Nearly 700 plant species in the United States face extinction by the year 2000, and those in rapidly developing parts of the nation are at particular risk, a conservation group says.

The private Center for Plant Conservation said 73 percent of the plants facing extinction are in southerly, subtropical regions with intense economic or population growth - Hawaii, California, Texas, Florida and Puerto Rico."The causes of extinction are by now quite familiar to us - destruction of habitat, population growth, urban, suburban and industrial sprawl, irresponsible resource extraction, overuse of public land," Donald Falk, the center's executive director, said Monday. "A consequence of that is this tremendously accelerated rate of extinction that we're seeing in these priority regions."

Falk said some of the endangered plants are relatives of important timber products, food groups, or plants used in horticulture and landscaping.

The center's 11/2-year study found that 253 species may become extinct within five years.