The process by which plants respire at night is called respiration. This process also takes place during the day time (it's just that we do not concentrate on it during the daytime). In fact, respiration is a continuous process. During the daytime, plants convert the light energy of sun into chemical energy (along with generation of oxygen and consumption of carbon dioxide) through the process of photosynthesis. The organic molecules formed during this process are consumed during the process of respiration, in which plants consume oxygen and produce carbon dioxide and water, along with energy (in the form of ATP molecules). Energy is required for the plant's growth, maintenance and other activities and is a constant necessity. Hence, respiration is a constant process, irrespective of light availability. It is only that during the daytime, photosynthesis ensures that plants are net producers of oxygen (produces more oxygen than respiration can consume), while at night, the absence of photosynthesis means that plants are net producers of carbon dioxide. This is why we focus on the respiration of plants at night.
Hope this helps.
The process is called respiration and is the reverse of photosynthesis. While Photosynthesis produces oxygen and is a building process, respiration in plants produces carbon dioxide and is a breakdown process.
The following attached flow chart will help understand the Process of RESPIRATION.