If transpiration stopped completely, how would a plant's homeostasis first be affected?

The answer is: the water would not be released from the leaves.

Transpiration is a water loss from a plant in the form of water vapor through small pores on the leaves. Plants take water from a soil through roots and release it into atmosphere through the leaves. This happens due to water gradient from the roots to the leaves. If for any reason transpiration stops, the water could not be released from the leaves, and consequently, the less water will be absorbed from the soil.