Plants are one of the few organisms that can manufacture their own food. Photosynthesis, the process for doing so, involves a complex series of chemical reactions that convert solar energy into glucose, a more usable energy form that plants can consume or store until needed. Each plant species is different with respect to the amount of sunlight and water needed to create ideal photosynthesis conditions, but all plants need both of these elements along with carbon dioxide.
Carbon dioxide is exhaled during animal respiration. It is also a byproduct of decaying organic matter. The gas is crucial for photosynthesis to occur. The outer layer of plant tissue, the epidermis, has tiny openings called stomata that take in carbon dioxide and release photosynthesis byproducts. Enough carbon dioxide is in the atmosphere to replenish a plant's need for food manufacturing purposes.
Plants use water to help break down carbon dioxide so its components can be combined with those in water to form sugar molecules. Plants absorb water through the root system, so watering as required for the particular species is critical to photosynthesis. The need is even more pronounced during hot weather and in warmer climates as the sun evaporates water from the ground and from leaves.
An energy source is needed to convert carbon dioxide and water into sugar. Sunlight provides that energy source. How much is necessary for ideal photosynthesis conditions is dependent on the plant species. Some plants require full sun, others are more shade tolerant. But all plants need the sun's energy. Plants take in this energy with pigments, like chlorophyll or carotenoids, which give plants their familiar green hues and absorb energy. The presence of these pigments in leaves is important because leaves are the manufacturing sites where photosynthesis takes place.
In colder climates, few plants manufacture food in the winter. The warmth of spring and summer are signals for plants to begin photosynthesis to create new growth and to store up energy for the following winter. While not a direct ingredient of photosynthesis, warmer weather provides a cue and is part of the ideal mix of conditions needed.