點解日間時,植物既光合作用率較呼吸作用率高,而晚間則相反?

係日間時..因為佢有足夠ge光度..photosynthesis係take place faster than respiration'
係晚間時..無光'光合作用stop左..only respiration take place.

It depends on the light intensity.
When daytime, the light intensity is high and the rate of photosynthesis is usually faster than the rate of respiration, so there are a net uptake( 吸入 ) of carbon dioxide and a net release of oxygen.
when it is night time, photosynthesis is stop and the respiration goes on. So there are an uptake of oxygen and a release of carbon dioxide from respiration.

晚上係冇陽光,植物進行唔到光合作用