Final answer:
Phenotypic modifications in response to environmental changes often depend on epigenetic modifications and enzyme regulation at the cellular level, which alter gene expression and biochemical reaction rates, respectively.
Step-by-step explanation:
Two important chronic modifications of phenotype exhibited in response to environmental change at the cellular level, often dependent on changes in the amounts of key enzymes, are epigenetic modifications and enzyme regulation. These modifications can be influenced by external factors such as stress, nutrient availability, heat, or ultraviolet light exposure, which lead to changes in gene expression and the cellular environment. Epigenetic modifications change how DNA is accessed and transcribed, while enzyme regulation adjusts the rates of biochemical reactions by controlling enzyme activity, which is dependent on factors such as pH and temperature.