THE TREASON trial of Anatoly Scharansky opening tomorrow in Moscow will tell whether the current cloud over Soviet-American relations will continue to darken or start to clear.It is startling, and acidential, that such large stakes should have settled on this one case, but it is so. The matter is, moreover, entirely Moscow's to decide.

Mr. Scharansky, who has been held incommunicado for 16 months, is hardly responsible for the timing of his trial. Behind that, one assumes, is a calculated Soviet decision to throw Jimmy Carter's human-rights interventions back in his teeth at what may well be a make-or-break moment in such crucial East-West negotitions as the strategic arms talks.

But Mr. Scharansky is, at close inspection, not an unlikely figure to be at the heart of this East-West encounter. As a Jew kept from emigrating, he touches what is, year in and year out, perhaps the most emotion-laden and politically freighted aspect of the human-rights cause. As a key member of the unofficial group formed to monitor Russia's compliance with its human-rights pledges at Helsinki, he figures in yet another head-on East-West collision. As a computer programmer with wide scientific contacts, he has an involvement with the exceedingly delicate area of the transfer of science and technology between East and West. As a defendant whose innocence of CIA connections Jimmy Carter has personally avowed, he becomes a figure whose treatment affects the prestige and credibility of the White House.

It is his scientific connection, by the way, that seems to have provided the basis for his being charged with "high treason in the form of espionage." The particulars of this charge have not yet been made public. But were it not for his scientific links, Mr. Scharansky presumably would have been charged only with anti-Soviet propaganda. This is the charge against a second prominent Jewish dissident, Alexander Ginzburg, whose trial is also to open tomorrow - separately, in Kaluga.

In a country that treated its citizens with respect, and under a government truly interested in pursuing detente, Mr. Scharansky and, for that matter, Mr. Ginzburg would not be facing trial at all. To that considerable extent, a great deal of damage has already been done. The Soviet regime still has it within its power, however, to limit further damage. It can expedite both trials and run them in a businesslike rather than a theatrical way. It can minimize the aspect of a Scharansky connection with the CIA and keep the trial away from President Carter. Most important, it can render a humane sentence - the maximum sentence for treason is death.

One can never say in Soviet-American relations that "everything" hangs in the balance of one event. Still, it is hard to exaggerate what hangs on the Scharansky trial. Yesterday, the cancellation of a high-level science and technology mission to Moscow (a mission meant to balance a similar one to Peking) was disclosed, along with an administration decision to review all other cooperative agreements with Moscow. Considerations of conscience and politics could yet make it impossible for Secretary of State Cyrus Vance to meet Foreign Minister Andrei Gromyko for the strategic arms control talks scheduled in Geneva on Wednesday. Mr. Vance personally denounced the trials yesterday.

The Soviet Union cannot be expected to stand still and say thank you when Jimmy Carter personally involves himself in particular human-rights cases in the Soviet Union. But it also cannot expect to trample wantonly on pledges and values it professes to uphold, and still enjoy the full benefits of cooperation with foreign states. The Scharansky trial puts the whole immediate future of Soviet-American relations in the Kremlin's hands.