This is not the right story — see comment by O.P.
Perhaps "The Ultimate Catalyst", a novelette by "John Taine", the pen name used by the mathematician Eric Temple Bell for his science fiction stories. It was first published in Thrilling Wonder Stories, June 1939, available at the Internet Archive.
The dinner party is a banquet hosted by the dictator of the fictional South American country of Amazonia:
The Dictator shoved his plate aside with a petulant gesture. The plate, like the rest of the official banquet service, was solid gold with the Dictator's monogram, K. I.—Kadir Imperator, or Emperor Kadir—embossed in a design of machine guns round the edge. And, like every other plate on the long banquet table, Kadir's was piled high with a colorful assortment of raw fruits.
This was the dessert. The guests had just finished the main course, a huge plateful apiece of steamed vegetables. For an appetizer they had tried to enjoy an iced tumblerful of mixed fruit juices.
There had been nothing else at the feast but fruit juice, steamed vegetales, and raw fruit. Such a meal might have sustained a scholarly vegetarian, but for soldiers of a domineering race it was about as satisfying as a bucketful of cold water.
"Vegetables and fruit," Kadir complained. "Always vegetables and fruit. Why can't we get some red beef with blood in it for a change? I'm sick of vegetables. And I hate fruit. Blood and iron—that's what we need."
The "ultimate catalyst" which turned the diners into plants was in the final course of beef-flavored fruit prepared as a surprise by the biochemist in an assassination plot:
"Three weeks ago I discovered the organic catalyst to hasten the replacement of the iron in your blood by magnesium and thus to change your animal blood to plant blood, chlorophyll. The catalyst is merely a chemical compound which accelerates chemical reactions without itself being changed.
"By injecting a minute trace of chloride of gold into the fruits, I—and the living plant—produced the necessary catalyst. I have not yet had time to analyze it and determine its exact composition. Nor do I expect to have time. For I have, perforce, taken the same medicine that I prescribed for you!"
"Not so much, but enough. I shall remain a thinking animal a little longer than the rest of you. That is the only unfair advantage I have taken. Before the sun sets we shall all have ceased to be human beings, or even animals."
[. . . .]
"Rooted to the spot, Kadir! You are taking root already. And so are the rest of you. Try to stand up like human beings! Kadir! Do you hear me? Remember that blue fungus we saw in the forest? I have good reason for believing that was your friend Juan. In less than an hour you and I and all these fools will be exactly like him, except that some of us will be blue, others green, and still others red—like the thing you stepped on!
