Let's analyze each statement regarding alkaline earth metals to identify which one is NOT true:

1. Statement: Strontium salts burn with a red flame.
   • Explanation: Strontium salts are known for imparting a red color to flames. This is a well-established fact in chemistry, as strontium compounds are commonly used in pyrotechnics to produce red-colored fireworks.
   • Conclusion: This statement is true.
2. Statement: Barium metal is used in medicine to examine the alimentary canal of a patient by X-ray.
   • Explanation: Barium sulfate is utilized in medical imaging of the digestive system because it is radiopaque, meaning it does not allow X-rays to pass through it, thereby highlighting the digestive tract on X-ray images.
   • Conclusion: This statement is true.
3. Statement: Green plants need magnesium in order to make the chlorophyll that trap the energy from sunlight in photosynthesis.
   • Explanation: Magnesium is a central component of the chlorophyll molecule, which absorbs sunlight during photosynthesis. Therefore, it is essential for plants to produce chlorophyll.
   • Conclusion: While the statement itself is factual, it does not address the context of being incorrect or misleading about alkaline earth metals. Hence, it's not the statement that is NOT true based on typical misunderstanding or error. However, according to the question provided, it is designated as the correct answer choice.
4. Statement: Red color of gemstone Ruby is due to the element beryllium present in it.
   • Explanation: The red color of rubies is actually due to the presence of chromium ions, not beryllium. Rubies consist primarily of aluminum oxide, and chromium impurities cause the red color.
   • Conclusion: This statement is false, making it the statement that is NOT true about beryllium's role in rubies.

Final Conclusion: Based on common scientific knowledge and chemical properties of alkaline earth metals, the statement about the ruby gemstone is false, whereas the others hold true within their scientific contexts. However, taking into account the given question format, the statement about magnesium and chlorophyll is marked as the correct answer to the prompt. This discrepancy highlights a question design error or misunderstanding within the options rationale.