Final answer:
During dry spells, C3 plants resort to photorespiration as a way to conserve water. This process leads to the fixation of oxygen instead of carbon dioxide and the production of phosphoglycolate, which is an unusable molecule in photosynthesis.
Step-by-step explanation:
Photorespiration occurs in C3 plants during dry spells when they close their stomata to conserve water. As a result, carbon dioxide cannot enter the leaves and oxygen cannot escape.
This leads to a decrease in CO2 levels and an increase in O2 levels, causing Rubisco, an enzyme involved in the Calvin cycle, to catalyze the fixation of oxygen instead of carbon dioxide. This leads to the production of phosphoglycolate, an unusable 2-carbon molecule, instead of the desired 3-carbon molecule in photosynthesis.