Plants actually don't give off carbon dioxide, they emit oxygen instead. It is due to the way their metabolism occurs. Plants take in carbon dioxide from the air, and turn it into sugars using energy from the sun and water. Plants build themselves out of carbon dioxide from the air. When a tree grows, it isn't formed from the soil. When we (humans) burn trees, we release this carbon dioxide back into the air. Our metabolism is in some ways the opposite of plants. We consume their sugar stores, combine it with oxygen, and break them back down into carbon dioxide, and release that into the air.

In the dark, plants breathe oxygen and give off carbon dioxide just like we do.

Plants absorb the air and water from their surroundings and use the carbon (from carbon dioxide) and hydrogen (from water) to build their cell walls (carbohydrates). They release the excess oxygen back into the air. Animals, like us, reverse this process. We breathe in oxygen and use it to break down the carbohydrates we eat, releasing carbon dioxide.

What you may be thinking about is the combustion of plants, which releases carbon dioxide. When burned, carbohydrates react directly with the oxygen in the air to create carbon dioxide and water. Many industrial processes involve burning plant matter (oil, coal, gas, wood, etc.) which produces and releases lots of carbon dioxide into the air. By contrast, very few processes (except plant growth) consume carbon dioxide to offset the increased in carbon dioxide that we produce.

In general, plants don't give off carbon dioxide unless they are burned. The natural respiration of plants actually pulls carbon dioxide out of the air. Using the energy from sunlight, the plants turn the carbon dioxide into sugars and starches, which are stored in the plant's roots, stems, and leaves.

When the plant dies, some carbon dioxide may be released as it decomposes. If a plant is burned, for example like firewood or biomass, it releases a lot of carbon dioxide in the process. If animals eat the plant, the animals will break down the sugars and starches for energy and they exhale the carbon dioxide that is produced by this respiration process.

Plants give off Carbon Dioxide for the same reason that we humans breathe out Carbon Dioxide every day. CO₂ is a waste product from a metabolic reaction called cell respiration. Basically, almost all organisms gain energy to do things by breaking down sugar in their bodies. You and I eat a candy bar, gain energy from the breaking down of the sugar and exhale CO₂. Plants use sunlight to convert light energy (attaching that energy to CO₂) into chemical energy (sugar) and then break down the sugar to grow and survive. Plants are always exhaling CO₂, but during the day they take in more CO₂ than they produce.