Re: Why are there more stomata on green parts of variegated leaves?

Why questions are often difficult to answer but you seem to have a very 
reasonable answer to explain your observation. To really confirm that the 
nongreen parts of your variegated leaves do not photosynthesize as much as the 
green parts, you would need to do one of the following:  

1. Starch test the leaves to show that there is little or no starch in nongreen 
areas and lots of starch in green areas. This may not work on all leaves 
because some plants do not accumulate starch in their leaves but export 
photosynthates as sucrose.

2. Extract chlorophyll from green and nongreen areas to show the nongreen areas 
have little or no chlorophyll while green areas have lots of chlorophyll. Just 
because a leaf is nongreen does not mean that there is not chlorophyll there. 
In some nongreen leaves, other pigments such as carotenoids and anthocyanins, 
simply mask the green chlorophyll.

3. Measure photosynthesis in leaf disks cut from green and nongreen areas of 
the leaf to show that photosynthesis occurs in green but not nongreen parts.


References

Hershey, D.R. 1995. Plant Biology Science Projects. New York: Wiley.


Starch testing leaves


Leaf disk photosynthesis technique