Final answer:
The growing season is the most vulnerable part of a plant's life cycle to high temperatures, which can shorten the season, increase transpiration rates, and fail to reduce pest populations.
Step-by-step explanation:
The most vulnerable part of a plant's life cycle to the effects of high temperatures is during its growing season. High temperatures can shorten the growing season, lead to increased water loss through transpiration, and sometimes allow pests to thrive due to milder winters not being cold enough to reduce pest populations. Plants have high and low thermal limits, and chronic exposure to temperatures above these can be lethal. For example, some plants may be killed by a short exposure to 38°C, while an exposure longer than 60 minutes to 36°C can also be lethal. Moreover, the mismatch between the flowering times of plants and the emergence of their insect pollinators due to the earlier onset of flowering can disrupt pollination and thus plant reproduction.