一道美国植物生理题请教

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A person is studying the relationship b/t photosythesis and respiration.He has a mutant in pea that grows slowly and has an elevated respiation rate compared to wild type.He found that the rate of photosynthesis is nearly identical in the mutant and wild type but the amount of sucrose that is synthesized by the mutant is only 30% of that in the wild type even there is no mutantion in any of the enzymes of sucrose synthesis.
Q:Where this mutantion might be (assume it is a single mutation) and what you would  do to test your hypothesis ?