The reason why orange leaves sink in salt water is because there is air in oranges. When orange leaves are put into water, water will enter those gaps, and the overall density is greater than the density of water.

According to a physical formula, buoyancy = volume of submerged liquid × density of liquid ×G, and g is a fixed number. Because you add salt to water, the salt dissolves, which increases the density of water, while the submerged volume of oranges remains unchanged, thus increasing buoyancy.