejected the most far-reaching aspect of Gov. Harry Hughes' energy conservation plan, killing by a 6-1 vote an executive order restricting the heating and cooling of all private non-residential builidngs in the state.

The special panel, which has the power to review all executive orders made by the governor while the legislature is not in session, had tentatively approved the heating and cooling measure last month when Hughes first unvieled it.

But today, three days before the order was to take effect, the legislators surprised Hughes by deciding that the order was premature and would create an unreasonable hardship for retail merchants in the state.

The order, which Hughes called the cornerstone of "the most stringent energy conservation program proposed by any state," would have required that all private non-residential buildings in that state set their thermostats at no lower than 80 degrees during warm months and no higher than 65 degrees during cold months.

A similar measure, applying only to state buildings, went into effect last month with the approval of the legislative panel. At the same time several other energy conservation measures went into effect including strict enforcement of the 55 mile-an-hour speed limit on state highways.

Hughes said he was disappointed by the legislative vote today, Which, according to press secretary Gene Oishi, "took all of us by surprise." Several members of the panel praised Hughes' intentions, but said they would prefer to wait until similar federal regulations take effect in July.

The panel's rejection of the Hughes proposal today came after a hearing at which several retail merchants and lobbyists for the clothing, restaurant and food and drug industries argued that they could not operate through the summer months with the 80 degree limit on air conditioning.

The merchants and lobbyists said the restrictions would drive away customers, resulting in declining sales and possible layoffs. One restaurant owner argued that his customers would rather stay at home and eat in the comfort of their own air-conditioning rather than "sweat it out in my establishment."

Plato Coundjeris, representing the Hess chain of women's clothing stores on the Eastern Shore, predicted that the restrictions would cause "a relative disaster" in the clothing industry. Coundjeris also claimed that the policy would be difficult to enforce. "Cheating would be rampant," he said. "Policing it would be almost an impossibility."

Hughes said he may seek a meeting with members of the legislative panel, but added: "I'm not sure what the next step is. I've got to do a little thinking about this." CAPTION: Picture, GOV. HARRY HUGHES . . . not sure what next step is