(FORTUNE Magazine) – The treaty signed by Presidents George Bush and Mikhail Gorbachev could mark a watershed in U.S.-Soviet trade relations -- but only if the charismatic Soviet leader moves decisively to stabilize his country. Says Robert Hormats, vice chairman of Goldman Sachs International: ''The major barriers to U.S.-Soviet trade are the lack of a competitive Soviet economy and increasing political instability.'' If the Soviets meet the conditions Bush and Congress have set -- emigration reforms and the end of the embargo on Lithuania among them -- they would be on the way to winning most-favored-nation status. This would give them preferential tariff rates for goods exported to the U.S., but that in itself won't help much, since it affects duties on manufactured articles much more than on raw materials. Says Soviet Deputy Prime Minister Yuri D. Maslyukov: ''For now, we are concentrating on exporting raw materials. But my hope is that joint ventures will play a great role in the future in developing sophisticated technology for export.'' But joint ventures, which reached their peak in June 1989, when 165 were signed, have slumped, particularly since December. In March they totaled a mere 42. Warns Soviet trade expert Jan Vanous: ''Western interest in joint ventures is directly related to the state of the Soviet economy. If it goes any further down the drain, so will interest in joint ventures.''