I asked myself the same question, since I'm doing incorporation experiments and I have to find the optimal conditions to have maximal photosynthetic activity. I therefore decided to measure oxygen production in different storing conditions.
From my experience:
Fresh protoplasts at RT: approx 100 umolO2 /mgChl * h
Fresh protoplasts on ice: approx 100 umolO2 /mgChl * h but rapidly decreasing to 20 after 20 min
After 24 h at...
...RT: completely broken and no oxygen evolution (thus confirming that chloroplasts swimming in a protoplast suspension possibly don't evolve oxygen)
...ice: same as fresh, activity rapidly decreasing after 20 min
...ice and then 4 h in RT: suprisingly the activity is similar to the fresh protoplasts. No decrease in oxygen production
The protoplasts are suspended in a 0,5M sorbitol buffer with the common additional salts (MgSO4, KNO3, KH2PO4,...) and I'm using barley.
It would be nice if you can replicate my experiment to confirm it in wheat.