Indirect method of measuring photosynthesis

where did you encounter the question?
i have heard of the standard one with photosynthometer
the non official one that i have heard is calculating the rate of oxygen bubbles by aquatic plants ( like elodea )...i wonder too what could the indirect one be

This is the question: Both methods (collecting oxygen via syringe or counting bubbles) are an indirect method of measuring photosynthesis, explain what this means

I tried searching it up but I couldn't find anything that made sense to m

Ah i see... they call them indirect methods cause these 2 mentioned ones aren't as reliable as the photosynothometer one...
Anyhow in both of the methods we would need to use a light source, an aquatic plant like a pondweed (in a beaker of water) and as it photosythesises it would release bubbles of oxygen...

We would need to calculate the rate of oxygen gas produced (since the size of oxygen gas produced is proportional to the rate of photosynthesis)

Two ways of measuring it as they said would be : collecting the oxygen gas using syringe ( so here we would obtain the volume of oxygen gas per minute)
and the next one:
count the bubbles of oxygen for the rate of oxygen per minute

(later on i assume the rate per minute could be converted to seconds)

I found this link helpful : https://www.saps.org.uk/secondary/teaching-resources/190-using-pondweed-to-experiment-with-photosynthesis-

thank you so so much