Final answer:
The best way to determine the source of water a plant uses in photosynthesis is by measuring the oxygen isotope ratio in the plant's tissues, as this can differentiate between soil water and atmospheric water.
Step-by-step explanation:
You asked how to determine whether the water a plant uses during photosynthesis comes from the soil or the atmosphere. The correct answer is b) By measuring the oxygen isotope ratio in the plant's tissues. This method involves analyzing stable isotopes of oxygen in the water molecules within the plant. Since the ratios of oxygen isotopes in soil water can be distinct from those in atmospheric water, researchers can trace the source of the water used by the plant based on these differences.
Plants undergo photosynthesis to produce glucose and oxygen by using sunlight, carbon dioxide, and water. Typically, plants absorb water through their roots from the soil, which is then used along with carbon dioxide from the atmosphere. The oxygen produced during photosynthesis is partly from the water taken up by the plant's roots. Using isotope ratio analysis, scientists can deduce which part of the water used in photosynthesis comes from soil moisture and which comes from atmospheric moisture.