Leaf discs buoyancy

[grayfalcon89]

I hope there is at least one person here who did this lab in their college. Well my class did this in our advanced biology class this week (i'm in high school) but search in google tells me that college people do so as well. Anyway, the lab is about leaf disc buoyancy which is basically by taking spinach leaf discs (you just take a straw and make small holes on it), vacuum out the oxygen so that they sink in the water, and then put them on water under photosynthetic lights and measure how many of them come up to float on the surface again.

 

Other leaves are OK but we used spinach and I think all leaves (as long as they're not hairy) are OK. But here's my Q (sorry for long intro and explanation of lab): I and my partner added pH buffer 8 to one of our cups and this increased the buoyancy speed greatly. The question is, I don't know how it happened. So, I looked at the ingredient of pH buffer:

 

Sodium phosphate

Dibasic CAS No. 7558-79-4

Pot. Phosphate, Monobasic CAS No.7778-77-0

 

Searching google tells me that sodium phosphate can increase the rate of photosynthesis.

 

http://biae.clemson.edu/biolab/PHSdisk.html

 

But again, this site didn't tell me how it happened so that's why I"m asking here for help... Thank you very much!