Answer: Heat stress can certainly cause the leaves to yellow. Too little water will make them wilt, so the yellowing is probably due to heat. Or, if your soil is alkaline, the yellowing could simply indicate a need for iron. There are both liquid and granular formulas of iron on the market. A few tablespoons of liquid in a gallon of water poured onto the root zone should be enough to help those leaves green up.
Best wishes with your trumpet vine.
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