Question

why is it usually difficult to demonstrate respiration in green plants

Answer 1

because during respiration co2 is released, but at the same time, photosynthesis goes on in which co2 is consumed by plants. You can keep the plant in dark to measure the rate of respiration.

Respiration rate is often measured by the amount of carbon dioxide gas evolved during respiration. Carbon dioxide is consumed during photosynthesis so if there is light available, the carbon dioxide would quickly be used in photosynthesis. You could keep the plant in the dark during the period of time your are measuring the respiration rate.