In the graphs titled "Rate of Photosynthesis Over Two 24 Hour Periods," besides light intensity and water, what major factor likely explains the differences between the daytime photosynthesis rates? A) carbon dioxide concentration B) temperature C) humidity D) oxygen concentration

What the question is really asking

You are comparing two daytime photosynthesis curves from the graph and choosing which additional limiting factor (other than light and water) most likely causes the rates to differ between the two days.

The main limiting factors of photosynthesis

For most plants, the big controllable limits on photosynthesis rate are:

• Light intensity
• Water availability (affects stomata and $CO_2$ entry)
• Temperature (controls enzyme reaction speeds)
• $CO_2$ concentration (the carbon source for sugars)

Since the prompt already removes light intensity and water, the best remaining general explanation is usually temperature.

Why temperature best explains different daytime rates

Photosynthesis depends on enzyme-catalyzed reactions (especially in the Calvin cycle). Enzyme activity changes with temperature, so a warmer (up to an optimum) or cooler day can shift the rate during daylight even if the timing of sunlight is the same.

Why the other options are less likely

• A) $CO_2$ concentration: It can limit photosynthesis, but in typical day-to-day outdoor conditions temperature swings are a more common “major factor” causing noticeable daytime differences when light and water are accounted for.
• C) Humidity: It mainly affects transpiration and stomatal opening indirectly, but water was already excluded, and humidity is not usually listed as a primary limiting factor.
• D) Oxygen concentration: $O_2$ is a product, not a main input; while high $O_2$ can increase photorespiration, it is not the most likely major cause of day-to-day daytime curve differences in this context.

Bottom line

Choose B) temperature because it directly changes the speed of photosynthetic reactions and commonly varies between days.