Final answer:
The mesophyll tissue of a leaf would be most directly affected by a non-functional rubisco enzyme because it contains chloroplasts where rubisco operates during photosynthesis.
Step-by-step explanation:
The most directly affected tissue by a non-functional rubisco enzyme would be the mesophyll of a leaf. Rubisco, which stands for ribulose-1,5-bisphosphate carboxylase/oxygenase, is a crucial enzyme involved in the first major step of carbon fixation in the light-independent reactions of photosynthesis, commonly known as the Calvin cycle. These reactions occur in the chloroplasts found within mesophyll cells of the leaf.
The mesophyll layer is divided into two types: the palisade mesophyll, which is closely packed and where most of the photosynthesis takes place, and the spongy mesophyll, which has many air spaces to facilitate the exchange of gases (CO2 and O2). Without functional rubisco, carbon dioxide cannot be efficiently converted into organic compounds in the calvin cycle, ultimately affecting the plant's ability to produce glucose and other sugars for energy.