Final answer:
The germination stage is most vulnerable to high temperatures as it affects a seed's ability to emerge from dormancy and sprout under appropriate conditions.
Step-by-step explanation:
The part of a plant's life cycle most vulnerable to the effects of high temperatures is germination. High temperatures can impact a seed's metabolic activity, potentially reducing its temperature tolerance and hindering the process of seed germination. During germination, seeds emerge from dormancy and require favorable conditions which can include moisture and appropriate temperatures. Extreme heat can disrupt these conditions, negatively affecting a seed's ability to successfully sprout and grow into a mature plant. It's crucial for seeds to germinate during conditions that support seedling survival; otherwise, they may not reach the vegetative growth stage.