Plant shrinks when places in saltwater due to osmosis of?

Water.

osmosis always refers to the diffusion of WATER down a concentration gradient.

diffusion can be applied to any substance;
osmosis only applies to the diffusion of water.

in the situation described, osmosis occurs by water diffusing out of the plant, down the diffusion gradient (because the salt water effectively has a lower concentration of water than in the plant) and into the salt water.