Hi! The white leaves contain fewer chloroplasts compared to the green leaves. Therefore when placed under the light, the green leaf will be able to photosynthesise at a higher rate than the white leaves, producing more net O2. O2 would be trapped within the air spaces of the leaves, allowing it to rise. The white leaves will sink instead since rate of respiration (which takes up O2) exceeds rate oh photosynthesis. There would be insufficient O2 to keep the leaves afloat.